Integrating Aquaponics with Macrobrachium amazonicum (Palaemonidae: Decapoda) Cultivation for the Production of Microgreens: A Sustainable Approach

Abstract

The use of aquaponic systems has grown in recent years, but few of these systems have integrated the production of prawns and short-cycle vegetables. This study evaluated the potential for producing microgreens (beet, amaranth, arugula, and red cabbage) integrated with Amazon River prawns (Macrobrachium amazonicum) in an aquaponic system. Four seeding densities (5, 10, 15, or 20 seeds/cell) were assessed in two treatments: one using prawn wastewater and the other using plain dechlorinated water (control). Water quality, prawn growth performance, and microgreen productivity were monitored over 13 days, revealing optimal conditions for both prawns and microgreens in the aquaponic system. Amaranthus paniculatus yielded 374.00 g/m² in prawn wastewater compared to 231.34 g/m² in the control, while Beta vulgaris produced 1734.39 g/m² in wastewater versus 1127.69 g/m² in the control. Similarly, Brassica oleracea (2180.69 g/m²) and Eruca sativa (2109.46 g/m²) had higher yields in the prawn aquaponics system. These findings demonstrate that integrating prawn cultivation in aquaponic systems significantly enhances microgreen production compared to traditional methods. This integrated approach not only improves yields but also offers a more sustainable production model. Significant variation in productivity and growth metrics among species treatments underscores the viability and need for more systematic aquaponic procedures.

1. Introduction

Microgreens have been heralded as a revolutionary specialty crop, a novel category of edible greens valued universally for their intense flavors, vibrant colors, and concentrated nutrient profiles. Harvested at the cotyledon or first true leaf stage, typically within 7–21 days after sowing, these miniature greens offer an ample spectrum of flavors, ranging from mild to spicy, depending on the type of plant. A number of studies have shown that microgreens often provide higher concentrations of nutrients, vitamins, and minerals than their mature counterparts, which makes them an attractive addition to any health-conscious diet [1].

Their rapid growth cycle and high yield make microgreens a potentially viable option for both small-scale urban agriculture systems and commercial farming operations [2]. Although most crops require little or no fertilizer, the application of some nutrients increases the yield of microgreens [3]. The wastewater from the aquaculture system thus could support robust growth in the microgreens [4].

Aquaponics is a production system that integrates traditional aquaculture with the hydroponic cultivation of plants, contributing to the sustainability of food production [5]. In this symbiotic arrangement, the hydroponic plants thrive due to the abundance of nutrients available in the wastewater from the aquaculture system [6,7]. As the plants absorb these nutrients from the water, they correct the composition of the water as it returns to the aquaculture tanks [8]. Aquaponic systems thus substantially reduce the reliance on chemical fertilizers for the cultivation of the plants, while also mitigating the potential environmental impacts caused by the farming of aquatic organisms [9,10]. Given these characteristics, aquaponics is an excellent circular bioeconomic model for the organic production of food [11].

Recent research has provided promising results for the cultivation of prawns in intensive production systems, such as the biofloc [12] and recirculating aquaculture (RAS) systems [13] and aquaponics [14]. Macrobrachium amazonicum, the Amazon River prawn, is widely distributed in the riverine and estuarine habitats of eastern South America, between Venezuela and Argentina [15]. This crustacean is a crucial resource for the subsistence of many Amazon communities that depend economically on artisanal fishing [16]. The species also has potential for aquaculture [17], which represents a potential alternative for the exploitation of M. amazonicum in an expanding market, while conserving natural stocks [10]. Macrobrachium amazonicum develops rapidly over a short life cycle, and is relatively robust, being able to adapt to a range of captive conditions [18,19]. While smaller than Macrobrachium rosembergii, M. amazonicum has the advantage of being a native species, thus avoiding potential environmental problems if animals escape from captivity [20]. Despite all these advantages, the farming of this native freshwater prawn is still in its infancy, and will require more advanced research to develop a viable technological package for efficient aquaculture operations [21,22].

Despite these promising features, technical data for the systematic implementation of aquaponic systems to produce microgreens are still very scarce. Further research is needed, for example, to determine how appropriate the specifications of different aquaponic systems are for the cultivation of different varieties of microgreens. The incorporation of freshwater prawns into aquaponic systems, together with the cultivation of vegetables represents a holistic approach toward the development of efficient and sustainable food production in Amazon basin. The emphasis here is on the combination of aquaculture and hydroponics for the conservation of resources and water, nutrient recycling, and environmental stewardship [14].

The present study investigated the feasibility and potential benefits of aquaponics combined with the cultivation of M. amazonicum for microgreens production, presenting a sustainable approach to meet the growing demand for crustaceans and nutrient-rich salad greens. By harnessing a synergistic combination of aquaponic and M. amazonicum cultivation, the study verified the potential of this integrated system for optimizing resource use, minimizing environmental impact, and enhancing overall productivity. Through a simple experimental design and comprehensive data analysis, valuable insights were gained into the practical aspects of implementing this innovative approach, contributing to the advancement of sustainable aquaponic practices.

