The Use of Some Species of Bacteria and Algae in the Bioremediation of Pollution Caused by Hydrocarbons and Some Heavy Metals in Al Asfar Lake Water

Abstract

Pollution is the biggest environmental and health threat in the world. Conventional treatments of polluted habitats require the removal of pollutants contaminating the environment, but removal methods are costly and involve high power consumption. This research aims to investigate the potential for bioremediation and proposes an alternative source for implementing it that is cheaper and more environmentally friendly. The phycobioremediation experiment used hydrocarbon- and heavy-metal-polluted water from Al Asfar Lake, AlAhsa, KSA. The isolation and characterization of the lake’s predominant microalgae and associated bacteria were carried out. Monoalgal cultures of the dominant genera of algae were employed for the treatment of contaminated water and soil samples. The concentrations of the heavy metals and hydrocarbons in these samples were determined before and after the treatments by using atomic absorption spectroscopy (for heavy metals) and gas chromatography (for hydrocarbons). From the initial assessments, the levels of manganese, copper, and chromium were high, with chromium being the highest. Three microalgal isolates were identified: two coccoid, with one being blue-green and the other green, and one filamentous cyanobacterium. These species were the most efficient in removing heavy metals and dangerous hydrocarbons. Molecular characterization revealed Chlorella sp. and Geitlarianema sp. to be the most promising for bioremediation. The present work sheds light on the prospect of using algal and bacterial consortia for optimized, safe, and eco-friendly pollution amelioration.

1. Introduction

The greatest problem of the highest priority today is environmental pollution, which includes air, water, and soil pollution mainly caused by anthropogenic activities. Activities like burning fossil fuels and applying pesticides, insecticides, and herbicides has tremendously increased the levels of pollution worldwide [1]. For instance, the contamination of water reservoirs affects water quality and intoxicates aquatic species. Therefore, there is a dire need for the introduction of proper and sustainable pollution management techniques that will help in remediation [2,3]. In that regard, microbial bioremediation represents a promising eco-friendly solution for environmental pollution, especially when using bacteria and microalgae to restore a contaminated niche [4]. Using the microbial bioremediation approach is far more advantageous than traditional physico-chemical techniques. The latter are expensive and hazardous. Biological remediation, on the other hand, is relatively inexpensive and eco-friendly. However, the time taken to remediate sites contaminated with heavy metals depends on the ability of the specific organisms employed in the process and the factors limiting their growth and bioremediating activity.

Various bacteria, fungi, and plants have shown their potential to biologically decontaminate polluted land and water and have been extensively reviewed. Algae are recognized as the most effective biosorbents because of their special metabolites/storage products. Some of the experimentally proven algal biosorbents belong to Spirogyra, Sargassum, and Lyngbea [5,6]. On the other hand, some heavy metals are poisonous to microorganisms, which represents a major limitation of microbial remediation approaches [7]. This impediment, however, is outweighed by the fact that some microbial strains possess the ability to develop coping/resistance mechanisms like metal detoxification [8].

Apart from photosynthetic aquatic microorganisms such as microalgae (including blue-green algae or cyanobacteria), some associated heterotrophic bacteria can also contribute to the elimination of contaminants. These unique microorganisms possess unique metabolic, enzymatic, and cellular functions [1,2,3,4]. Microbial bioremediation based on algae and bacteria in particular is an attractive method for the restoration of polluted sites. Al Asfar Lake, Eastern Province, Kingdom of Saudi Arabia, is a site with huge development potential as a recreational lake, but eco-friendly efforts are needed to clean up the lake from wastes from agricultural drainage and industrial and domestic effluents in order to eliminate both organic and inorganic pollutants. Pollutants that are predominant in the lake are heavy metals and hydrocarbons. Manganese (Mn), copper (Cu), and chromium (Cr), in particular, are higher than their standard levels. Many researchers have discussed the risks of heavy metals and possible remediation options ([3,4,7,8] and references therein). Heavy metals are harmful to health, and repeated exposure can lead to neurological damage, respiratory disorders, and cancer. Among all the existing forms of chromium pollutants in the environment, Cr (VI), i.e., a hexavalent ion, is the most toxic form because of its strong oxidizing ability. However, the other form of chromium, Cr (III), i.e., a trivalent ion, is highly reactive to carboxyl and thiol groups and therefore disrupts the structure and function of various enzymes. Cr ionic complexes within the cells form electrostatic interactions with the phosphate groups in DNA molecules to negatively affect essential cellular processes such as transcription, translation, and replication. These ions are produced due to various industrial activities, such as oil well drilling, which is quite relevant to Eastern Province, KSA, where there are many oil drilling sites. Industries like paper manufactories, electroplating, and textiles all contribute to pollution.

