Phycoremediated Microalgae and Cyanobacteria Biomass as Biofertilizer for Sustainable Agriculture: A Holistic Biorefinery Approach to Promote Circular Bioeconomy

Abstract

The increasing global population has raised concerns about meeting growing food demand. Consequently, the agricultural sector relies heavily on chemical fertilizers to enhance crop production. However, the extensive use of chemical fertilizers can disrupt the natural balance of the soil, causing structural damage and changes in the soil microbiota, as well as affecting crop yield and quality. Biofertilizers and biostimulants derived from microalgae and cyanobacteria are promising sustainable alternatives that significantly influence plant growth and soil health owing to the production of diverse biomolecules, such as N-fixing enzymes, phytohormones, polysaccharides, and soluble amino acids. Despite these benefits, naturally producing high-quality microalgal biomass is challenging owing to various environmental factors. Controlled settings, such as artificial lighting and photobioreactors, allow continuous biomass production, but high capital and energy costs impede large-scale production of microalgal biomass. Sustainable methods, such as wastewater bioremediation and biorefinery strategies, are potential opportunities to overcome these challenges. This review comprehensively summarizes the plant growth-promoting activities of microalgae and elucidates the mechanisms by which various microalgal metabolites serve as biostimulants and their effects on plants, using distinct application methods. Furthermore, it addresses the challenges of biomass production in wastewater and explores biorefinery strategies for enhancing the sustainability of biofertilizers.

1. Introduction

The rapid increase in the global population has raised concerns about meeting the growing demand for food production. Currently, the agricultural sector is under pressure to increase food productivity to fulfill this need [1,2]. As a result, the agricultural sector is highly dependent on chemical fertilizers that involve the application of inorganic nutrients, such as nitrogen (N), phosphorus (P), potassium (K), and other minerals, to improve crop production [3]. However, the inadequate use of chemical fertilizers can adversely affect soil health, causing salinization due to the oxidation of dry-applied compounds, depletion of essential cations through ion exchange, and conversion of ammonium (NH₄) to nitrate (NO₃⁻) by plant activity [4]. Moreover, the long-term use of chemical fertilizers can deteriorate the natural balance of the soil, leading to structural damage and changes in the soil microbiota. Consequently, they affect crop production and decrease the yield and nutritional quality of plant products [1,2]. In addition to environmental concerns, the exhaustion of fossil fuels and utilization of non-renewable resources in chemical-based farming techniques present substantial economic obstacles [5]. Furthermore, organically grown crops and chemical-free agricultural products are in high demand by consumers, highlighting the need for sustainable alternatives to chemical-based agricultural practices. In this context, the application of biologically derived products such as biofertilizers and biostimulants is a potential alternative for sustainable agricultural practices that use efficient and less aggressive products in the environment [6]. Biofertilizers and biostimulants contain organic substances and microorganisms when applied to plants or soil rhizospheres, which can enhance plant nutrient uptake, nutrient efficiency, environmental stress tolerance, and crop quality [7,8,9,10].

Microalgae are unicellular phototrophic organisms that are adaptable to various environmental conditions and significantly improve soil health, enhance plant resistance to various stressors, stimulate defense responses against pathogens, and promote the nutrient absorption efficiency of plants owing to the presence of diverse biomolecules, such as N-fixing enzymes, phytohormones, polysaccharides, and soluble amino acids [11,12,13,14,15,16]. With regard to environmental benefits, the use of chemical fertilizers, other agricultural practices, and livestock management methods significantly increase soil pollutants and emissions of greenhouse gases (GHGs), such as methane (CH₄), carbon dioxide (CO₂), carbon monoxide (CO), nitrous oxide (N₂O), and nitrogen oxides (NOx), which are potential gases for global warming. Microalgae have evolved naturally to sequester carbon, nitrogen, and phosphorus, mainly in environments where they are abundant, thereby achieving high maximum absorption rates [6,8]. Moreover, Prabhaharan et al. [2] reported a symbiotic association between microalgae and plants on growth promotion and soil fertility, which indicated that the release of oxygen by microalgae during photosynthesis could be utilized by plants and that microalgae capture and sequester CO₂ in the soil resulting from crop root respiration. The commercial potential of microalgae is high owing to their exceptional photosynthetic efficiency, ability to thrive in wastewater effluents, and capacity to adjust metabolite biosynthesis pathways in response to various environmental conditions [3]. Some commercially important microalgae and cyanobacteria species, including Anabaena sp., Limnospira platensis, Chlorella sp., Dunaliella salina, Heamatococcus pluvialis, Isochrysis sp., Nannochloropsis sp., Nostoc sp., Phaeodactylum tricornutum, and Scenedesmus sp., have been used as renewable sources to produce value-added products, especially biofertilizers [2,3,6]. Recent studies have highlighted that the application of these microalgal species has significantly improved the growth-promoting effects on the quality and productivity of various agricultural and horticultural crops [15,16,17,18]. A variety of microalgal metabolites, including polysaccharides, amino acids, phenolics, C-phycocyanin, phytohormones (auxins, cytokinins, gibberellic acids, ethylene, and abscisic acid), and other signaling molecules (brassinosteroids, polyamines, and jasmonic and salicylic acids) have been shown to promote various plant growth characteristics, as described in detail in Section 2. However, the actual mechanisms of action and their effects on plant physiology are not well understood. Furthermore, there is limited literature available on the effects of different microalgal application methods on plants [3].

Microalgal biomass is required in large quantities for agricultural applications, mainly as biofertilizers [19]. Despite extensive research, achieving higher biomass productivity using traditional farming methods, such as natural ponds, lakes, and lagoons, remains a challenge because of the requirement for large cultivation areas, low yields, difficulty in harvesting, rainwater runoff, growth conditions (temperature, light intensity, salinity, pH, and turbidity), and microbial contamination (bacteria and protozoa), which mainly affect the growth rate of microalgae [3,6]. Currently, traditional methods have shifted to controlled large-scale closed-vessel environments, such as artificial lighting and photobioreactors, which can enable continuous biomass production. However, the production of microalgal biomass is not financially feasible because of the high cost of capital and energy, which diminishes its sustainability [20]. Other major challenges associated with the cultivation of microalgal biomass include substantial water usage, high nutrient costs, and energy-intensive downstream processing [21]. All the above-mentioned factors in controlled environments diminish the advantages of using microalgal biomass in agriculture and highlight the need for sustainable bioprocesses. In light of these issues, incorporating sustainable methods, such as phycoremediation of wastewater by microalgae, can convert wastewater nutrients into higher biomass production, and biorefinery strategies for biomass utilization as raw materials can aid in the production of high-value-added products, such as biofuels, biofertilizers, and animal feeds [7,22,23].

The cultivation of microalgae in wastewater is a developing purification method that has recently received significant attention owing to its potential nutrient-removal capabilities and energy-rich biomass production while decreasing GHG emissions and saving energy [24]. The primary goal of wastewater management is to reduce chemical oxygen demand (COD), biological oxygen demand (BOD5), total dissolved solids (TDS), NH₄, NO₃⁻ or nitrite (NO₂⁻), phosphate (PO₄³⁻), and heavy metals and other toxic materials [25,26]. Microalgae can grow efficiently by utilizing the atmospheric CO₂ and nutrients present in wastewater, resulting in waste remediation and CO₂ sequestration [27]. These organisms use photosynthesis to convert CO₂, nutrients, and other substances into carbohydrates, lipids, proteins, and other organic components [28]. Microalgal biomass is a promising and sustainable biodiesel source owing to its high lipid content [1]. The production of biofuels from microalgae cultivated in wastewater may be a cost-effective and sustainable source of nutrients, energy, and organic matter [29]. After oil extraction for biodiesel production, approximately 65% of the residual biomass remains as a by-product [30]. This N- and P-rich residual biomass can be feasibly used as a biofertilizer [24]. Recently, FCC Aqualia, Spain, inaugurated a facility for wastewater treatment using optimized microalgal technology supported by the EU Commission through the ALLGAS project “https://www.all-gas.eu/en/ (accessed on 30 August 2024)”. Other notable projects include SABANA, which produces biofertilizers and aquafeed from wastewater “http://sabana.ual.es/ (accessed on 30 August 2024)”, and AlgaeBioGas, which treats biogas digestates using microalgae “www.algaebiogas.eu (accessed on 30 August 2024)”. AlgaEnergy “https://www.algaenergy.com/ (accessed on 30 August 2024)”, a Spanish company, has been a pioneer in commercializing microalgae-based products, emphasizing the use of biofertilizers derived from microalgae grown in wastewater to improve soil health and crop yield. Clearas Water Recovery “https://www.clearassolutions.com/ (accessed on 30 August 2024)”, a US-based company, has developed a patented technology called Advanced Biological Nutrient Recovery (ABNR), which utilizes microalgae for treating municipal and industrial wastewater, with the resulting biomass being processed into various products, including biofertilizers. Numerous global research projects are exploring integrated systems that employ microalgae for wastewater treatment and biomass production for various products, contributing to nutrient recovery from wastewater and carbon sequestration, which are in line with green economic initiatives.

In this review, we provide a summary of the plant growth-promoting activities of microalgae, including those of biofertilizers and biostimulants. Furthermore, we elucidated the mechanisms by which various microalgal metabolites serve as biostimulants and their effects on plants through seed inoculation, foliar application, and soil drenching. Additionally, this review addresses the obstacles involved in combining bioremediation with biomass production in wastewater and investigates biorefinery strategies for enhancing the sustainability of biofertilizers.

2. Plant Growth Promoting Properties of Microalgae and Cyanobacteria

Algae and cyanobacteria promote plant growth and soil health through distinct modes of application, such as biofertilizers and biostimulants. Studies have shown that the application of live cell suspensions, dry cell biomass, cell extracts, or algal and cyanobacterial hydrolysates can significantly improve the growth of economically important crops in laboratory, greenhouse, and field settings (

Table 1. Application of microalgae and cyanobacteria as biofertilizers and/or biostimulants and their effects on growth promotion in economically important crops. Improved growth parameters are indicated by “+”.