2. Materials and Methods

The present study was conducted in Laboratory of Amazonian Aquaculture Biosystems (BIOAQUAM) located on the campus of the Federal Rural University of Amazonia (UFRA) in Belém, Pará, Brazil (1°27′30″ S, 48°28′12″ W). The data were collected during the rainy season, in the austral summer months of February and March 2024. The laboratory consists of a rectilinear convective-roof greenhouse with dimensions of 8.0 m wide by 12.0 m long, a low-density polyethylene (LDPE) cover, and 50% shading screens on the sides, where the aquaponics systems were installed.

2.1. Plant Material

Seeds of arugula (Eruca sativa Mill. cv. Surya), red cabbage (Brassica oleracea var. capitata), amaranth (Amaranthus paniculata L. cv. Asteca), and beet (Beta vulgaris L.) were sourced from Isla Sementes (https://www.isla.com.br/produtos/sementes, accessed on 3 June 2024) for the cultivation of microgreens using a floating hydroponic system. The seeds were sown on 29 February 2024, in polystyrene containers (0.67 m × 0.34 m) filled with coconut fiber, which are denominated “cells” here. The seeds were sown at four densities—5, 10, 15, and 20 seeds per cell—which were irrigated using two types of water: (i) wastewater from the cultivation of Amazon River prawns (Macrobrachium amazonicum) in an integrated aquaponic system; and (ii) plain dechlorinated water (control). Each treatment has twelve replications and was distributed in a 128-cell floating tray, with the four plant species. The seed cells were first covered with moistened coconut fiber and placed in a floating tray with no direct contact with water under greenhouse conditions optimized for germination (temperature: 28.89 ± 1.15 °C, relative humidity: 84.29 ± 4.04%). During this process, the growing medium was initially immersed in water for 15 min, and then removed until germination was complete, which was observed two days post-sowing in all four species. Following germination, the units were transferred to a floating hydroponic tray (0.89 m × 1.19 m). The plants were harvested manually on 12 March, 13 days after the seeds were sown. At the end of the experiment, the values of the productivity of the plants were extrapolated to determine the productivity of plant biomass per square meter.

The microgreens were cultivated in four production units (Figure 1). Each unit comprised a 100 L settling tank and a 1000 L prawn cultivation tank (800 L effectively used) with continuous aeration. The hydroponic bed was composed of a 150 L bed placed above the prawn tank, where a 128 cell Styrofoam tray was placed, with a 100 L equalizing tank equipped with a pump (3000 L/h) for water circulation. The experiment used four independent aquaponic units, each equipped with a recirculating water system. Dechlorinated water was added continually to compensate for evaporation throughout the experiment. For the control units, the setup was identical, except for the absence of prawn cultivation. This ensured that any differences in microgreen growth could be attributed to the presence of prawns rather than variations in the system configuration or water quality management.

2.2. Analysis of the Water

Samples of the water were collected daily during the experiment to estimate the physicochemical parameters of the water and its nitrogen content. To maintain oxygen levels within adequate limits, a 1.0 cv radial blower with a stone diffuser was used to aerate the prawn cultivation tank. The water temperature, dissolved oxygen (DO), electric conductivity (EC), and pH of the water were monitored daily. The oximeter (YSI ProODO, Yellow Springs, OH, USA, ±0.01 mg/L) was calibrated to the saturation point in the air, while the pHmeter (AKSO^®, Porto Alegre, RS, Brazil, ±0.01) was calibrated using commercial standard buffers at pH 7 and 4 (AKSO, Porto Alegre, RS, Brazil), and the electrical conductivity meter (TDS&EC, São Paulo, SP, Brazil, ±2% FS) was calibrated with a conductivity solution of 1413 μS/cm (AKSO, Porto Alegre, RS, Brazil). At the end of each week, a sample of water was collected from each tank for the analysis of total ammonia, nitrite, and nitrate concentrations, using a commercial LabconTest kit (Alcon, Camboriú, SC, Brazil) and analyzed in duplicate with a spectrophotometer (Kasuaki, model IL-593, São Paulo, Brazil), using a 630 nm wavelength for the ammonia, 540 nm for the nitrite, and 220–270 nm for the nitrate.

2.3. Prawn Growth

Wild Amazon River prawns were collected in natural channel at Mosqueiro, district of Belém, Pará, using a matapi trap (an indigenous trap made of stripes of palm stalk tied together with liana), and acclimated in two experimental tanks of 1000 L (n = 100). In this period, prawn juveniles were fed three times a day, at 5% of their biomass, with commercial pellets, 0.4 mm, NUTRIPISCIS STARTER^® (45% protein and 9% lipid).

To evaluate the growth and survival analyses, the M. amazonicum were counted, measured and weighted (n = 100 juveniles per tank, 200 per treatment, initial length and weight 56.00 mm and 1.75 g, respectively) before being transferred to experimental tanks until 13 days after assay, when the study was concluded. A precision balance BEL ± 0.1 mg (São Paulo, SP, Brazil) was used for weight and length used digital pachymeter (ZTTO, São Paulo, SP, Brazil). Weight gain was obtained by subtracting the final from initial weight.