Exposure to high contents of Cr may damage the size and diversity of the microbial population. The high toxicity and mutagenic nature of Cr have raised serious concerns for human health, which range from nasal irritation to cancer. The release of different forms of Cr due to the use of Cr-plated utensils induces disorders such as ulcers, dermatitis, bronchitis, hemorrhage, skin cancer, and DNA damage. Another heavy metal is copper, which is considered an essential trace element in minute concentrations. Therefore, it is crucial for various physiological activities but is largely toxic for various organisms in high concentrations. Increased accumulation of Cu in the environment has been observed over the past few decades. The high ductility and thermal conductivity of copper make it an ideal candidate for engines, and most of the agricultural practices require tractors and other mobile vehicles for agronomic practices. Human industrialization has also led to substantial increases in Cu levels in both the terrestrial and aquatic environments. Additionally, the use of fungicides and pesticides, which contain copper in addition to other salts, like Bordeaux mixture, continuously add Cu to the agricultural runoff.

Exposure to high levels of Cu is toxic to the human body, and such effects become pronounced with frequent exposure. The copper ions Cu(I) and Cu(II) catalyze the formation of reactive oxygen species (ROS) through the Haber–Weiss reactions, culminating in the formation of free radicals that can injure all forms of biomolecules. However, their buildup to even tolerable levels causes severe irritation and corrosion on the mucous membranes of many organs. This can lead to angiogenesis, infection, and inflammatory responses, which, in the context of the CNS, can lead to irritation and, thus, depression. Fatally high concentrations of copper also lead to the formation of necrotic zones in the liver and kidneys.

With regard to hydrocarbons, petroleum, and polyaromatic hydrocarbons (PAHs), in particular, those present in wastewater are lethal to all forms of life in water bodies, interfere with the whole ecosystem, and should be eliminated [9]. Organic pollution sources include oil extraction and movement and the incomplete combustion of petroleum products. Among all the pollutants, oil is the most significant contributor of organic pollution for both types of settings: terrestrial and water. By the same measure, deforestation and the burning of trees also cause the release of hydrocarbons into the air and erase the natural carbon dioxide sink. The increase in the level of organic pollution from domestic, industrial, agricultural, and commercial discharge is a major concern according to the environmentalists.

Therefore, attempts must be made to isolate and characterize microbial strains from polluted ecosystems. Such microbes will most probably be adapted and resistant to the environment in question due to their origin from contaminated sites. The present study focuses on trying to pinpoint some of the most abundant microbial strains that make up algal–bacterial consortia in Al Asfar Lake. It also looks at the current status of heavy-metal contamination of lake water and the bio-remediation potential of microbial consortia. Also, this research focuses on the detection of hydrocarbons and the possibility of their biodegradation. The main purpose is to offer a cost-effective, eco-friendly biological solution to the pollution of the lake, which in turn contributes to both environmental restoration and sustainability.

2. Methodology

2.1. Collection of Algal Samples

Water samples containing algae were collected from Asfar Lake, Al-Hassa, Kingdom of Saudi Arabia, in December 2022.

2.2. Isolation, Purification, and Identification of Algal Samples

Algal biomass was extracted from the water samples by using centrifugation at 3000 rpm; the supernatant was discarded, whereas the biomass pellet was spread on BG-11 medium (

Table 1. BG-11 medium composition.

Component	Stock Solution (g·L−1 dH2O)	Quantity Used/L
Fe citrate solution	–	1 mL
Citric acid	6	1 mL
Ferric ammonium citrate	6	1 mL
NaNO3	–	1.5 g
K2HPO4·3H3O	40	1 mL
MgSO4·7H2O	75	1 mL
CaCl2·2H2O	36	1 mL
Na2CO3	20	1 mL
Na2EDTA·H2O	1.0	1 mL
Trace metal solution	–	1 mL
Trace element solution composition
Component	Stock Solution (g·L−1 dH2O)	Quantity Used/L
H3BO3	–	2.860 g
MnCl2·4H2O	–	1.810 g
ZnSO4·7H2O	–	0.22 g
CuSO4·5H2O	79.0	1 mL
Na2MoO4·2H2O	–	0.391 g
Co(NO3)2·6H2O	49.4	1 mL

) solidified by agar for isolation and purification. Pure monoalgal cultures were established by repeated streaking on agar plates. The initial identification of cultures was performed by using light microscopy according to Raklami [10].