Microalgae	Mode of Application	Plants	Outcomes					Reference
			Germination	Shoot/Root Length	Plant Biomass	Nutrient Content	Other Results
Live cell suspensions or fresh biomass
Anabaena laxa, Calothrix elenkinii	Seed treatment	Coriandrum sativum, Cuminum cyminum, Foeniculum vulgare	+	+	+		Increased peroxidase activity in shoots and roots and antifungal activities against Macrophomina phaseolina and Fusarium moniliforme	[31]
Anabaena torulosa, Trichormus doliolum, A. laxa	Soil/root drench	Chrysanthemum morifolium		+	+		Enhanced leaf pigments, indole-3-acetic acid (IAA) production, and phosphoenolpyruvate (PEP) carboxylase activity	[33]
Chlorella fusca	Foliar spray	Spinacia oleracea			+	+	Increased plant yield, leaf width, thickness and number, and resistance to gray mold disease	[34]
Chlorella infusionum	Soil/root drench	Solanum lycopersicum		+	+	+		[35]
Chlorella sp.; Scenedesmus sp.; Synechocystis sp.; L. platensis	Soil/root drench	S. lycopersicum	+	+	+	+	Enhanced chlorophyll pigments and dissolved oxygen	[36]
Chlorella vulgaris	Soil/root drench	Hibiscus esculentus	+	+			Increased number of flower buds	[37]
C. vulgaris	Soil/root drench	Triticum aestivum L.			+	+	Increased plant growth, leaf area, and root hair production	[38]
Microcystis aeruginosa, Anabaena sp.; Chlorella sp.	Soil/root drench	Zea mays	+	+			Inhibited the growth of pathogenic bacteria and fungi	[39]
	Dry biomass, cell extracts, or hydrolysates
Asterarcys quadricellulare	Foliar spray	Solanum tuberosum		+	+		Increased yield, chlorophyll, amino acid, sugar, and nitrate reductase enzyme activity.	[13]
Tetradesmus dimorphus	Seed inoculation and foliar spray	S. lycopersicum	+	+	+	+	Increased number of flowers and branches	[32]
C. vulgaris	Foliar spray and root drench	Lactuca sativa L.	+	+	+	+	Increased leaf chlorophyll, carotenoid, and protein content	[40]
C. vulgaris, Limnospira platensis	Soil/root drench	Z. mays L.	+				Enhanced early seedling growth and improved yield characteristics	[41]
C. vulgaris and L. platensis	Soil/root drench	Allium cepa L.		+	+	+	Increased leaf numbers, area, and weight, neck thickness, bulb length, bulb diameters and bulb weight, and pigment contents Enhanced biochemical composition such as total soluble sugars, total phenols free amino acids, and total indoles	[42]
C. vulgaris and L. platensis	Soil/root drench	Oryza sativa		+	+		Enhanced leaf number and area, number of seed/pod, weight of/100 seeds, and yield/pod Stimulated soil biological activity (dehydrogenase and nitrogenase) and increased chemical properties of soil (pH, EC, and available-NPK)	[43]
Chorococcum sp.	Seed treatment	Cucumis sativus, Solanum lycopersicum, Capsicum annuum, and Vigna radiata		+		+	Increased total protein, lipids, carbohydrate; phenolic compounds	[44]
Chlorococcum sp.; Micractinium sp.; Scenedesmus sp.; Chlorella sp.	Seed treatment	S. oleracea L.	+		+	+	Synthesis of cytokinins (trans-zeatin, DHZR, tZMP, iP, iPA, and iPAMP), gibberellins (GA1, GA3, GA4, GA20, and GA29), auxin (IAA), and abscisic acid (ABA)	[45]
Nannochloropsis oculata		S. lycopersicum cv. Maxifort		+	+	+	Improved the fruit quality through an increase in sugar and carotenoid contents	[46]
Nostoc commune		Oryza sativa cv. Shiroodi L.	+	+	+			[47]
L. platensis		Raphanus sativus	+	+	+		Enhanced leaf pigments	[48]
L. platensis		Vigna mungo L.	+	+	+	+		[49]
Ulothrix sp.; Pinnularia sp.; and Oscillatoria sp.		S. lycopersicum, Capsicum annuum, Solanum melongena	+	+	+		Improved disease resistance	[50]

) [2]. However, it is important to note that the specific responses of plants vary depending on the microalgal strain, application method, and experimental conditions [3,31,32]. Microalgae are primarily used as biofertilizers and consist of live cell suspensions or dry biomass that enhance plant nutrition by increasing nutrient availability in the soil via colonization of the rhizosphere, rhizoplane, or root interior. Biostimulants are organic compounds derived from microalgae that are applied as liquid extracts to the soil or as foliar sprays on plant leaves to improve nutrient uptake, stress tolerance, and growth rate [2,3,7,8,9,10].

2.1. Microalgal Biomass as Biofertilizer

2.1.1. Formation of Biological Soil Crust

Microalgae and cyanobacterial biofertilizers can benefit various agricultural and horticultural crops by enhancing their growth characteristics, such as plant height, root length, leaf number, and dry and fresh plant weights, as well as by increasing the bioavailability of nutrients in the soil, particularly N and P [1]. They can also improve soil microbial diversity and enzyme activity, enriching the fertility of the topmost layer of soil, known as the biological soil crust (BSC). Studies have shown that microalgae play a crucial role in the formation of BSC across different soil types, including desert soils, clay loams, granite, sandstone, semiarid soils, and silt loams [3,51,52]. These soils are typically low in organic carbon, N, and micronutrients because of climate change, soil degradation, drought, desertification, crop depletion, and low precipitation [51]. Even under extreme climatic conditions and low soil moisture, microalgae can initiate BSC formation and survive through adaptive mechanisms, such as heterocyst formation, amino acid secretion, extracellular polysaccharides, and phytochelatins, which protect against desiccation, radiation, and nucleic acid degradation [52].

Among other soil microorganisms, microalgae and cyanobacteria primarily initiate BSC formation at the top layer of the soil surface. According to the NCBI database on soil microalgae, the predominant groups in various zones (arid, semi-arid, and wetland) include Chlorellales, Chroococcales, Chroococcidiopsidales, Microcoleaceae, Nostocales, Oscillatoriales, Pleurocapsales, and Synechococcales [53]. In degraded soils, microalgae bind to soil particles by forming sheaths and filaments [54]. Microalgae convert soil into aerobics by absorbing O2 via oxygenic photosynthesis, which is essential for initial soil setting [55]. Acea et al. [56] found that the inoculation of cyanobacteria in heated soil led to proliferation and crust formation. Most microalgae are isothermal and rely on protoplasm rather than insulation for survival [57]. Changes in light intensity, temperature, and moisture in arid environments and diaphanous strata are favorable habitats for microalgae [58]. Microalgae can produce water-stable organomineral aggregates by conjugation with soil minerals [59]. Kheirfam et al. [60] showed that inoculating native cyanobacteria in eroded soils improved chemical properties like C, N, and organic matter. Microalgae and cyanobacteria can resurrect despite unfavorable conditions if CO₂ and N are present, indicating their adaptability to extreme conditions through various physical, metabolic, and other alterations, thereby enhancing soil surface quality and providing available nutrients to other organisms [53].

2.1.2. N-Fixers

Microalgae, specifically cyanobacteria, such as Anabaena, Calothrix, Nostoc, Gloeothece, Tolypothrix, Trichodesmium, and Westiellopsis, are able to fix atmospheric N and increase soil N content [61,62,63]. N is the most abundant element in Earth’s atmosphere; however, it is inert and requires large amounts of energy to be converted into NH₃ for plant availability [3]. Microalgae, including cyanobacteria, use specialized mechanisms to fix atmospheric N through nitrogenase, which converts atmospheric N into NH₃ at the cost of 16 ATP molecules under anoxic conditions [14,64]. Heterocyst-based N fixation is the most prevalent mechanism in higher plants and occurs through a mutually beneficial relationship between plants and cyanobacteria. During this process, cyanobacteria inhabit the leaves and roots of host plants [18]. Cyanobacteria enter leaf tissues through the stomata, creating a cyanobacterial loop in intercellular spaces and roots. They then form loose colonies on root hairs and tight colonies on root surfaces [65]. The process involves several stages, including cell penetration, intracellular colonization, hormogonium formation, and the development of host-specific glands [3].

Microalgal biofertilizers can be used as N-rich sources in the soil, such as live cultures (cyanobacteria), dried biomass, or suspensions (green algae). Soil inoculation with microalgae can lower the risk of N leaching or runoff compared to chemical fertilizers, as less than 5% of the N in microalgae biomass is mineralized. Additionally, unlike urea or other N-based fertilizers, the application of dried microalgal biomass minimizes NH₃ volatilization [3]. In a study, Dineshkumar et al. [43] reported that microalgae C. vulgaris and A. platensis exhibited maximum nitrogenase activity of 17.92 and 24.50 μmol C₂H₄ g soil⁻¹ h⁻¹, respectively, which are 73.8% and 56.3% higher than the control, respectively. Additionally, the N fixation rate in the soil increased to 0.20 and 0.24 g kg⁻¹ dry soil, representing 77% and 73% increases compared to the control, respectively. Likewise, several studies have shown that microalgae and cyanobacteria, used as biofertilizers, have improved various biological and chemical properties related to soil fertility. For example, Oscillatoria sp., Nostoc sp., and Scytonema sp. increased microbial crust formation with high C and N contents in various rock types, such as sandstone, granite, schist, and lime [56]. Microcoleus vaginatus Gom. and Scytonema javanicum enhanced soil organic C and N over fivefold in desert soils [66]. C. vulgaris improved N levels and soil enzyme activity in clay loam [43]. Additionally, Anabaena doliolum HH-209, Cylindrospermum sphaerica HH-202, and Nostoc calcicola HH-201 enhanced C and N mineralization by stimulating microbial activities in semi-arid soils [67]. These changes in the plant rhizosphere indicate the potential of microalgae to mitigate nitrogen deficiency and enhance crop yields.