2.4. Plant Growth

The growth of the microgreens was evaluated through a series of measures. At the end of the experiment, the roots were washed, and the following measurements were obtained for each plant or cell: total length, root length, leaf count (number of leaves), plant count (number of surviving seedlings), and the weights of the root and the aerial part of the plant. Weights were acquired using a precision balance BEL ± 0.1 mg. The roots and aerial parts were then placed in individually labeled paper bags that were heated to a constant temperature of 67 °C for 72 h, to remove the moisture completely. Once processed, both parts were weighed to determine their respective biomass.

2.5. Statistical Analysis

The statistical analysis began with the verification of the homoscedasticity and normality of the variation in the data, to ensure the conditions for parametric analyses. A one-way Analysis of Variance (ANOVA) followed by Tukey’s post hoc test was used to identify significant differences (p < 0.05) between treatment groups of water. A two-way ANOVA was also used to compare the total length, root length, aerial length, number of leaves, stem diameter, root weight, total weight, and dry weight among the different seedling densities and between the two types of water (p < 0.05). All statistical analyses and plots were obtained using GraphPad Prism 10, to ensure the rigorous and accurate evaluation of the experimental data.

3. Results

3.1. Water Quality

The monitoring of the quality of the water demonstrated that adequate conditions for plant growth and optimal conditions for raising prawns were available throughout the study period (Figure 2). The mean water temperature in the control tanks was 30.03 ± 2.86 °C, while it was 29.26 ± 0.72 °C in the treatment tanks. In the treatment groups, the mean dissolved oxygen concentration was 8.14±0.13 mg/L, whereas in the control groups, it was 7.71 ± 0.70 mg/L. The mean electrical conductivity was 71.8 ± 8.80 µS/cm in the treatment groups, and 54.87 ± 3.48 µS/cm in the control groups. At the beginning of the experiment, the mean concentration of NH₃ in the treatment groups was 0.315 ± 0.007 mg/L, while that of NO₃ was 12.73 ± 0.04 mg/L, and NO₂ was 0.97 ± 0.33 mg/L, in comparison with 0.410 ± 0.021 mg/L, 3.83 ± 4.5 mg/L, and 0.67 ± 0.01 mg/L, respectively, in the control groups. At the end of the experiment, the means in the treatment groups were 2.84 ± 3.16 mg/L (NH₃), 18.22 ± 1.42 mg/L (NO₃), and 0.11 ± 0.01 mg/L (NO₂), in comparison with 0.245 ± 0.19 mg/L, 3.785 ± 4.41 mg/L, and 0.09 ± 0.01 mg/L, respectively, in the control groups. At the beginning of the experiment, the pH was 7.69 in the treatment groups and 7.54 in the control, decreasing to 6.08 and 6.55, respectively, by the end of the study.

3.2. Prawn Growth

The process of prawn cultivation in this experiment involved stocking 100 individuals in a 1000 L tank, utilizing 800 L of water. Initially, the prawns had a mean length of 56.00 mm and a mean weight of 1.75 g. Over a period of 13 days, under controlled conditions, the prawns exhibited significant growth, with their mean length increasing to 63.39 mm and their mean weight rising to 2.12 g. The survival rate of the prawns throughout the experiment was approximately 90%, indicating favorable conditions for prawn cultivation.

3.3. Plant Development

The plant development parameters are presented here per family—Amaranthaceae and Brassicaceae—each with two cultivars.

3.3.1. Brassicaceae

In the case of arugula, Eruca sativa, significant variation was found among the different seeding densities in all parameters except stem diameter (

Table 1. Growth parameters recorded in the present study for E. sativa in the two types of water (treatments = prawn wastewater vs. control (plain dechlorinated water)) at different seeding densities (number of seeds/cell). Treatments: T1—5 seeds per cell, T2—10 seeds per cell, T3—15 seeds per cell, T4—20 seeds per cell; controls: C1—5 seeds per cell, C2—10 seeds per cell, C3—15 seeds per cell, C4—20 seeds per cell. Numbers followed by different letters are statistical different between the densities p ≤ 0.05.