2.3. Morphological Examination and Establishment of Monocultures

Through the use of light microscopy, it was revealed that the water samples were predominantly full of three algal species. Those isolates were isolated and labelled C1, C2, and F; two of them were unicellular coccoids; one was a cyanobacterium and one a green alga. The cyanobacterium was an unbranched non-heterocystous filamentous strain.

2.4. Isolation and Purification of Bacterial Strains

The heterotrophic bacteria associated with the algal isolates from Al Asfar Lake were isolated by using nutrient agar. Pure strains were grown from the colonies by isolating them on solid medium and keeping them in a refrigerator to run the experiments later.

2.5. Detection of Heavy Metals by Chemical Analysis of Samples

The water samples of Al Asfar Lake were analyzed chemically by using ICP-OES and PerkinElmer Titan MPS for the presence of heavy metals, which included Li, Co, Ni, Zn, Cd, As, and Pb. The ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy) system and PerkinElmer Titan MPS equipment are both made by PerkinElmer, USA.

2.6. Chemical Element Analysis after Incubation with Algal Consortium

A unified amount of algal biomass of each species, determined gravimetrically, was added to the filtered lake water samples and incubated for 2 weeks. The elemental composition of water before and after incubation with algae was determined by using the ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy) system.

2.7. Molecular Identification

Microalgae that showed the highest bio-remediating activity were identified by using a polyphasic approach which combines the morphological characters with molecular characterization and phylogenetic inference of the isolates. The molecular method included PCR using universal algal primers [11], with the template being genomic DNA that was previously extracted by using method in [12]. Those primers targeted part of the 23S rDNA gene within the cyanobacterial genome. This was followed by phylogenetic analysis and tree reconstruction to reveal their evolutionary background.

2.8. Polyaromatic Hydrocarbon Analysis

The water samples were tested for the presence of polyaromatic hydrocarbons, and the samples of water were incubated with the algal biomass for a month [13].

3. Results

3.1. Microscopic Examination of Predominant Microbial Consortium Members

The experimental findings of this study identify three microalgal strains and their heterotrophic bacteria from Al Asfar Lake. The five microalgal strains comprised two unicellular coccoid forms: a blue-green alga (cyanobacterium) and a green alga; a third was a filamentous non-heterocystous cyanobacterium. The filamentous strain, which had an unbranched thin trichome with rounded ends, did display motility though gliding. These strains were identified as C1 (the first coccoid unicellular type), C2 (the second coccoid unicellular type), and F1 (the filamentous lytic type) and proved to be cultivable on liquid BG-11 medium.

Apart from microalgae, four heterotrophic bacterial strains affiliated to the algal cultures were also identified and given codes A1, A2, B, and C1. After Gram staining, bacterial strains A1, A2 and C1 were found to be Gram-negative, and strain B was found to be Gram-positive. Incubation with spore stain agents showed that all the bacterial isolates were vegetative cells, but none of the cells had endospores. Therefore, these results enrich the information and knowledge concerning the microbial consortia connected with algae in aquatic ecosystems.

Table 2. Colony characteristics of bacteria associated with algal strains in microbial consortium.

Bacterial Pathogen Strain (Code)	Colony Color	Size of Colony (Diameter)	Shape	Texture	Elevation
A1	Orange	2 mL	Round	Shiny, smooth	Flat
A2	Yellowish	1 mL	Round	Shiny, smooth	Flat
B	Light, orange	2 mL	Round	Shiny, smooth	Flat
C1	White	2.5 mL	Round	Shiny, smooth	Flat
C2	White	1.5 mL	Round	Shiny, smooth	Flat

lists some of the morphological characteristics of bacteria in the microbial consortium.

3.2. Bioremediation of Water Samples from Al Asfar Lake

The chemical analyses of water from Al Asfar Lake without algal treatment by using ICP showed the presence of some heavy metals above standard levels, namely, Mn, Cu, and Cr. The latter was present in much higher concentration than the standard level.