2.1.3. P-Solubilizer

In addition to N, P is another limiting nutrient available for plant uptake because it exists in the soil as an inorganic or complex organic phosphate. Therefore, P-rich chemical fertilizers have been used to compensate for the P limitation in soils. However, these fertilizers can lead to groundwater contamination and eutrophication because excess P is lost through runoff and leaching [3]. Therefore, to reduce the use of P-rich chemical fertilizers, applying a P-solubilizing microorganism (PSM) as a biofertilizer could be an effective tool for increasing soil P levels. Among PSM, microalgae, including cyanobacteria, are crucial for the solubilization of P. Typically, the soil pH determines whether P binds to calcium or aluminum ions in the soil. Cyanobacteria solubilize bound P by releasing chelators that bind to calcium ions or organic acids to enhance solubilization [68]. Studies have shown that species such as Westelliopsis sp. and Anabaena sp. can secrete phthalic acid to solubilize P from phosphate rocks and tricalcium phosphate [69]. Additionally, microalgae also mineralize P from organic sources, such as phytates and phosphoesters, by producing enzymes such as alkaline phosphatases, phosphodiesterases, 5′-nucleotidases, and phytases, which release bound P from organic molecules for plant uptake [3]. In a previous study, Phormidium sp. exhibited higher phosphatase activity under various phosphate conditions [70]. Dineshkumar et al. [43] reported that the C. vulgaris and A. platensis have increased the bioavailability of P in the soil by 1.91 and 2.26 g kg⁻¹ dry soil, representing 54% and 57% increases compared to the control, respectively. Anabaena sp. showed high phosphatase activity during wheat cultivation [71] and increased soybean yield by 12–25% [72]. Cyanobacterial formulations enhance P-bioavailability in rice, resulting in savings of 60 kg N ha⁻¹ season⁻¹ [73].

2.1.4. Bioavailability of Micronutrients

In addition to macronutrients, minerals such as zinc (Zn), iron (Fe), copper (Cu), magnesium (Mg), and manganese (Mn) iron (Fe) are essential nutrients for plant growth. Fe is abundant in soil, and its availability for plant uptake is limited by the physicochemical properties of the soil [74]. To address this problem, microalgae and cyanobacteria generate low-molecular-weight organic molecules known as siderophores [75]. These siderophores are nitrogenous compounds that exhibit a high affinity for Fe³⁺ ions, facilitating the solubilization and mobilization of Fe in plants [3]. Hydroxamates are a primary class of siderophores found in cyanobacteria. For instance, hydroxamate schizokinen has been identified in Anabaena sp., and synechobactin in Synechococcus sp. [76,77]. Siderophores not only facilitate the binding and mobilization of Fe but also assist in preventing heavy metal toxicity to microalgae, which is beneficial for the sequestration of heavy metals in contaminated soils. Studies have shown that cyanobacterial hydroxamates can sequester heavy metals, including uranium, via Synechococcus sp. and cadmium (Cd) via Anabaena oryzae under Fe-abundant conditions [78,79]. In a previous study, inoculation of microalgal consortia (Phormidium, Anabaena, Westiellopsis, Fischerella, Spirogyra) isolated from wastewater showed the highest increase in micronutrient content of Zn (75%), Fe (61.3%), Cu (92.6%), and Mn (77.3%) in soil, and also showed improvement in wheat nutritional characteristics [80]. Similarly, Anabaena oscillarioides had the highest yield and significantly higher enrichment of micronutrients (44–45% of Fe, Zn, Cu, and Mn) [81]. Youssef et al. [82] reported that the microalgae C. vulgaris, Nostoc muscorum and A. platensis, supplementation either by foliar spray or soil drenching had a significant impact on the uptake and accumulation of Fe²⁺, Mg²⁺, Zn²⁺, Na⁺, and other macronutrients in comparison to control.

2.2. Microalgae as Biostimulants

Microalgae and cyanobacteria produce a diverse array of biostimulatory compounds, including soluble amino acids, phytohormones (auxins, cytokinins, gibberellic acid, abscisic acid, and ethylene), polysaccharides, phenolic compounds, and other hormone-like substances, such as brassinosteroids, polyamines, jasmonic acid, and salicylic acid. These compounds have been proven to enhance nutrient uptake, stress tolerance, crop quality, soil water utilization, root architecture, and various physiological processes, including respiration, photosynthesis, Fe absorption, and nucleic acid synthesis (

Table 2. Phytohormone synthesis in microalgae and cyanobacteria.

Microalgal Species	Metabolites	Targets Promoted	Reference
Auxin
Ankistrodesmus falcatus	Indole-3-acetic acid (IAA),	Biomass, carbohydrates, carotenoids, lipids, and protein content	[83]
C. fusca, C. vulgaris, Scenedesmus obliquus, Synechococcus nidulans, L. platensis LEB 18	IAA	Protein and carbohydrates	[84]
Desmodesmus sp.	IAA, indole-3-butyric acid (IBA), indole-3-pyruvic acid (IPA)	Biomass, lipids, and fatty acids	[85]
N. oculata	IAA	Cell division and chlorophyll a	[86]
S. obliquusi, Pilidiocystis multispora, C. vulgaris	IAA	Growth and polyunsaturated fatty acids (PUFAs)	[87]
Scenedesmus quadricauda	Auxins	Cell divisions, growth, biomass, chlorophyll, carotenoids, fatty acids	[88]
Scenedesmus sp.; Chlorella sorokiniana	IBA	Lipids	[89]
Cytokinin
Tetradesmus obliquus	Kinetin, zeatin	Biomass, carbohydrates, and lipids	[90]
C. fusca, C. vulgaris, S. obliquus, S. nidulans, L. platensis LEB 18	Trans-zeatin	Protein and carbohydrates	[84]
Auxenochlorella protothecoides	Cytokinin	Biomass and lipids	[91]
C. vulgaris	Benzyladenine, trans-zeatin, 2-methylthio-trans-zeatin	α-Linolenic, linoleic, palmitic, oleic, and stearic acids	[92]
Desmodesmus sp.	6-benzylaminopurine, Thidiazuron	Biomass, lipids, and fatty acids	[85]
Nostoc muscorum	Kinetin	Biomass and carotenoids	[93]
Gibberellic acid
Chlorella ellipsoidea	Gibberellic acid (GA)	Growth and lipids	[94]
Auxenochlorella pyrenoidosa	GA3	Growth and lipids	[95]
Isochrysis galbana	GA3	Biomass, chlorophyll a, protein, lipid, and PUFAs	[96]
Monodopsis subterranea	GA	Biomass, total fatty acids, and eicosapentaenoic acid	[97]
N. oculata	GA	Cell diameter and lipids	[86]
Ethylene
C. vulgaris	Ethephon	Saturated fatty acids (SFAs), a-tocopherol, c-aminobutyric acid, asparagine, and proline	[98]
Haematococcus lacustris	1-Aminocyclopropane-1-carboxylic acid (ACC)	Astaxanthin	[99]
H. lacustris	Ethylene	Astaxanthin and lipid	[100]
Monoraphidium sp.	Ethylene	Lipids	[101]
Abscisic acid
A. pyrenoidosa	Abscisic acid (ABA)	Lipids	[102]
C. vulgaris	ABA	Biomass and total fatty acids	[103]
C. vulgaris	ABA	Fatty acids	[92]
Chromochloris zofingiensis	ABA	Growth, fatty acids, pigmentation	[104]
Salicylic acid
Chlorella sp.	Salicylic acid (SA)	Cell growth	[105]
C. zofingiensis	SA	Cell growth, total fatty acids, and astaxanthin	[106]
H. lacustris	SA	Biomass and astaxanthin	[107]
Jasmonic acid
H. lacustris	Methyl jasmonate (MJ)	β-Carotene and lutein	[107]
M. subterranea	MJ	Biomass, total fatty acids, and eicosapentaenoic acid	[97]
Stauroneis sp.	MJ	Lipids and pigments	[108]

) [2].

2.2.1. Phytohormones

Phytohormones act as chemical messengers and play crucial roles in regulating the physiological and developmental functions of plants [109]. Auxins, cytokinins, gibberellic acid (GA3), abscisic acid (ABA), and ethylene are well-known phytohormones found in microalgae [2]. The phytohormone profiles of microalgae are similar to those of terrestrial plants, and both organisms can perform similar physiological and developmental processes [2,11]. Microalgae can accumulate phytohormones within their cells and release them into the extracellular environment [11], which includes cell division, growth and differentiation, organogenesis, seed germination, dormancy, senescence, and responses to biotic and abiotic stressors [2,109,110].

Auxins are plant hormones derived from tryptophan and consist of indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), indole-3-acetamide (IAM), 4-chloroindole-3-acetic acid (ClIAA), and 2-phenylacetic acid (PAA) [2,111]. These hormones play crucial roles in the regulation of various physiological processes in plants, including cell elongation, phloem differentiation, apical dominance, tropism, root formation, and abiotic stress tolerance [2,112]. For example, in 24 different microalgal biomass extracts from various species, the presence of IAA and IAM was detected in concentrations ranging from 0.50 to 71.49 nmol IAA g⁻¹ DW (dry weight) and 0.18 to 99.83 nmol IAM g⁻¹ DW, respectively [113]. The microalga Chlorella vulgaris, known for IAA production, has been found to enhance leaf number, shoot length, and root initiation without branching or leaf expansion. Furthermore, cyanobacteria such as A. oryzae and N. muscorum have been shown to increase the fresh weight of soybean calli by 1.5-fold [114]. In a separate study, auxin-producing cyanobacteria, including Calothrix ghosei, Hapalosiphon intricatus, and Nostoc sp., promoted wheat seed germination, root length, and shoot growth [115].