	Growth Performance
Treatments	NP (cm)	TL (cm)	RL (cm)	APL (cm)	NL (cm)	SD (cm)	RW (g)	APW (g)	DAPW (g)	DRW (g)	TW (g)	TDW (g)
Densities (seeds/cells)
5	4.50 ± 0.60 a	21.19 ± 3.94 a	14.41 ± 4.46 a	6.78 ± 2.33 a	16.72 ± 2.33 a	1.11 ± 0.19 a	0.35 ± 0.10 a	0.93 ± 0.35 a	0.06 ± 0.02 a	0.04 ± 0.04 a	1.12 ± 0.55 a	0.10 ± 0.04 a
10	8.61 ± 1.34 b	23.81 ± 3.24 ab	15.81 ± 2.92 ab	7.99 ± 1.11 b	30.83 ± 4.69 b	1.18 ± 0.21 a	0.53 ± 0.14 b	1.62 ± 0.54 b	0.04 ± 0.02 a	0.05 ± 0.04 a	2.16 ± 0.60 b	0.09 ± 0.03 a
15	14.11 ± 2.02 c	26.26 ± 5.55 b	17.93 ± 5.17 b	8.33 ± 1.60 b	45.89 ± 5.59 c	1.07 ± 0.15 a	0.61 ± 0.16 b	2.02 ± 0.68 b	0.07 ± 0.01 b	0.02 ± 0.00 b	2.63 ± 0.73 b	0.08 ± 0.02 a
20	17.44 ± 1.95 d	24.56 ± 4.17 b	15.95 ± 3.88 ab	8.62 ± 2.15 b	59.11 ± 7.88 d	1.08 ± 0.16 a	1.34 ± 0.66 b	2.50 ± 0.67 c	0.08 ± 0.02 b	0.02 ± 0.01 b	3.24 ± 0.72 c	0.10 ± 0.02 a
Type of water
DA	11.61 ± 5.36	23.36 ± 4.27	16.65 ± 3.72	6.71 ± 1.31	36.25 ± 15.80	1.01 ± 0.15	0.89 ± 0.66	1.39 ± 0.53	0.06 ± 0.02	0.03 ± 0.03	1.90 ± 0.83	0.09 ± 0.03
WPC	10.72 ± 5.05	24.55 ± 4.98	15.40 ± 4.86	9.15 ± 1.79	40.03 ± 17.59	1.21 ± 0.16	0.53 ± 0.18	2.14 ± 0.87	0.06 ± 0.03	0.03 ± 0.03	2.67 ± 1.03	0.09 ± 0.03
p-value
DA	0.78 ns	0.12 ns	0.09 ns	0.66	0.09 ns	0.08 ns	0.14 ns	0.001	0.12 ns	0.11 ns	0.19 ns	0.10 ns
WPC	<0.001	<0.001	0.007	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
Interaction	0.79 ns	0.16 ns	0.14 ns	<0.001	0.008	<0.001	0.021	<0.001	0.17 ns	0.56 ns	0.66 ns	0.22 ns

NP—number of plants, TL—total length, RL—root length, APL—aerial part length, NL—number of leaves, SD—stem diameter, RW—root weight, APW—aerial part weight, TW—total weight, DAPW—dry aerial part weight, DRW—dry root weight, TDW—Total Dry Weight, DA—dechlorinated water, WPC—water prawn cultivation, ns—not significant.

). No significant variation was found between the two water treatments (prawn wastewater vs. control) in the total plant length and the number of plants surviving, although significant differences were observed in root length, shoot length, the number of leaves, stem diameter, total fresh weight, shoot weight, and root weight among the different seeding densities (Table 1).

In the cultivation of red cabbage, Brassica oleracea, significant variation was found in all the parameters analyzed among the different seeding densities (

Table 2. Growth parameters recorded in the present study for B. oleracea in the two types of water (treatments = prawn wastewater vs. control (plain dechlorinated water)) at different seeding densities (number of seeds/cell). Numbers followed by different letters are statistical different between the densities p ≤ 0.05.

	Growth Performance
Treatments	NP (cm)	TL (cm)	RL (cm)	APL (cm)	NL (cm)	SD (cm)	RW (g)	APW (g)	DAPW (g)	DRW (g)	TW (g)	TDW (g)
Densities (seeds/cells)
5	3.89 ± 0.57 a	18.34 ± 2.95 a	8.74 ± 5.13 a	6.22 ± 0.75 a	12.94 ± 2.86 a	1.14 ± 0.22 a	0.40 ± 0.08 a	0.75 ± 0.22 a	0.05 ± 0.02 a	0.01 ± 0.01 a	1.15 ± 0.26 a	0.06 ± 0.02 a
10	7.33 ± 1.00 b	22.86 ± 2.87 b	15.54 ± 2.61 b	7.32 ± 1.09 b	23.39± 4.08 b	1.17 ± 0.11 a	0.57 ± 0.15 a	1.42 ± 0.30 b	0.07 ± 0.01 a	0.02 ± 0.00 a	1.99 ± 0.27 b	0.09 ± 0.01 a
15	12.06 ± 2.15 c	23.65 ± 2.73 b	15.31 ± 2.54 b	8.34 ± 1.20 c	36.00 ± 7.06 c	1.14 ± 0.15 a	0.69 ± 0.14 ab	2.15 ± 0.58 bc	0.09 ± 0.01 b	0.03 ± 0.01 a	2.84 ± 0.54 c	0.12 ± 0.02 b
20	15.33 ± 2.11 d	22.49 ± 3.26 b	14.34 ± 2.49 b	8.15 ± 1.10 c	43.33 ± 5.46 c	0.99 ± 0.19 b	0.81 ± 0.31 c	2.56 ± 0.71 c	0.11 ± 0.04 b	0.03 ± 0.01 a	3.37 ± 0.80 d	0.15 ± 0.04 b
Type of water
DA	9.50 ± 4.70	20.48 ± 3.85	12.12 ± 5.50	6.66 ± 0.91	26.67 ± 11.13	1.06 ± 0.20	0.69 ± 0.25	1.37 ± 0.59	0.08 ± 0.03	0.02 ± 0.01	2.06 ± 0.80	0.10 ± 0.04
WPC	9.81 ± 4.62	23.19 ± 2.74	14.84 ± 2.08	8.35 ± 1.17	31.17 ± 13.79	1.15 ± 0.16	0.55 ± 0.22	2.07 ± 0.93	0.09 ± 0.04	0.02 ± 0.01	2.62 ± 1.08	0.11 ± 0.05
p-value
DA	0.80 ns	0.17 ns	0.08 ns	0.66	0.08 ns	0.08 ns	0.15 ns	0.001	0.12 ns	0.14 ns	0.10 ns	0.10 ns
WPC	<0.001	<0.001	0.0006	<0.001	<0.001	<0.0001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
Interaction	0.89 ns	0.17 ns	0.12 ns	<0.001	0.008	<0.0001	0.0241	<0.001	0.15 ns	0.56 ns	0.66 ns	0.22 ns