Table 3. Analysis of chemical element in water from Al Asfar Lake by using ICP.

Chemical Element	Concentration Average	Concentration RSD	Concentration SD
Mn	1.678	35.1%	0.6 ppm
Co	0.994	0.5%	0.0 ppm
Cu	0.769	92.7%	0.7 ppm
Zn	1.059	0.3%	0.0 ppm
Cd	1.037	0.3%	0.0 ppm
Pb	1.060	1.2%	0.0 ppm
As	0.753	0.6%	0.0 ppm
Fe	0.887	0.2%	0.0 ppm
Ni	1.032	0.3%	0.0 ppm
Cr	0.588	259.5%	1.5 ppm
B	1.0090	0.3%	0.0 ppm

shows the original concentrations of elements in lake water, and

Table 4. Bioremediation of metals in water samples from Al Asfar Lake metals based on algal treatment. C1 denotes the coccoid cyanobacterium; C2 denotes the coccoid green alga; F3 denotes the filamentous non-heterocystous cyanobacterium. For each algal isolate, different volumes of cultures were used in the bioremediation experiment. Values reported here are the average concentrations detected. The value 0.00 ppm indicates that the element concentration is below the detection limit of the device. Every element has its own detection limit below which it cannot be detected by the device.

Sample	(C1) 10 mL Culture	(C1) 15 mL Culture	(C2) 10 mL Culture	(C2) 15 mL Culture	(F) 10 mL Culture	(F) 15 mL Culture
Mn	0.511 ppm	0.220 ppm	0.126 ppm	0.081 ppm	0.362 ppm	−0.668 ppm
Co	0.004 ppm	0.002 ppm	0.002 ppm	0.001 ppm	0.002 ppm	0.003 ppm
Cu	0.105 ppm	−0.068 ppm	0.282 ppm	−0.096 ppm	0.149 ppm	0391 ppm
Zn	0.029 ppm	0.000 ppm	0.030 ppm	0.036 ppm	0.082 ppm	0.100 ppm
Cd	0.002 ppm	0.017 ppm	0.000 ppm	0.000 ppm	0.000 ppm	0.000 ppm
Pb	0.018 ppm	0.008 ppm	0.019 ppm	0.020 ppm	0.027 ppm	0.018 ppm
As	0.011 ppm	0.008 ppm	0.006 ppm	0.008 ppm	0.007 ppm	0.005 ppm
Fe	0.060 ppm	0.053 ppm	0.058 ppm	0.062 ppm	0.027 ppm	0.037 ppm
Ni	0.004 ppm	0.0 03 ppm	0.002 ppm	0.004 ppm	0.003 ppm	0.001 ppm
Cr	−0.103 ppm	−1.116 ppm	−1.642 ppm-	0.172 ppm	−0.400 ppm	−2.307 ppm
B	1.427 ppm	1.544 ppm	1.426 ppm	1.676 ppm	1.737 ppm	1.419 ppm

shows the element concentration after incubation with algal samples at two different culture volumes for each algal strain. It was notable that water samples were already green in color, indicating the profuse growth of algae. A unified filtered volume of each water sample of Al Asfar Lake was incubated with two volumes (10 and 15 mL) of a one-month-old algal culture from each isolate type corresponding to 0.1 and 0.15 gm fresh weight of algal biomass. The treated water samples were incubated for a month and reanalyzed for their element contents (

Table 5. Hydrocarbon composition of water sample from Al Asfar Lake (control).

Compound Name	%	Retention Time (Rt), min
Phenol, 2,4-bis(1,1-dimethylethyl)	0.94	7.597
Cycloheptasiloxane, tetradecamethyl	3.62	7.801
Cyclooctasiloxane, hexadecamethyl	7.69	9.2
Cyclononasiloxane, octadecamethyl	8.15	10.365
Cyclodecasiloxane, eicosamethyl	7.38	11.423
Cyclooctasiloxane, hexadecamethyl	8.7	12.635
Cyclononasiloxane, octadecamethyl	11.72	13.723
Tetracosamethyl-cyclododecasiloxane	13.56	14.759
Tetracosamethyl-cyclododecasiloxane	17.48	16.067
Tetracosamethyl-cyclododecasiloxane	20.76	17.836

). A control sample that was not treated with algal cultures was also analyzed. The addition of the three algae caused a noticeable decrease in the metal concentration but with differential impacts (

Table 6. Hydrocarbon composition of water sample treated with Chlorella sp.