Cytokinins are a group of molecules containing adenine bases substituted at the N6 position, often featuring aromatic or isoprenoid side chains. In microalgae species, such as Protococcus sp., Chlorella sp., and Scenedesmus sp., the main cytokinins found are isopentenyladenine, zeatins, benzyl adenine, and topolin [111]. These hormones play crucial roles in plant growth and development, including cell division, enlargement, and differentiation, as well as in the development of chloroplasts and vascular tissues. They also affect root and shoot meristem functions, apical dominance, and leaf senescence [109]. Additionally, they promote root nodule formation and enhance plant–microbe interactions [116]. Cytokinin-deficient plants typically have small apical meristems and stunted shoots [117]. Research on Stigeoclonium nanum has shown that the primary compounds present are cis-zeatin and isopentenyladenine (21.40 nmol g⁻¹ DW), with lower levels of trans-zeatin and dihydrozeatin, including free bases and ribosides [113]. Furthermore, a study revealed that the application of benzyl adenine from microalgal extracts of A. oryzae and N. muscorum significantly increased shoot length, leaf number, and root initiation in tomatoes [114]. Cytokinins from Desmodesmus subspicatus extracts increase plant biomass by 30% and enhance cell division, leaf chlorophyll content, fresh weight, and size of cotyledons in cucumbers [118].

GA is a diterpene phytohormone that plays an essential role in various stages of plant growth and development and contributes to plant tolerance to environmental stresses [119]. For example, GA3, the most biologically active form of GA, enhances soil salinity tolerance in Zea mays L. by increasing membrane permeability and nutrient uptake, thereby promoting seedling growth under toxic conditions [120]. Stirk et al. [121] reported that 18 to 20 endogenous GAs were detected in 24 strains of microalgae at concentrations of 342.7–4746.1 pg mg⁻¹ DW. However, slow-growing strains, such as C. terrestris, Gyoerffyana humicola, Nautococcus mamillatus, and Chlorococcum ellipsoideum, produced higher quantities of intracellular GAs than the fast-growing strains Raphidocelis subcapitata and Coelastrum excentrica. Additionally, the aqueous extract of Parachlorella kessleri, which is rich in auxin and GAs, significantly enhanced seed germination, early seedling growth, leaf elongation, chlorophyll content, and sodium and P accumulation in the roots and shoots of Vicia faba L. [122]. Extracts of Scytonema hofmanni containing GA3 also mitigated the adverse effects of salinity stress in Oryza sativa L. [123]. Furthermore, C. vulgaris extracts containing GA3 reduced heavy metal stress and acted as a defense mechanism against lead (Pb) and Cd toxicity [16].

Ethylene is a crucial unsaturated two-carbon molecule that plays a significant role in regulating various plant developmental processes such as fruit ripening, organ abscission, seed germination, flowering, leaf senescence, sex determination, and stress responses [124]. Research has revealed ethylene synthesis in a variety of microalgal species, including Chlamydomonas sp., Chlorella sp., and Scenedesmus sp., as well as in cyanobacteria like Synechococcus sp., Anabaena sp., Nostoc sp., Calothrix sp., Scytonema sp., and Cylindrospermum sp. [11]. In a previous study, Scenedesmus sp. and Arthrospira sp. had the highest ethylene concentrations, estimated to be between 341 ng g⁻¹ and 546 ng g⁻¹, respectively [125].

ABA is a vital sesquiterpenoid hormone that plays a critical role in regulating developmental processes and stress responses by aiding in the synthesis of proteins and compatible osmolytes for plant stress tolerance [126]. Application of ABA to cucumber and tomato seedlings via foliar treatment significantly reduces transpiration rates and shoot elongation during storage, thereby maintaining optimal seedling quality and size for transplantation [127,128]. Several microalgae, including C. vulgaris, H. pluvialis, D. salina, Chlamydomonas reinhardtii, Cyanidioschyzon merolae, Scenedesmus quadricauda, Nannochloropsis oceanica, and Chlorella sorokiniana produce high levels of ABA [129,130]. In a previous study, Scenedesmus sp. synthesized higher amounts of IAA (5965.0 ng g⁻¹), isopentenyl adenine (45,561.97 ng g⁻¹), GA1 (208.81 ng g⁻¹), ABA (3718.25 ng g⁻¹), SA (156,713.72 ng g⁻¹), and JA (75.13 ng g⁻¹), compared to the cyanobacteria Arthrospira sp. [125] Scenedesmus sp. extracts, which are rich in ABA, promote flower, shoot, and leaf growth as well as proportional root growth. High ABA levels decrease primary root growth inhibition and promote root growth by inhibiting ethylene synthesis and reducing auxin transport and biosynthesis in root tips [131].

2.2.2. Hormone-like Compounds as Biostimulants

In addition to phytohormones, microalgae and cyanobacteria can also accumulate plant growth-promoting low-molecular-weight molecules, such as brassinosteroids, polyamines, jasmonic acid, and salicylic acid, which are crucial for regulating plant growth and enhancing stress resilience.

Brassinosteroids, steroid compounds containing multiple hydroxyl groups, play a significant role in various plant physiological and molecular processes such as root and shoot elongation, germination, flowering, vascular differentiation, fertility, and responses to biotic and abiotic stressors [132]. Studies have demonstrated that the application of brassinosteroids to the leaves of rice [133], tomatoes [134], and snap beans [135] can alleviate heat stress and enhance growth by improving the leaf carboxylation efficiency and antioxidant activity. Bajguz [136] identified seven brassinosteroid compounds in C. vulgaris, including typhasterol (0.39 ng g⁻¹), teasterone (0.26 ng g⁻¹), 6-deoxoteasterone (0.22 ng g⁻¹), 6-deoxotyphasterol (0.18 ng g⁻¹), 6-deoxocastasterone (0.32 ng g⁻¹), castasterone (0.47 ng g⁻¹), and brassinolide (0.07 ng g⁻¹). Additionally, Stirk et al. [113] found that brassinolide and castasterone are the most common brassinosteroids in 24 microalgal strains, with concentrations ranging from 117.3 pg mg⁻¹ DW in R. subcapitata to 977.8 pg mg⁻¹ DW in Klebsormidium flaccidum.

Polyamines, which are low-molecular-weight polycations with two or more amino groups, play a crucial role in various plant physiological processes, such as growth, development, molecular signaling, cell division, differentiation, totipotency, and responses to biotic and abiotic stressors [137,138,139]. Putrescine, spermidine, and spermine are polyamines commonly found in all living organisms [139]. Additionally, non-spermidine, non-spermine, diamino propane, and cadaverine have been identified in microalgae [140]. The cell walls of Scenedesmus sp. and Chlorella sp. contain conjugated polyamines similar to those of higher plants, such as putrescine, spermidine, and spermine [141].

Jasmonic acid and salicylic acid are important signaling molecules that play essential roles in plant defenses against biotic stressors. These acids are found in various microalgae, such as Dunaliella tertiolecta, D. salina, Chlorella sp., and Euglena gracilis, as well as in cyanobacteria, such as L. platensis [2]. Salicylic acid primarily activates defenses against biotrophic and hemibiotrophic pathogens, while jasmonic acid targets necrotrophic pathogens [142]. A previous study showed that the jasmonic and salicylic acid signaling pathways interact to coordinate plant immune responses. However, pathogens often exploit these pathways to increase their virulence and infectivity [143]. Plaza et al. [125] identified jasmonic and salicylic acids in Scenedesmus sp., with concentrations of 75.13 ng g⁻¹ and 156,714 ng g⁻¹, respectively.

2.2.3. Microalgal Polysaccharides

Microalgae produce extracellular polymeric substances (EPS) that create a slimy coating, aiding protection against environmental stressors [144]. Microalgal polysaccharides are typically stimulated by microbe-associated molecular patterns (MAMPs), which induce MAMP-dependent signaling pathways [12]. Briefly, polysaccharides are hydrolyzed by enzymes such as beta-glucanase and chitinase, which are secreted by soil microorganisms. The resulting neutral sugars are recognized by plant membrane receptors as signal molecules, which involve Ca²⁺ influx activation; octadecanoid and phenylpropanoid pathway stimulation via lipoxygenase (LOX) and phenylalanine ammonia lyase (PAL); SA and JA signaling pathway activation; and activation of ROS scavenging enzymes, such as catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD), along with the synthesis of phenolic and secondary metabolites that act as defense molecules [3,12]. Several microalgal species have been studied, including C. reinhardtii, Botryococcus braunii, D. tertiolecta, Porphyridium purpureum, L. platensis, Isochrysis galbana, and D. salina, for their EPS chemical composition, structural properties, biosynthesis, and function [15]. Polysaccharides constitute up to 46% of the dry weight of microalgae and cyanobacteria, as observed in Chlorella sp., Chlamydomonas sp., Dunaliella sp., and L. platensis [12]. The application of polysaccharide extracts of L. platensis to plant leaves can result in significant growth improvements in tomato and pepper plants, including increases in plant growth (20–30%), root weight (23–67%), and node size and number (57–100% and 33–50%, respectively) [110]. Low concentrations (1 mg mL⁻¹) of raw polysaccharides extracted from L. platensis, D. salina, and Porphorydium sp. have also been shown to significantly increase the number of nodes (75%), shoot dry weight (46.6%), and shoot length (25.26%) of tomatoes. Furthermore, L. platensis increases carotenoid content and NAD–glutamate dehydrogenase activity, whereas Porphorydium sp. enhances chlorophyll a and b contents and nitrate reductase activity [145].

2.2.4. Proteins and Amino Acids

Similar to polysaccharides, other biostimulant compounds such as proteins and amino acids extracted from microalgal biomass have been reported to promote crop productivity. Microalgae can provide proteins to plants, resulting in energy saving during metabolic processes. This occurs by providing N to the plants, some of which are available in the form of proteins within the microalgal biomass [1]. The accumulation of proteins and amino acids in biomass can vary depending on the environmental factors and cultivation conditions. In controlled environments, such as laboratories, microalgae accumulate higher essential amino acid contents in their biomass [146]. Amino acids increase the chlorophyll content, which promotes plant growth [17] and contributes to the biosynthesis of polyamines, which are involved in cell division, flower and fruit development, and the differentiation of leaves, flowers, and roots [147]. Phenylamine aids in the synthesis of phenolic compounds, which enhance plant defense mechanisms against stress [148]. The higher levels of amino acids, such as arginine and tryptophan, in L. platensis make them viable for biostimulant applications and serve as precursors for polyamines and auxins, respectively. Furthermore, amino acids such as glycine, betaine, and proline can act as osmoprotectants and antioxidants to mitigate heavy metal and salinity stress [149]. Microalgae, including cyanobacteria, have biomass proteins of up to 63%, with amino acids comprising 40–48% of the total proteins [3]. The use of L. platensis protein-rich extracts on the leaves of red beets resulted in enhanced early seedling growth, higher chlorophyll content, and improved nutrient composition [17]. Similarly, it increased the number of flowers and fresh and dry weights of Petunia [125].