NP—number of plants, TL—total length, RL—root length, APL—aerial part length, NL—number of leaves, SD—stem diameter, RW—root weight, APW—aerial part weight, TW—total weight, DAPW—dry aerial part weight, DRW—dry root weight, TDW—Total Dry Weight, DA—dechlorinated water, WPC—water prawn cultivation, ns—not significant.

). Overall, there was a slight enhancement in the growth parameters when prawn wastewater was used to irrigate the plants, with some significant differences. The potential for the improvement of growth performance can also be observed in the comparison of a number of parameters at different seeding densities in the two water treatments (Table 2).

3.3.2. Amaranthacea

In the aquaponic cultivation of the amaranth, Amaranthus paniculatus, significant variation was found among seeding densities in leaf number, shoot length, leaf diameter, and fresh shoot weight (

Table 3. Growth parameters recorded in the present study for A. paniculatus in the two types of water (treatments = prawn wastewater vs. control (plain dechlorinated water)) at different seeding densities (number of seeds/cell). Treatments: T1—5 seeds per cell, T2—10 seeds per cell, T3—15 seeds per cell, T4—20 seeds per cell; controls: C1—5 seeds per cell, C2—10 seeds per cell, C3—15 seeds per cell, C4—20 seeds per cell. Numbers followed by different letters are statistical different between the densities p ≤ 0.05.

	Growth Performance
Treatments	NP (cm)	TL (cm)	RL (cm)	APL (cm)	NL (cm)	SD (cm)	RW (g)	APW (g)	DAPW (g)	DRW (g)	TW (g)	TDW (g)
Densities (seeds/cells)
5	2.13 ± 0.93 a	23.55 ± 9.12 a	15.53 ± 8.41 a	8.03 ± 1.24 a	9.94 ± 5.25 a	1.24 ± 0.27 a	0.13 ± 0.06 a	0.29 ± 0.16 a	0.01 ± 0.01 a	0.01 ± 0.00 a	0.42 ± 0.20 a	0.02 ± 0.01 a
10	3.52 ± 1.47 a	22.78 ± 5.91 ab	13.93 ± 5.72 a	8.85 ± 0.96 a	16.78 ± 6.37 b	1.18 ± 0.18 a	0.17 ± 0.08 a	0.43 ± 0.15 b	0.02 ± 0.01 a	0.01 ± 0.00 a	0.60 ± 0.22 b	0.02 ± 0.01 a
15	5.17 ± 1.89 a	23.05 ± 7.79 ab	14.10 ± 7.48 a	8.95 ± 1.11 a	23.46 ± 8.37 c	0.97 ± 0.14 b	0.23 ± 0.10 b	0.53 ± 0.23 b	0.02 ± 0.01 a	0.01 ± 0.00 a	0.76 ± 0.32 b	0.03 ± 0.01 a
20	5.54 ± 2.43 a	20.41 ± 5.39 b	12.00 ± 4.99 a	8.41 ± 1.46 a	23.83 ± 9.58 c	0.92 ± 0.19 c	0.20 ± 0.09 ab	0.45 ± 0.25 b	0.02 ± 0.01 a	0.01 ± 0.01 a	0.65 ± 0.32 b	0.03 ± 0.02 a
Type of water
DA	4.42 ± 2.18	26.36 ± 7.15	17.79 ± 6.74	8.58 ± 1.23	20.49 ± 8.99	1.02 ± 0.20	0.18 ± 0.08	0.46 ± 0.20	0.02 ± 0.01	0.01 ± 0.01	0.64 ± 0.27	0.03 ± 0.01
WPC	4.12 ± 2.29	18.03 ± 3.79	9.40 ± 3.01	8.63 ± 1.28	18.05 ± 9.69	1.11 ± 0.25	0.20 ± 0.10	0.41 ± 0.24	0.02 ± 0.01	0.01 ± 0.00	0.61 ± 0.32	0.02 ± 0.01
p-value
DA	0.88 ns	0.33 ns	0.39 ns	0.19 ns	0.67 ns	0.53 ns	0.88 ns	0.47 ns	0.11 ns	0.17 ns	0.19 ns	0.22 ns
WPC	<0.001	0.63 c	0.61 c	0.05	<0.001	<0.001	0.01	0.02	0.01	0.02	0.01	0.02
Interaction	0.04	<0.001	<0.001	0.84 c	0.01	0.02	0.89 c	0.11 c	0.14 c	0.22 c	0.13 c	0.11 c

NP—number of plants, TL—total length, RL—root length, APL—aerial part length, NL—number of leaves, SD—stem diameter, RW—root weight, APW—aerial part weight, TW—total weight, DAPW—dry aerial part weight, DRW—dry root weight, TDW—Total Dry Weight, DA—dechlorinated water, WPC—water prawn cultivation, ns—not significant.