Compound Name	%	Retention Time (Rt), min
Phenol, 2,6-bis(1,1-dimethylethyl)	1.42	7.593
7-(1,5-Dimethylhexyl)-10,13-dimethyl2,3,4,7,8,9,10,11,12,13,14,15,16,17-tetradecahydro-1H-cyclopenta[a]phenanthren3-ol	72.14	14.341
Cholest-5-ene, 3.beta.-chloro	26.44	14.711

). The most bio-remediating algal samples were the green coccoid alga and the filamentous cyanobacterium. They caused the almost complete removal of metals that were previously present above standard concentrations.

The monoalgal cultures are not axenic, and they are associated with heterotrophic bacteria. The removal of metals with the algal–bacterial consortium was the best with bioremediating strains C2 and F.

Briefly, in the case of lake water samples before algal bioremediation, manganese, copper, and chromium (Cr) were all higher than standard values (Table 5). Following algal treatments, all these elements were almost completely removed, especially when applying a higher volume of algal cultures (Table 4). The most efficient algal strains in bioremediation were the filamentous cyanobacterium and the green algal strain.

3.3. Characterization of Taxonomic Identity of Most Efficient Bio-Remediating Algal Strains Using Molecular Analysis and Phylogenetic Inference

According to the molecular characterization, the most bio-remediating algae was found to be a non-heterocystous filamentous blue-green alga (cyanobacterium) and a coccoid green alga. The taxonomic identity of the isolates was also established by molecular techniques and phylogenetic trees. The BLAST search returned 99%. The sequence was highly related to Chlorella sp.

It is also necessary to mention that BLAST determines the pairwise homologous alignments of a subject range with a particular sequence and an aligned sequence from the database, while the search in multiple sequences is beyond its capabilities. Rather, implicit homology is built by how the database sequences match up with the query. The case with sequences that only partially match is solved in BLAST by the inclusion of only the sequence with the highest score in the phylogenetic tree (Figure 1).

The BLAST search matched Geitlerinema sp. with 82.91% similarity. BLAST computes pairwise alignment between a query and database sequences but does not perform multiple alignments. An implicit alignment is constructed based on database sequence alignment to the query [14]. In cases where sequences barely overlap, only the higher-scoring sequence is included in the tree (Figure 1).

The BLAST search matched Chlorella sp. with 99.40% similarity. BLAST computes pairwise alignment between a query and database sequences but does not perform multiple alignments. An implicit alignment is constructed based on database sequence alignment to the query [6]. In cases where sequences barely overlap, only the higher-scoring sequence is included in the tree (Figure 2).

3.4. Polyaromatic Hydrocarbon Analysis

The hydrocarbons found in the water of Al Asfar Lake are listed in

Table 7. Hydrocarbon composition of water sample treated with filamentous Geitlernema sp.

Compound Name	%	Retention Time (Rt), min
Phenol, 2,4-bis(1,1-dimethylethyl)	10.44	7.595
Cyclooctasiloxane, hexadecamethyl	5.11	9.197
Cyclononasiloxane, octadecamethyl	7.4	10.362
Cyclodecasiloxane, eicosamethyl	6.49	11.421
Cyclooctasiloxane, hexadecamethyl-	8.26	12.633
Tetracosamethyl-cyclododecasiloxane	11.44	13.721
Tetracosamethyl-cyclododecasiloxane	13.89	14.755
Tetracosamethyl-cyclododecasiloxane	17.53	16.064
Tetracosamethyl-cyclododecasiloxane	19.44	17.833

. The following chemicals were detected in the case of the control (Table 5) but were not present in the case of the sample treated with Chlorella sp. (Table 6), indicating its bioremediating activity that resulted in the removal of Cycloheptasiloxane, tetradecamethyl; Cyclooctasiloxane, hexadecamethyl; Cyclononasiloxane, octadecamethyl; Cyclodecasiloxane eicosamethyl; Cyclooctasiloxane, hexadecamethyl; Cyclononasiloxane, octadecamethyl; Tetracosamethyl-cyclododecasiloxane; Tetracosamethyl-cyclododecasiloxane; and Tetracosamethyl-cyclododecasiloxane.