2.2.5. Phenolic Compounds

Phenolic compounds derived from amino acids, such as phenylamines, have been found to enhance plant defense against stressors [148]. In recent studies, L. platensis [150] and Nannochloropsis sp. [151] have been used to extract essential and non-essential amino acids. Scenedesmus sp. exhibited significant antioxidant capacity, as well as total phenolic, flavonoid, and carotenoid contents. The antioxidant capacity was measured using two assays, 2,2-diphenyl-1-picrylhydrazyl and ferric-reducing antioxidant power, yielding 3.71 ± 0.11 μmol Trolox eq. g⁻¹ DW and 47.01 ± 3.14 μmol Trolox eq. g⁻¹ DW, respectively. The total phenolics, flavonoids, and carotenoids were quantified at 5.40 ± 0.28 mg gallic acid eq. g⁻¹ DW, 1.61 ± 0.76 mg quercetin eq. g⁻¹ DW, and 0.61 ± 0.05 mg g⁻¹, respectively [152]. The aqueous and methanolic extracts of D. salina had the highest total phenolic content at 8.78 ± 1.49 mg GAE g⁻¹ DW and 1.30 ± 0.37 mg GAE g⁻¹ DW, respectively. The methanolic extract of Mychonastes homosphaera recorded 9.04 ± 0.68 mg GAE g⁻¹ DW, and its aqueous extract had 3.00 ± 0.30 mg GAE g⁻¹ DW. Phaedactylum tricornutum showed the highest carotenoid (fucoxanthin) and phenolic (protocatechuic acid) contents and 65.5% antioxidant activity, outperforming Nannochloris sp., Tetraselmis suecica, and Microchloropsis gaditana, with antioxidant activities of 51.1% and 56.8%, respectively [153].

2.2.6. C-Phycocyanin

C-phycocyanin (C-PC) is a phycobiliprotein complex mainly produced by cyanobacteria, such as Anabaena fertilissima, L. platensis, Galdieria sulphuraria, Phormidium sp., Nostoc sp., and Synechocystis spp. In the food industry, C-PC is commonly used as a natural colorant (blue protein pigment) and antioxidant supplement [3]. In addition to its nutraceutical characteristics, C-PC has recently been shown to possess biostimulatory properties. The application of a phycocyanin-rich L. platensis extract as a biostimulant in hydroponically grown lettuce increased early maturity growth and yield by 12.5% and improved antioxidant flavonoids (quercetin and luteolin) [154]. In another study, C-PC treatment of tomatoes increased germination percentage, germination index, seedling growth, dry weight, and seedling length vigor index, as well as enhanced the content of secondary metabolites such as total phenolics, flavonoids, and shikimic acid [155]. Moreover, C-PC altered the overall bacterial diversity, especially of Actinobacteria and Firmicutes, and reduced the potentially pathogenic bacterial community in the hydroponic nutrient solution of lettuce. These results indicated that C-PC possesses prebiotic properties that can boost the growth of plant-promoting bacterial communities by inhibiting harmful bacteria and enhancing plant growth [154]. The primary advantage of C-PC as a biostimulant is that its molecular structure is well-characterized, and downstream processes, such as extraction and purification, are standardized and commercialized. In addition, C-PC is water soluble, making it highly scalable for use as a biostimulant. However, one major challenge with using C-PC under open conditions is its sensitivity to light and short half-life at high light intensities. Despite this, C-PC can be effectively utilized as a biostimulant under controlled atmospheric growth conditions such as hydroponics and other vertical farming systems [3].

3. Application Methods of Microalgae and Cyanobacteria-Based Biofertilizers and Biostimulants

As previously mentioned, microalgae and cyanobacteria are utilized as biofertilizers and/or biostimulants in different forms, including fresh or dry biomass, cellular extracts, and hydrolysates, to promote plant growth and increase crop yield (Table 1). They can be applied by mixing soil with live or algal biomass, treating seeds with cellular extracts (seed priming), foliar spraying on plant leaves, or root drenching [65]. The method of application depends on the specific needs of the crop, such as nutrient supplementation, addition of micronutrients, or prevention of diseases. Furthermore, the crop type plays a crucial role in determining the application method, considering whether it is sown directly or raised in a nursery before being transplanted into the field [3].

3.1. Seed Inoculation

Seed inoculation with microalgae typically involves three steps: priming, coating, and dipping [156]. Seed priming is a treatment before sowing that involves the controlled hydration of seeds to improve germination rates and promote root formation in plants [157]. Seed coating involves the application of microalgal biomass by spraying or coating to create a uniform layer on the seeds, whereas seed dipping or soaking treatments involve immersing the seeds for a specific period between 18 and 24 h before sowing. These methods have the advantages of increasing germination, enhancing the germination index, seedling vigor, and shoot and root length, and reducing pathogenic seed microflora [158]. Studies have shown that inoculation of seeds with C. vulgaris accelerates the germination of Hibiscus esculentus by 3 days and its maturity by 8 weeks, resulting in increased pod yield and plant height [37]. In a separate study, seeds treated with Acutodesmus dimorphus culture and extract concentrations (0.75 g mL⁻¹) exhibited faster tomato seed germination, occurring two days earlier than in the control group [32]. In addition, spinach seedlings were treated with soluble and insoluble microalgal fractions derived from whole-cell and lysed-cell extracts of Chlorococcum sp., Micractinium sp., Scenedesmus sp., and Chlorella sp. showed up to 1.7-fold and 1.6-fold (day 5) improvement in germination, green cotyledon emergence by up to 2-fold (day 6), and increased seedling biomass by up to 2.1-fold and 1.9-fold (day 9) compared to water and seaweed extracts, respectively [45].

3.2. Foliar Application

The use of microalgae in foliar spraying is a highly efficient method for increasing crop productivity because this approach allows plants to respond quickly to added nutrients, outperforming other treatments [159]. Foliar spraying, commonly applied through fertigation or aerial sprays, significantly enhances nutrient-use efficiency [160]. Nutrients supplied through foliar adsorption, cuticular penetration, leaf cell uptake, translocation, and utilization are promptly absorbed by leaves. Alternatively, nutrient absorption through stomatal penetration via diffusion through stomatal pores is also possible [161]. Nonetheless, the effectiveness of nutrient absorption by plant leaves depends on the physical and chemical properties of the spray formulation, including the pH, surface tension, and adhesion to the leaf surface. Furthermore, the molecular size, charge, and solubility of nutrients and other compounds in the spray formulation determine their penetration efficiency [162]. The application of Chlorella fusca to spinach and Chinese chives via foliar spraying improved plant height, fresh weight, leaf number, thickness, width, and crop yield. It also reduces the severity of gray mold disease in Chinese chives. Moreover, mineral content, such as K, calcium (Ca), magnesium (Mg), P, Fe, and manganese (Mn), was higher in chlorella-treated spinach than in untreated spinach [34]. The use of Asterarcys quadricellulare microalga-biomass sprays increased potato yield and led to biochemical changes, including improved chlorophyll, amino acid, sugar, and nitrate reductase activity. These results demonstrated the efficacy of A. quadricellulare in stimulating plant growth, development, and N-assimilation, making it a promising input for sustainable potato production [13].

3.3. Soil and Root Drenching

The soil drenching method involves the direct application of live microalgal suspensions or dry biomass formulated with suitable carrier materials to the soil or plant. Effective carriers for cyanobacteria and microalgae in soil-drenching applications include agricultural and agro-industrial wastes such as peat, vermiculite, wheat straw, and animal manure [160]. Microalgal biomass has the potential to effectively fertilize the soil by releasing nutrients in a form that roots can access at a rate sufficient for plant growth. In a study conducted on wheat growth in two nutrient-deficient soils, the application of wet and spray-dried C. vulgaris over an 8-week period resulted in increased wheat growth, including root hair production, comparable to that in mineral-fertilized soils [38]. These findings suggest that algal biomass can serve as a viable nutrient source for agriculture in marginal soils. In addition, inoculation of corn seeds with monocultures of Microcystis aeruginosa, Anabaena sp., and Chlorella sp. significantly enhances corn seed germination, early seedling development, and plant metabolic processes [39]. Furthermore, the application of C. vulgaris extract as a foliar spray and root drenching to lettuce seedlings positively affected their growth and enhanced dry matter, chlorophyll, carotenoid, and protein content in the edible parts [40]. These methods mainly regulate the primary and secondary metabolism by coordinating the C and N metabolic pathways, which is crucial for their effectiveness. Notably, foliar application primarily affected the enzymes involved in N metabolism, whereas root drenching mainly affected enzymes related to carbon (C) metabolism.

4. Microalgae and Cyanobacteria Biomass Production from Wastewater and Its Composition

Recent studies on the production of microalgal-based biofertilizers can be divided into two broad categories. First, a portion of studies have reported the assimilation of nutrients from wastewater by using microalgae to produce higher biomass yields. This novel method is a cost-effective way to reduce the cost of biomass production and mitigate the environmental impact of wastewater discharge. Later, the biomass cultivated from wastewater was converted into valuable bioproducts, such as biofertilizers, which are anticipated to promote a circular economy in agriculture. Second, various formulations of microalgal biofertilizers have been developed using conventional methods and have been evaluated for their effectiveness in agricultural settings [163]. Phycoremediation and biomass production in wastewater have primarily been used for biofuel production, thereby significantly enhancing the economics of biomass cultivation [164]. Microalgae can grow in various types of agricultural wastewater, including food processing, dairy, distillery, aquaculture, palm oil effluents, piggeries, slaughterhouses, leather tanneries, and dyeing facilities [165,166,167,168,169]. They can effectively assimilate over 80% of wastewater nutrients, including NH₄, NO₃⁻, NO₂⁻, PO₄³⁻, organic carbon, and other minerals, resulting in higher biomass productivity [22,170,171]. Cultivated biomass has been utilized in the development of biofertilizers for crop development (

Table 3. Microalgae and cyanobacteria biomass with plant growth promoting characteristics of biofertilizer potential and bioremediation efficiency in different wastewater as culture media.