), although no significant differences were observed in total length, root length, or fresh root weight. Significant variation in total length, root length, and leaf number was recorded between the treatments (prawn wastewater vs. control), but not in plant number, shoot length, shoot diameter, or fresh shoot weight (Table 3).

In the cultivation of the beet, Beta vulgaris, significant variation was found among the different seeding densities in some, but not all parameters, specifically, total length, root length, stem diameter, and root weight (

Table 4. Growth parameters recorded in the present study for B. vulgaris in the two types of water (treatments = prawn wastewater vs. control (plain dechlorinated water)) at different seeding densities (number of seeds/cell). Treatments: T1—5 seeds per cell, T2—10 seeds per cell, T3—15 seeds per cell, T4—20 seeds per cell; controls: C1—5 seeds per cell, C2—10 seeds per cell, C3—15 seeds per cell, C4—20 seeds per cell. Numbers followed by different letters are statistical different between the densities p ≤ 0.05.

	Growth Performance
Treatments	NP (cm)	TL (cm)	RL (cm)	APL (cm)	NL (cm)	SD (cm)	RW (g)	APW (g)	DAPW (g)	DRW (g)	TW (g)	TDW (g)
Densities (seeds/cells)
5	5.88 ± 0.83a	29.17 ± 3.10 a	21.72 ± 3.06 a	7.45 ± 0.93 a	15.50 ± 3.95 a	1.16 ± 0.15 a	43.68 ± 20.45 a	0.28 ± 0.09 a	0.03 ± 0.01 a	0.02 ± 0.02 a	1.86 ± 0.37 a	0.05 ± 0.02 a
10	11.71 ± 1.86 b	29.78 ± 3.37 a	21.23 ± 3.58 a	8.55 ± 0.80 ab	26.92 ± 5.27 b	1.14 ± 0.14 a	1.43 ± 0.25 b	0.42 ± 0.11 b	0.04 ± 0.02 a	0.02 ± 0.01 a	1.85 ± 0.35 a	0.06 ± 0.02 a
15	17.83 ± 2.44 c	30.75 ± 2.18 a	21.81 ± 2.17 a	8.94 ± 0.87 ab	38.08 ± 5.89 c	1.05 ± 0.18 a	2.01 ± 0.41 c	0.58 ± 0.14 c	0.05 ± 0.02 a	0.02 ± 0.01 a	2.58 ± 0.53 b	0.07 ± 0.02 a
20	23.04 ± 2.78 d	30.55 ± 2.65 a	21.40 ± 2.85 a	9.16 ± 1.19 a	46.54 ± 5.47 d	1.01 ± 0.14 a	2.30 ± 0.35 d	0.67 ± 0.12 d	0.08 ± 0.14 a	0.03 ± 0.01 a	2.97 ± 0.45 b	0.10 ± 0.13 a
Type of water
DA	14.54 ± 6.88	30.66 ± 3.44	22.65 ± 3.09	8.22 ± 0.98	30.69 ± 13.31	1.07 ± 0.16	23.03 ± 146.74	0.48 ± 0.19	0.06 ± 0.10	0.02 ± 0.01	23.50 ± 146.72	0.07 ± 0.10
WPC	14.69 ± 6.68	29.26 ± 2.41	20.43 ± 2.35	8.83 ± 1.25	32.83 ± 12.15	1.11 ± 0.17	1.69 ± 0.67	0.49 ± 0.19	0.04 ± 0.02	0.02 ± 0.02	2.18 ± 0.85	0.07 ± 0.02
p-value
DA	0.32 ns	0.68 ns	0.56 ns	0.29 ns	0.03	0.55 ns	0.70 ns	0.87 ns	0.10 ns	0.19 ns	0.22 ns	0.55 ns
WPC	<0.001	0.57 ns	0.94 ns	0.001	<0.001	0.05	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
Interaction	0.48 ns	0.01	<0.001	0.10 ns	0.24 ns	0.08 ns	0.53 ns	0.98 ns	0.62 ns	0.52 ns	0.99 ns	0.12 ns

NP—number of plants, TL—total length, RL—root length, APL—aerial part length, NL—number of leaves, SD—stem diameter, RW—root weight, APW—aerial part weight, TW—total weight, DAPW—dry aerial part weight, DRW—dry root weight, TDW—Total Dry Weight, DA—dechlorinated water, WPC—water prawn cultivation, ns—not significant.