On the contrary, the sample treated with the filamentous cyanobacterium (Geitlernema sp.) showed the presence of most of those polyaromatic hydrocarbons that were detected in the control, showing its very limited bioremediating ability (Table 7), which only resulted in the removal of Cycloheptasiloxane, tetradecamethyl.

4. Discussion

The study investigates the effects of algal treatment on reducing and bioremediating heavy metals in water samples from Al Asfar Lake. The control sample showed higher than normal concentrations of manganese, copper, and cadmium. Through algal treatment, the levels of all those metals dropped, indicating the complete or near complete removal of those metals. The results suggest that algal treatment effectively removed those elements from the water samples. Indeed, ref. [5] reported that phycobioremediation using Chlorella sp. for cleaning up contaminated water reservoir was feasible, efficient, and eco-friendly.

The study shows that algae have strong bioremediation capacity for some heavy metals present in the water samples from Al Asfar Lake, particularly the non-heterocystous filamentous blue-green alga (cyanobacteria) and the green alga. These algae show promise for cleaning up the environment by drastically lowering the concentrations of heavy metals like Mn, Cu, and Cr. The use of indigenous microorganisms, including green microalgae, to eliminate toxic levels of Cr pollutants has gained particular attention during recent decades [6]. The use of biological techniques produces little to zero secondary waste during bioremediation operations [6]. Moreover, the employment of both Gram-positive and Gram-negative bacteria is known to decontaminate the chrome-polluted environment in the vicinity of tanneries and electroplating industries [6]. Singh et al. (2013) [15] have demonstrated the excellent potential of Bacillus cereus to remove 72% of Cr (VI) at a concentration of 1000 μg/mL chromate. Interestingly, these bacteria were able to maintain their Cr removal potential in a broad range of temperature and pH [15]. On the other hand, the bioremediation of copper is possible by utilizing certain Cu-tolerant microorganisms [16]. Some specific bacteria and microalgae reduce Cu levels by immobilizing the ions through various bioremediation strategies, such as bioleaching to remove ex situ solids which contain Cu and bioaugmentation to increase the rates of in situ Cu removal [16].

Several bioremediating mechanisms are used by microalgae and cyanobacteria to clean up heavy-metal pollution. These include both extra- and intracellular strategies, such as adsorption, the binding of metal ions to chemicals on the outer surface, and/or-the intake of metal ions within their microbial cells, followed by internal detoxification; these cellular strategies limit the toxic effects of heavy metals. These mechanisms are summarized in [17] (and references within).

To remove heavy metals extracellularly, the composition of the cell wall and the presence of extracellular polymeric substances play crucial roles [17,18]. The large biomolecules in the cell wall, such as carbohydrates, lipids, and proteins, perform multiple functions, including giving an overall negative charge to the cell wall because of functional groups such as amino, carboxyl, phosphate, and phenol groups. [17]. The attachment of heavy-metal cations thus occurs on the cell surface and is not dependent on cellular metabolism [17]. However, the removal strategy largely varies with the genus and species of algae utilized [17]. In some algae, essential ions such as calcium, potassium, and magnesium bind to the cell surface but are reversibly substituted in the presence of excessive contents of heavy metals by a process known as ‘ion exchange’ [17]. The intracellular accumulation of heavy metals, however, depends on cellular metabolism [17]. The transport of heavy metals ions is largely dependent on active and/or passive transport pathways [17]. The intracellular uptake strategy, on the other hand, involves the active transport of heavy metals via negatively charged groups of biopolymers on the cell surface. The hydrophilic nature of heavy metals allows for their transport across the cell membrane with the help of specific intrinsic proteins known as metal transporters. Afterwards, the detoxification of heavy metals occurs via various pathways, including the production of metal binding proteins such as phytochelatin to chelate heavy metals, the interaction of heavy metals with thioneins for the formation of metallothioneins, the formation of complex interactions between heavy metals and poly-phosphate complexes, the sequestration of heavy-metals ions into vacuoles to reduce toxic effects, and increased activity of metal efflux systems to expel metal ions in case of extremely dangerous heavy-metal levels [17,18]. The intracellular detoxification and compartmentalization of heavy-metal ions is followed by the transformation of heavy-metal ions from the most toxic state to a relatively less toxic state, which may be performed by enzymes or a biochemically mediated mechanism. For example, the reduction of hexavalent chromium to a trivalent oxidation state makes chromium comparably less toxic and stable [17,18].