Microalgal Species	Wastewater			Biomass		Promoted Plant Growth Parameters and Soil Properties	Reference
	Culture Medium	Characteristics	Removal Efficiency	Production	Composition
Chlorella minutissima, Nostoc muscorum, Scendesmus sp., and Scendesmus consortium	Domestic wastewater	EC: 3.14 dS m−1, NH4+: 39.5 mg L−1, NO3−: 2.38 mg L−1, P: 3.68 mg L−1, COD: 149.75 mg L−1, BOD5: 99.5 mg L−1, TDS: 2196 mg L−1	NH4: 92%, NO3: 87%, PO43−: 85%, COD: 81%, BOD5: 90%, TDS: 96%	0.14–0.45 g L−1	N: 2–6%, P: 0.5–1%, K: <0.5%, LP: 11.33–81.23 mg L−1	-	[29]
C. minutissima	Domestic wastewater	EC: 3.52 dS m−1, NH4+: 5.60 mg L−1, NO3−: 3.06 mg L−1, P: 3.54 mg L−1, K: 5.50 mg L−1, COD: 157 mg L−1, BOD5: 114 mg L−1, DO: 3.50 mg L−1, TDS: 2416 mg L−1	EC: 92.9%, NH4+: 48.2%, NO3−: 88.9%, P: 67.5%, K: 66.4%, COD: 80.5%, BOD5: 93.2%, TDS: 94.4%	1.26 ± 0.07 g L−1 FW and 0.44 ± 0.04 g L−1 DW	N: 6.0%, P: 1.0%, K: 0.48%	Leaf length, leaf and root biomass in spinach. Yields with and without husk and cob length in baby. Soil organic carbon, nitrogen and phosphorous. Dehydrogenase, urease, and nitrate reductase activity.	[10]
Chlorella sp., Scendesmus sp., and Scendesmus consortium	Domestic wastewater	TN: 61.47 mg L−1, NH4+: 37.64 mg L−1, NO3−: 16.58 mg L−1, P: 7.42 mg L−1, COD: 446.25 mg L−1, TOC: 208.15 mg L−1	TN: 85–94%, NH4+: 95–98%, NO3−: 84–96%, P: 89–95%, COD: 78–88% TOC: 81–86%	1.78 g L−1	N: 7.21–7.81%, P: 1.55–1.72%, K: 0.75–1.06%, Ca: 0.21–0.28%, Na: 1.08–1.18%, Mg: 0.11–0.17%, S: 0.21–0.27% Fe: 0.30–0.36% Chlorophyll: 27.03 μg mL−1 Protein: 175 μg mL−1 Lipid: 34.83% dry cell weight	Shoot and root length, fresh and dry weight, and yields of tomato. Increased macro (N, P, K, Ca) and micro-nutrients (Mg, Fe) of tomato.	[30]
Scenedesmus sp.	Domestic wastewater	NH4+: 38.6 mg L−1, NO3−: 17.1 mg L−1, PO43−: 9.24 mg L−1, COD: 142.2 mg L−1	-	0.68 g L−1	N: 7.45%, P: 1.6%, K: 0.7%, S: 0.3%, Na: 1.41%, Ca: 0.14%, Mg: 0.12%, Fe: 0.3%, Mn: 210 ppm, Cu: 6.8 ppm, Zn: 34 ppm, Lipid: 24.1%	Plant height, root weight, and yields in rice. Increased NPK content in grain and straw of rice.	[24]
Chlorella sp., Scenedesmus sp., Chlorococcum sp., Chroococcus sp.	Domestic wastewater	-	-	-	-	Plant fresh and dry weight, root length, spike and grain weight, and nutrient contents (NPK) of wheat. Increased soil nutrients NPK Higher acetylene-reducing activity.	[174]
Phormidium sp., Anabaena sp., Westiellopsis sp., Fischerella sp., Spirogyra sp.
Scenedesmus sp.; Chlorella vulgaris	Dairy cattle wastewater	TOC: 623.3 mg L−1, DOC: 361.7 mg L−1, NH3: 141.8 mg L−1, TKN: 174 mg L−1, TP: 1144.1 mg L−1, SP: 629.4 mg L−1, VSS: 623.9 mg L−1, TSS: 729.9 mg L−1, tCOD: 3106.3 mg L−1, sCOD: 1015 mg L−1	TOC: 83.9%, DOC: 82.4%, NH3−: 99.8%, TKN: 78.4%, TP: 53.2%, SP: 66.9%, VSS: 57.4%, TSS: 55.7%, tCOD: 35.4%, sCOD: 55.7%	7.1 gm−2 day−1	TP: 1992 mg L−1, TKN: 1657.4 mg L−1, Cu: 0.53 mg L−1, B: 0.55 mg L−1, Mo: <0.05 mg L−1, Zn: 0.005 mg L−1	Pasture yield, dry matter, ash, nutrients (P, Ca, Zn, Mn, B).	[9]
Chlorella pyrenoidosa	Paddy-soaked rice mill wastewater	NH4+: 147- 154 mg L−1, PO4: 67–70 mg L−1, S:30–38 mg L−1, C: 640–760 mg L−1, TS: 4554–4640 mg L−1, SS: 106–160 mg L−1, DS: 4448–4480 mg L−1, COD: 960–1280 mg L−1, BOD: 680–851 mg L−1	NH4+: 69.39%, PO4: 64.76%	0.11 g L−1 d−1	Carbohydrate: 8.84%, lipid: 32.12%, protein: 34.15%	Seed germination, chlorophyll, fresh shoot and root weight, dry shoot and root weight, and root-to-shoot biomass ratio of okra.	[25]

) [22,172,173].

Over the last few decades, several studies have been conducted on the use of microalgae for wastewater phytoremediation [175,176], including dairy wastewater [165], distillery wastewater [166], domestic wastewater [177], heavy metal-containing wastewater [178], palm oil mill effluent [167], pharmaceutical and xenobiotic wastewater [179], piggery wastewater [168], poultry slaughterhouse wastewater [169], starch wastewater and alcohol wastewater [170], sugar beet processing wastewater [180], swine wastewater [181], textile wastewater [182], and winery wastewater [183], and biorefinery-based approaches have been proposed [169]. Wastewater is a highly suitable resource for microalgal biomass production for various reasons: (i) it is a cost-effective growth medium; (ii) it supports large-scale biomass and biofuel production; (iii) it contains high amounts of essential nutrients; and (iv) it allows the incorporation of microalgal cultivation into existing wastewater treatment technologies [184]. Despite its numerous benefits, microalgae-based remediation poses several challenges that must be addressed.

4.1. Nutrient Removal

Several studies have demonstrated that microalgal species, including Chlorella sp. [164,169,176,185], Chlamydomonas sp. [175], Scenedesmus sp. [186,187], and Neochloris sp. [169], can efficiently absorb nutrients and remove toxic pollutants and heavy metals from different types of wastewater (Table 3). According to Singh et al. [171], C. vulgaris showed a removal efficiency of 98.4% total phosphorus (TP) and 87.9% total nitrogen (TN) in domestic wastewater and produced 1.5 times more lipids up to 14.31 mg L⁻¹ d⁻¹ compared to microalgae grown in standard culture. Chlorella sp. has been shown to effectively remove up to 100% Fe and Mn, 95% Ca, 87% aluminum (Al), 81% Zn, and 98% Mg from domestic wastewater [188]. In a comparative study, native and commercial strains of C. sorokiniana were used to treat palm oil mill effluent. The results have shown that both strains significantly produced the maximum growth rates of 0.24 and 0.23 day⁻¹, respectively, as well as higher biomass yields of over 100 mg L⁻¹ day⁻¹ [189]. Additionally, Chlorella pyrenoidosa produced biomass up to 3.01 g L⁻¹ DW and 127.71 mg L⁻¹ d⁻¹ of lipids, reducing the concentrations of COD, TN, and TP by 75.78%, 91.64%, and 90.74%, respectively, in mixed wastewater (alcohol: starch wastewater = 0.053:1, v/v) [170]. C. pyrenoidosa and Chlorella sp. showed higher biomass production (0.57 to 0.96 g L⁻¹) and protein production (53.55 to 60.32%), as well as removal efficiencies of TN (74–94%), TP (74–94%), and COD (44–86%) in soap and detergent wastewater [26]. In another study, Chlorella sp. decolorized indigo, straight blue, remazol brilliant orange, and crystal violet dyes by 89.3, 79, 75.3, and 72.5%, respectively [190].

In addition to Chlorella sp., the microalgae Scenedesmus obliquus removed 83.1–99.8% of TN and 97.6–98.9% of TP from primary and settling wastewater tanks [186]. It also achieved maximum growth rates of 0.42 d⁻¹ and total lipid yields of 0.38 and 0.33 g L⁻¹ in primary and secondary settling tanks of domestic wastewater, respectively [22,191]. Similarly, Scenedesmus sp. reduced 93% of COD and produced 18.45 mg L⁻¹ of total pigments, 11.46 mg L⁻¹ of lutein, and 2.64 g L⁻¹ of biomass in potato processing wastewater [187]. Moreover, a consortium of C. vulgaris, S. obliquus, and Pseudokirchneriella subcapitata achieved 80% COD, 83% CO₂, 74% TN, and 85% TP removal efficiencies from anaerobically digested swine wastewater [192]. In another study, a mixture of S. quadricauda and T. suecica produced biomass and fatty acids of 0.36 and 0.65 g L⁻¹ and C18:1 and C18:3n-3, respectively. Individually, the microalgae S. quadricauda alone removed 92% of TN, 100% of TP, 100% of sulfate, and 77% of organic carbon from dairy wastewater [165]. According to Chinnasamy et al. [193], the consortia formulated with 15 native microalgae species removed 96% of nutrients and produced 9.2–17.8 tons ha⁻¹ yr⁻¹ of biomass and 6.82% of lipids in carpet wastewater.