). Significant variation among seeding densities was found in the number of plants, shoot length, shoot weight, root weight, and the number of leaves. Similarly, no significant variation was found between water treatments in the number of germinated plants, fresh shoot weight, fresh root weight, number of leaves or stem diameter. However, total length, root length, and shoot length were enhanced significantly in the prawn wastewater treatment (Table 4).

3.3.3. Productivity

The productivity of the plants, measured in grams per square meter, was compared among seeding densities and between treatment and control groups. In all four cases, the 20 seeds per cell density was the most productive. At this density, Amaranthus paniculatus yielded 374.00 g/m² in the prawn wastewater system, in comparison with only 231.34 g/m² in the plain dechlorinated water, while Beta vulgaris produced 2180.69 g/m² in the prawn wastewater system, and 1127.69 g/m² in the plain water. Similarly, Brassica oleracea produced 1734.39 g/m² and Eruca sativa produced 2109.46 g/m² in the prawn wastewater cultivation system, but they only produced 1108.20 g/m² and 1232.50 g/m², respectively, in the plain dechlorinated water system.

4. Discussion

The findings of the present study are broadly consistent with the results of a number of previous studies of microgreen cultivation in aquaponic systems. For example, Danaher et al. (2018) [23] reported that maintaining the dissolved oxygen (DO) concentration above 6 mg/L is crucial for optimal microgreen growth, which is consistent with the higher DO levels (8.14 mg/L) observed here in the treatment group, which contributed positively to the health of both the microgreens and the prawns. Tyson et al. (2008) [24] emphasized that aquaponic systems benefit from higher DO levels due to the enhanced root respiration and nutrient uptake, which is consistent with the results of the present study, which indicated better growing conditions in the treatment group.

Temperature management is also crucial. Moraru et al. (2022) [4] determined that the optimal temperature range for the growth of microgreens was 18–24 °C, but that they can tolerate higher temperatures over the short term. The temperatures in the present study (29.26 °C in the treatment tank and 30.03 °C in the control) were higher than this optimal range, but were still adequate for growth, indicating that microgreens can adapt to higher temperatures, within certain limits. This adaptability may be crucial in integrated aquaponic systems, in which temperature regulation may be more challenging.

The management of nitrogenous compounds, specifically ammonia (NH₃), nitrate (NO₃), and nitrite (NO₂), is also crucial to guarantee the health and productivity of aquaponic systems. In the present study, the concentrations of these compounds varied considerably between the treatment and control systems, which had important implications for the growth of both the plants and the prawns. Initially, the treatment (0.315 mg/L) and control tanks (0.410 mg/L) had similar levels of ammonia, but while the mean concentration in the treatment tanks increased to 2.84 mg/L, it decreased to 0.245 mg/L in the control tanks. The increase in ammonia in the treatment tank can be attributed to the bioload produced by the prawns, in particular from their metabolic waste [14,25]. The levels of nitrate, a key nutrient for plant growth, were significantly higher in the treatment tanks both at the beginning (12.73 mg/L) and end (18.22 mg/L) of the experiment, in comparison with the control tanks (3.83 mg/L and 3.785 mg/L, respectively). The high levels of nitrate recorded in the treatment tanks indicate enhanced nitrification activity, through which the ammonia is oxidized to nitrite and then to nitrate, by nitrifying bacteria. This process is essential in aquaponic systems because it converts toxic ammonia into less harmful nitrates, providing a nutrient-rich substrate that is conducive to plant growth.

Electrical conductivity, which reflects the ion concentration in the water, is also an important parameter. Resh (2022) [26] noted that higher conductivity (50–200 μS/cm) is beneficial for the absorption of nutrients in both hydroponic and aquaponic systems. The higher conductivity (71.8 μS/cm) of the treatment group, in comparison with the control (54.87 μS/cm) in the present study, indicates a more nutrient-rich environment, which favors the development of microgreens, which is consistent with the findings of previous studies, e.g., [26], and further validates the improved plant growth conditions in the treatment group.

In the present study, the pH levels of the water declined significantly over the experimental period. Initially, the mean pH was 7.69 in the treatment group and 7.54 in the control, but by the end of the experiment, these values had dropped to 6.08 and 6.55, respectively. This trend toward increasing acidity is consistent with the pattern observed in recirculating aquaculture systems (RAS), in which the CO₂ produced by the prawns and the biological activity of the biofilter play crucial roles. The CO₂ produced by the respiration of the prawns reacts with the water to form carbonic acid, thus lowering the pH. The nitrification process in the biofilter, which converts ammonia to nitrate, generates hydrogen ions (H+), which further contributes to the decrease in the pH [27].

The observed decrease in the pH is a common challenge in RAS, given that the maintenance of an optimal pH is crucial for the health of both aquatic animals and plants. Acidic conditions can have a negative impact on prawn metabolism and growth, as well as nutrient availability and uptake in plants. Bregnballe (2022) [28] emphasizes that effective pH management in RAS requires the monitoring and adjustment of the water chemistry to mitigate these acidification processes. The integration of hydroponics, as in the case of aquaponics, can buffer fluctuations in the pH through the uptake of nitrates by the plants, which can reduce the acidity slightly. These findings underscore the importance of continuous monitoring and the active management of water quality parameters to ensure the sustainability of both the aquaculture and hydroponic components of integrated systems, thus ensuring optimal conditions for both the animals and the microgreens. The results of the present study are consistent with the findings of the previous research, highlighting the interconnectedness of the biological and chemical processes in aquaponic systems, and the need for balanced management practices to support sustainable production systems.