Furthermore, the study highlights the activity of algae in the removal/breakdown of polycyclic aromatic hydrocarbons, or PAHs, underscoring their significance in bioremediation initiatives.

Polycyclic aromatic hydrocarbons (PAHs) are the largest group of hydrocarbons with two or more fused benzene or pentacyclic rings [19]. These hydrocarbons are naturally present in coal, crude oil, and gasoline [19]. Coal-tar pitch, creosote, and asphalt produced from fossil fuels are also rich in PAHs [20]. Natural production of PAHs also occurs during forest fires and volcano eruptions. Relatively less soluble in water, PAHs are highly soluble in lipids [20]. A fraction of PAHs may decompose under ultraviolet light [21]. Some microorganisms are also capable of degrading aromatic hydrocarbons [21].

PAHs have attracted serious attention in recent years because of their detrimental effects on human health [20]. In fact, eight PAHs are typically regarded as possible carcinogens (Car-PAHs) [20]. These include chrysene, dibenzo(a,h)anthracene, benzo-anthracene, benzo-fluoranthene, indeno(1,2,3-cd)pyrene, benzo(k)fluoranthene, benzo(a)pyrene (B(a)P), and benzo(g,h,i)perylene [20]. Even PAHs which are not carcinogenic may still behave as synergists [20]. The increased production of PAHs is due to their ever-spreading use in research, medicines, dyes, plastics, and pesticides [19]. Indeed, our findings indicate their presence in large quantities in Al Asfar Lake water, and only Chlorella sp. was able to remove/breakdown these harmful chemicals.

The results suggest that algae, and more specifically Chlorella sp. and a non-heterocystous filamentous blue-green alga (Geitlerinema sp.), have great potential for the bioremediation of water samples from heavy metals, but Chlorella sp. alone has great potential for hydrocarbon bioremediation.

With regard to hydrocarbon pollution, some hydrocarbon compounds are reported to be pollutants, such as Tetracosamethylcyclododecasiloxane, which is a pollutant present in the process of textile enhancements and pretreatments with ozone (https://www.chemicalbook.com/ChemicalProductProperty_EN_CB61436019.htm, accessed on 25 June 2024).

In addition, another chemical detected in the analysis was Octamethylcyclononasiloxane, which is used as an antimicrobial agent (https://amp.chemicalbook.com/ChemicalProductProperty_EN_CB41410749.htm, accessed on 25 June 2024). Compounds like Cyclononasiloxane are reported to result from some plant extracts and are also found in agricultural waste [22]. PAHs may be adsorbed by several particles [19]. These particle-bound PAHs are believed to be significantly hazardous substances to the well-being of humans, as they can enter in their bodies through breathing [19]. The commercial production of naphthalene—a PAH with the lowest boiling point and the highest volatility—to make mothballs and other chemicals has certainly escalated its contribution to organic pollution [20]. Although naphthalene occurs mostly in the vapor phase and its deposition on different surfaces is small if compared with other PAHs, the use of camphor balls (made with naphthalene) in the wardrobe has increased its levels in domestic environments [20]. Thus, the carcinogenic or mutagenic effects caused by increasing quantities of PAHs are a serious concern for human health [20]. Tetracosamethyl-cyclododecasiloxane was the most prevalent PAH (20.76%) among the control samples, which showed a range of PAHs. A distinct PAH composition, dominated by a molecule present in 72.14% of the sample, was detected in sample C2. Sample F3 likewise revealed a mixture of PAHs, with Tetracosamethyl-cyclododecasiloxane (19.44%) being the most common component. Hydrocarbon processing industries release a broad range of organic pollutants, including isoalkanes and aromatic hydrocarbons [23].

Overall, this research reports the significant ecological role of algal consortia in bioremediating freshwater environments [14]. Accordingly, this could prospectively be used to better deal with the conservation and management of similarly polluted aquatic habitats.

5. Conclusions

This research addressed the growing environmental concerns regarding the pollution of Al Asfar Lake and provided an eco-friendly solution through the use of algae as bio-remediators. Algae have the ecological merits of being photosynthetic, mitigators of global warming, and removers of several heavy metals and hydrocarbons. Through their use, the ecological balance of the lake would be restored, and via their renewable growth, they would significantly contribute to the long-term sustainability of this vital water resource. The success of this study could pave the way for scalable, low-cost environmental restoration initiatives in similar ecosystems.