These data clearly indicated that microalgae and cyanobacteria can efficiently remove significant nutrients from wastewater while producing higher biomass yields. Unlike municipal and industrial effluents, agricultural wastewater contains fewer toxic compounds and is more readily available in rural areas, making it suitable for cultivating microalgae for biofertilizers [163]. However, it is important to note that, in some cases, the nutrient removal efficiency of microalgae can be low because of unbalanced nutrient profiles and suspended organics in the wastewater medium. Several novel solutions have been proposed and applied to address this issue. First, mixing wastewater from different sources can balance nutrient profiles, enhance biomass yield, and remove C and P [194]. Secondly, chemical oxidation, such as Fenton–Fe and hypochlorite oxidation, can treat wastewater by converting suspended organics into dissolved nutrients for algal growth [195,196]. Thirdly, co-culturing microalgae with bacteria can promote nutrient recovery, as bacteria degrade suspended organics into low-molecular-weight compounds that microalgae can assimilate, whereas microalgal photosynthesis provides oxygen for bacterial metabolism, enhancing both nutrient recovery and biomass yield [28]. Moreover, biochemical triggers can also stimulate microalgal metabolism, such as phytohormones that can change biosynthetic pathways or act as metabolic precursors. These growth-promoting substances are often produced by specific microalgae-associated bacteria, such as Allorhizobium sp., Bosea sp., Emticicia sp., Flavobacterium sp., Neorhizobium sp., Pararhizobium sp., Rhizobium sp., and Sphingomonas sp. [197]. Understanding microalgae–bacterial interactions is crucial for enhancing treatment performance, including total nutrient removal. Microalgal and bacterial interactions can range from symbiotic to competitive and are based on energy and nutrient exchange, signal transduction, and gene transfer [198].

4.2. Biomass Composition

The key components present in wastewater, such as C, N, P, K, micronutrients, vitamins, and trace elements, have a great influence on microalgae growth, biomass production, pollutant removal rates, and the synthesis of intracellular compounds, such as carbohydrates, proteins, and lipids [22,23,29]. Municipal or domestic wastewater discharged from households is often suitable for microalgae-based wastewater treatment because of its low N, P, and COD concentrations [28,177,188]. In contrast, industrial wastewater contains various contaminants, such as heavy metals, antibiotics, oil, grease, and hazardous chemicals, making it unsuitable for microalgae-based treatment, although certain microalgae species can absorb and adsorb toxic heavy metals [22,179,182,185]. However, wastewater generated from various agricultural activities and livestock production, such as crop cultivation, livestock breeding, and agricultural product processing, including farmland drainage wastewater and animal manure wastewater [22,180,183,187], are considered the most promising sources for microalgal biomass production because of their high nutrient concentrations, turbidity, and concentrations of insoluble organic compounds [22].

The fertilization effect of the microalgal biofertilizer was attributed to the high concentration of N in the biomass. N-rich biomass is an excellent source for microalgal biofertilizer production because it is essential for plant growth and development [3]. However, NH₃ in wastewater, which is crucial for N accumulation and protein synthesis in microalgal biomass, can also be toxic and hinder microalgal growth or lead to cultivation failure [23]. Previous studies have used various pretreatment methods to address this issue. For example, pretreatment involving nitrification and NH₃ stripping has been shown to reduce NH₃ concentration in wastewater by nitrifying bacterial activity and air bubbling, creating a more favorable environment for microalgae growth [14,43,64,163]. Additionally, researchers have explored the use of zeolites to absorb NH₃ in the early stages of wastewater treatment and release it into the wastewater in the final stage to support the growth of microalgae [163]. This approach not only alleviates NH₃ toxicity but also addresses N deficiency during the later stages of microalgal cultivation.

5. Challenges Associated with Biomass Cultivation in Wastewater

5.1. Biological Challenges

The application of microalgae in wastewater treatment involves a single species (monoculture), multiple species (polyculture), or a combination of microalgae and bacteria (microalgae–bacterial consortia). Various laboratory- and pilot-scale conditions have been used to study different microalgal species for wastewater treatment using natural and synthetic wastewater [165,192,199]. Fundamental studies have been conducted on a laboratory scale using monoculture microalgae to understand the removal mechanisms of pollutants as well as to evaluate the effects of biotic and abiotic factors that can influence the growth of microalgae in sterilized wastewater media [23,200,201]. However, in real-life conditions, maintaining a monoculture of microalgae in wastewater settings is challenging because of the presence of various algal species, bacteria, and other toxic microorganisms that inhibit the growth of monocultured microalgae of interest and their bioremediation processes [196]. The use of microalgal polycultures in wastewater treatment offers significant advantages over monoculture. First, it increases the nutrient uptake efficiency, which results in higher biomass production. Second, it enhances the robustness, scalability, self-reliance, and viability of the bioremediation process owing to the diverse nutritional requirements and adaptation capabilities of different species [22,173,183]. However, it is important to note that interactions among microalgal species may lead to competition for nutrients and the release of allelochemicals under unfavorable conditions, such as nutrient deprivation, high temperatures, low light intensity, and elevated pH levels, which could impede their growth [23,168,202].

Recently, the presence of bacteria in microalgal cultures has transitioned from contamination to recognition as a promising beneficial symbiotic relationship for various applications, particularly wastewater treatment. Microalgae convert solar energy into chemical energy to produce O₂ and organic matter. At the same time, bacteria use O₂ for respiration, break down organic compounds, and provide microalgae with CO₂, growth-promoting compounds, and vitamins such as B12, B1, and B7, which enhance microalgae biomass production [28]. Furthermore, the surface of microalgae serves as a habitat for bacteria, protecting them from adverse environmental conditions and providing extracellular metabolites that promote bacterial growth. Bacteria also release chemical signals, such as N-acyl-homoserine lactones and IAA, which can promote cooperative relationships by mediating ecological niche formation and inducing microalgal growth [203]. Consequently, consortia of microalgae and bacteria offer several advantages for wastewater treatment, including enhanced uptake of conventional pollutants, reduced stress from high pollutant concentrations (such as bacterial consumption of NH₃), toxicity to microalgae, increased microalgal biomass production, easier microalgal harvesting through bacterial EPS-induced flocculation, and better environmental resistance and robustness of the consortia [23]. However, microalgal–bacterial consortia can face challenges, such as (i) nutrient competition between microorganisms; (ii) bacterial shading of microalgae, which impedes photosynthesis; (iii) bacteria releasing substances that are harmful to microalgae; (iv) microalgae-producing antibacterial compounds; and (v) inhibition of bacterial growth due to increased pH and O₂ concentrations from microalgal photosynthesis [23,199,203]. Therefore, it is critical to establish and optimize strains based on the purpose and conditions of interest. Additionally, understanding how pollutant concentrations and other characteristics, including the biotic and abiotic factors of each wastewater type, influence microalgal removal mechanisms and efficiency is essential for optimizing treatment. These considerations are critical for the development of microalgal communities that can remove nutrients, heavy metals, antibiotics, and pathogens during wastewater treatment.

5.2. Environmental Challenges

The performance of microalgae- and cyanobacteria-based systems in wastewater treatment depends on several environmental factors, such as pH, temperature, light intensity, hydraulic retention time, CO₂ and O₂ concentrations, nutrient composition, and turbidity, which are essential for achieving the maximum removal efficiency of nutrients and biomass production [23,200,201].

Optimizing the pH is a crucial factor for nutrient assimilation, photosynthesis, and C sequestration. Microalgae showed optimal growth at pH 7.0–9.0, while cyanobacteria Anabaena variabilis thrived at pH 7.4–8.4, with significant declines above pH 9, and failed to survive at pH 9.7–9.9 [204]. Studies have indicated that C absorption is minimal at pH 6.0–7.0 and that C concentrations are below 9.52 mmol L⁻¹. Notably, the input of high CO₂ flue gas (10–20%) acidifies the culture medium, which in turn inhibits microalgal growth [27]. In high-rate algal ponds, the increased pH can be compensated by deeper respiration, which can be regulated by adding more organic material to enhance respiration [204]. Valizadeh and Davarpanah [20] found that the optimal pH for COD removal from dairy wastewater is 8.0. Further increases in pH can significantly decrease microalgal growth and metabolism, resulting in inhibition of nutrient assimilation. Microalgae have been found to grow at temperatures ranging from 5 to 40 °C, with an optimal temperature between 15 and 30 °C. However, the optimal temperature may vary from species to species; for instance, the optimal temperature for Nannochloropsis oculata is 20 °C, and that for C. vulgaris is 30 °C. Temperatures beyond the optimal limit inhibit C sequestration by reducing the enzymatic activities of Rubisco and carboxylase, which play essential roles in photosynthesis and photorespiration [23].

Optimizing light intensity is another challenging factor in wastewater treatment because of the high levels of suspended particles and turbidity. These factors significantly reduce light penetration and affect photosynthesis, which in turn leads to inhibition of microalgal growth. Gonçalves et al. [205] found a direct correlation between increased light intensity and improvements in the growth rate, biomass yield, and nutrient removal efficiency in autotrophic metabolism. Continuous light exposure resulted in significant NO₃⁻ removal efficiency in C. kessleri compared to a periodic light–dark cycle [206]. Furthermore, the use of red and blue light has been shown to improve nutrient removal and biomass production in Scenedesmus spp. compared with single light [207]. Although optimizing the light intensity, color, or duration can be cost-effective in microalgae cultivation, it may not be economically feasible [208].

The absorption of C, P, and N by microalgae depends on the nutrient levels in wastewater. For optimal cell development, it has been suggested that the mass ratio of N to P (N/P) ranges from 5 to 30 [209]. Low N levels inhibit P absorption, which is essential for protein synthesis and assimilation. Similarly, low P levels can decrease ATP production, which is important for C concentration mechanisms [210]. High concentrations of suspended solids, turbidity, and NH₃ can negatively affect microalgal growth [211]. Therefore, it is essential to conduct sedimentation pretreatment in wastewater to enhance the effectiveness of the remediation process [212]. Depending on the specific microalgal species, it may be essential to perform further pretreatments, such as filtering or dilution, to resist the harmful effects of NH₃ toxicity, particularly in the range of 25–1000 μmol NH₄ L⁻¹ in wastewater and achieve optimum cell development [213].

5.3. Economic Challenges

Microalgae and cyanobacteria require efficient operational functionality and costly treatment processes to maximize nutrient removal and biomass production in wastewater [214]. Therefore, industrial-level applications have not advanced primarily because of the significant economic costs associated with large-scale implementations [215]. Two challenges in producing microalgal biomass as a raw material for biofertilizer production are the high cost of artificial media and low biomass yield. Researchers have explored the extensive use of microalgae in wastewater treatment to reduce production costs and produce high biomass yields for various applications, particularly biofertilizers [3]. This approach makes industrial microalgae production feasible by utilizing reusable resources such as wastewater as a culture medium. Moreover, this integrated strategy combines CO₂ reduction, wastewater treatment, and biofertilizer production to enhance the sustainability and cost-effectiveness of microalgal farming, which aligns with the principles of circular bioeconomy [10,215].

Studies on the life cycle assessment of microalgae production have shown that photobioreactors require approximately ten times more energy for pumping and aeration than high-rate microalgae ponds that use paddlewheels [216]. Lundquist et al. [217] highlighted the importance of reducing electricity consumption from the grid for air compressors and pumps. This can be achieved by accurate aeration rate measurements, proper pump dimensioning, and effective air compression [218]. A more efficient alternative involves introducing dissolved C directly into the medium, such as inorganic C or organic substrates, which reduces operational costs and inefficiencies in C supply from aeration. Furthermore, adding waste sources rich in soluble C can enhance environmental benefits by recycling resources, reducing material costs, and supporting a circular economic paradigm [218]. The primary challenge in microalgae application is the harvesting method, which is complicated because of the operational conditions and the high energy required to separate microalgal biomass from various types of wastewater effluents [219]. Factors such as low microalgae concentration, high growth rates, microscopic cell sizes, and negative cell surface charges complicate the separation process, thereby increasing the harvesting costs. The development of a cost-effective microalgal harvesting system remains a challenge, and several ongoing studies are being conducted. Recent studies have suggested a two-stage procedure for concentrating microalgal biomass: flocculation, followed by sedimentation, with bioflocculation being preferable because of its reduced chemical costs and ability to maintain biomass efficiency [22,220].

6. Integrated Biorefinery Approaches to Produce Microalgae and Cyanobacteria-Based Biofertilizer toward a Circular Economy

The circular economy concept has gained increasing attention in recent years owing to the extensive use of non-renewable natural resources. Microalgae and cyanobacteria play multifaceted roles in the circular economy, particularly in the development of biofertilizers. As previously discussed, the cultivation of microalgae in cost-effective media such as wastewater can effectively facilitate nutrient sequestration and produce higher biomass yields [52]. Currently, chemical fertilizers derived from finite resources are the primary source of nutrients for global agriculture [1,4]. Consequently, incorporating sustainable nutrient sources from microalgal biomass into agricultural practices significantly improves sustainability and advances the circular economy. Furthermore, the use of microalgae in wastewater treatment serves as an efficient and sustainable method for converting CO₂ into O₂, which helps to mitigate environmental impacts [3].

In the circular economy concept, biorefineries offer a practical way to reduce costs across different processes by maximizing the recovery of bioproducts [221]. The primary goal of microalgal biorefineries is to improve the extraction of valuable elements from biomass, thereby making their use economically viable [222]. Microalgal biomass is currently used in various industries, such as cosmetics, food and feed, nutraceuticals, and pharmaceuticals. Furthermore, they have been used in wastewater treatment, biofuel production, and biofertilizer production [223]. Although there have been discussions on integrating microalgal biofertilizer production into biorefinery processes, many opportunities still need to be explored. Figure 1 illustrates the potential integration of microalgal biofertilizer production into a circular economy and a biorefinery.

Several studies have indicated that the use of microalgae in biorefineries is associated with the production of various biofuels, including biomethane, biohydrogen (obtained through biomass digestion), bioethanol (derived from carbohydrate extraction), and biodiesel (extracted from lipids). In biodiesel production, microalgal biomass is recovered from cultivation, lipids are extracted from the biomass used for biodiesel production, and the remaining biomass can be used as biofertilizer [1]. In a previous study, the microalgae Chlorella minutissima, Scendesmus spp., and cyanobacteria N. muscorum were utilized in a biorefinery approach, resulting in phycoremediation potential with higher pollutant removal efficiencies, yielding the highest lipid and dry biomass, which contained N (5.46%) and P (0.85%) as potential biofertilizers. Using this biomass as manure could save approximately USD 5584 ha⁻¹ yr⁻¹ in chemical fertilizer costs [29]. In another study, de-oiled microalgal biomass waste of Scenedesmus sp. contained N (7.45%), P (1.6%), K (0.7%), and other nutrients that increased plant growth parameters, such as height, root weight, shoot weight, and yield parameters of rice, as well as improved NPK content in the grain and straw of rice plants [24]. Similarly, deoiled algal biomass waste used as a biofertilizer resulted in greater improvements in tomato shoot (44%) and root (89%) lengths, fresh (95%) and dry (53%) weights, macro-and micronutrients (N, 61%; P, 179%; K, 71%; Ca, 38%; Mg, 26%; and Fe, 11%), and tomato yield (174%) [30]. Microalgal biomass digested from different wastewaters has shown NPK levels above 7% and a C/N ratio below 15, enabling its use as fertilizer [224].

In recent decades, microalgae-based wastewater treatment plants (WWTPs) have been increasingly explored for their potential to produce biofertilizers while simultaneously treating wastewater. Below are some examples of real-world and pilot-scale microalgae-based wastewater treatment facilities around the world:

1. All-gas Project, Spain “https://www.all-gas.eu/ (accessed on 30 August 2024)”: This pilot-scale facility is part of the European Union’s All-gas project. This study aimed to demonstrate the large-scale production of bioenergy from wastewater. The facility uses raceway ponds for microalgae cultivation. Microalgae treat wastewater by absorbing nutrients that are then converted into biomass. The biomass is harvested and can be further processed into biofertilizers.

2. Western Treatment Plant, Melbourne, Australia “https://www.melbournewater.com.au/ (accessed on 30 August 2024)”: This pilot-scale project integrates microalgal cultivation with conventional wastewater treatment processes. Microalgae absorb excess nutrients, such as nitrogen and phosphorus, from wastewater, helping to reduce nutrient levels in the effluent. Harvested microalgae biomass has been explored for various applications, including as a biofertilizer. The pilot project aimed to assess the feasibility of scaling up the process and improving the sustainability of wastewater treatment.

3. CSIR-NEERI, Chennai, India “https://www.neeri.res.in/ (accessed on 30 August 2024)”: The Council of Scientific and Industrial Research—National Environmental Engineering Research Institute (CSIR-NEERI) operates a pilot-scale microalgae-based wastewater treatment system. The focus is on treating industrial and municipal wastewater while producing valuable by-products such as biofertilizers. The microalgae used in this system are cultivated in open ponds, where they effectively remove contaminants from the wastewater. The biomass produced was tested for its potential as a biofertilizer, given its nutrient-rich composition.

4. The Chinese Academy of Sciences, China “http://english.qibebt.cas.cn/ (accessed on 30 August 2024)”: The Institute of Oceanology at the Chinese Academy of Sciences in Qingdao has developed a pilot-scale microalgae-based wastewater treatment facility. This project focused on the dual goals of wastewater treatment and biofertilizer production. The system was designed to treat wastewater from various sources, including agricultural runoff. Microalgae effectively remove nutrients and heavy metals, and the resulting biomass is processed into biofertilizers.

5. INRAE Algal Culture Facility, France “https://www.inrae.fr/en (accessed on 30 August 2024)”: The National Research Institute for Agriculture, Food, and Environment (INRAE) in Toulouse operates a pilot-scale facility focused on microalgae-based wastewater treatment. The facility explored the use of different algal species to optimize the removal of nutrients from wastewater. The facility operates under controlled conditions, allowing for precise monitoring of the treatment process. The microalgal biomass generated was evaluated for its effectiveness as a biofertilizer.

7. Conclusions

The increased use of biofertilizers and natural biostimulants aims to decrease reliance on chemical fertilizers and non-renewable resources. Microalgae and cyanobacteria have shown the potential for promoting plant growth and offering systemic immune resistance to various environmental stressors. However, the success of microalgae-based biofertilizers largely depends on the efficient production of low-cost biomass and minimal energy consumption. To enhance the commercial feasibility of these agro-technologies, it is crucial to integrate the bioremediation and biorefinery models. Cultivating microalgae and cyanobacteria in nutrient-rich sewage wastewater is a novel method to manage pollution, generate biomass, extract lipids, produce biofertilizers, control pollution, generate biofuels, and produce organic fertilizers. Algal and cyanobacterial biomass are stable, transportable, and concentrated in manure nutrients, showing a higher NPK content than available organic fertilizers and a lower nutrient content than chemical fertilizers. Only approximately 5% of microalgal N is available as mineral N at application, minimizing N loss to leaching and runoff, thus reducing eutrophication. These biofertilizers enhance the soil nutrient content and plant growth, thereby promoting sustainable organic agriculture. The zero-waste biorefinery approach benefits both the economy and the environment, positioning microalgae as a solution for sustainable phycoremediation, biodiesel production, and organic manure utilization while indirectly mitigating climate change by reducing GHG emissions. WWTPs have been increasingly explored for their potential to produce biofertilizers while simultaneously treating wastewater. Real-world and pilot-scale microalgae-based wastewater treatment facilities worldwide have demonstrated a growing interest in leveraging microalgae for sustainable wastewater treatment and biofertilizer production across various scales and regions. Further studies are required to improve the pilot-scale process and evaluate the long-term fertilization potential of various crops under field conditions.