In the present study, the germination rates and plant survival were assessed in Brassica oleracea, Eruca sativa, Beta vulgaris, and Amaranthus paniculatus in the context of two types of water—wastewater from the freshwater prawn aquaculture and plain, dechlorinated water. Brassica oleracea had a germination rate of 74.4% in the wastewater treatment, and 78.85% in the control, reflecting a robust performance under both conditions. Eruca sativa presented even higher germination rates, reaching 83.85% in the wastewater group, and 90.55% in the control. The highest germination rates were recorded in Beta vulgaris, however, reaching 93.3% in the wastewater and 91.1% in the control, highlighting the adaptability of this species. By contrast, Amaranthus paniculatus presented the lowest germination rates, with 27.9% in the wastewater group and 27.75% in the control, indicating either that it was not well suited to the experimental conditions or that the system requires further adjustment.

A number of previous studies have reinforced the importance of adequate water quality and nutrient management in aquaponic systems used for the cultivation of microgreens. Environmental factors, such as nutrient concentrations and water quality, have a critical effect on microgreen growth and nutrient uptake, which is consistent with the high germination rates observed in Brassica oleracea and Beta vulgaris in the present study [29]. In addition, Pattillo et al. (2022) [30] emphasized the variability in the design of aquaponic systems and the impact of this on plant productivity, which indicates that the optimization of conditions in line with the requirements of each plant species is essential for maximizing yields. The observed differences in germination rates among the study species highlight the need for the taxon-based tailoring of aquaponic management configurations to ensure the most efficient production of different microgreen cultivars.

The mean weight per cell recorded for each cultivar was influenced by a range of factors that impacted the performance of each plant. Significant variation was recorded in most of the parameters in the different cultivars, in particular, the number of plants, which increased with seedling density. Previous studies have shown that higher seeding densities generally result in a greater number of plants per cell and a greater overall weight yield [4,31]. This pattern was not observed in Amaranthus paniculatus, however. Even so, similarities in the total length, root length, and root weight were found among the different seeding densities, which indicate that this cultivar may not respond to increased seeding density in the same manner as the other species [32].

In the present study, coconut fiber, an abundant and reasonably priced substrate in northern Brazil, was used for the production of microgreens. Coconut fiber has a number of properties that enhance the capillarity of the substrate, and over the 13-day cultivation period, it proved effective for the cultivation of Beta vulgaris, Eruca sativa, and Brassica oleracea. The yields per square meter for these species were 1.734 g/m², 2.109 g/m², and 2.181 kg/m², respectively, when irrigated with prawn wastewater. The yields in the control were only 1.233 kg/m² for B. vulgaris, 1.108 kg/m² for E. sativa, and 1.128 kg/m² for B. oleracea. These findings contrast with those of other previous studies, in which perlite and peat-based substrates were used, and yields were lower. While effective, peat-based substrates are not only costly, but are also not reusable. Perlite, which has strong capillary action, also produced lower yields in a number of studies, with a mean yield of only 1.12 kg/m² [33]. In addition, the highest yield reported by Brlek (2019) [34] was 1.0 kg/m² using a commercial substrate mixed with perlite, while burlap yielded only 0.6 kg/m². El-Nakhel et al. (2021) [33] found that growing parsley microgreens on a peat-based substrate over 21 days produced yields consistent with similar studies of fennel microgreens. These comparisons indicate that the type of substrate has a significant influence on the productivity of microgreens.

The productivity of microgreens in aquaponic systems may be influenced by a number of different factors, including seeding density, water quality, and nutrient availability. In the present study, the productivity of Amaranthus paniculatus, Beta vulgaris, Brassica oleracea, and Eruca sativa was measured under controlled conditions. The highest yields produced by Amaranthus paniculatus were achieved with a seeding density of 20 seeds per cell, resulting in a mean yield of 374 g/m². By comparison, Beta vulgaris produced a significantly higher yield of 1734.39 g/m² at the same seeding density. The species of the family Brassicaceae, Brassica oleracea and Eruca sativa produced even higher yields, of 2180.69 g/m² and 2109.46 g/m², respectively, under the same seeding conditions.

Resh (2022) [26] and Somerville et al. (2014) [35] highlighted the impact of water quality and nutrient management on microgreen yields in aquaponic systems. Resh [26] noted that optimal nutrient concentrations and balanced pH levels are crucial for maximizing plant growth. Similarly, the higher yields recorded in the present study for Beta vulgaris and Brassica oleracea can be attributed to the favorable nutrient uptake facilitated by the higher electrical conductivity and nitrate levels found in the prawn wastewater. Somerville et al. [35] also emphasized the role of dissolved oxygen levels in root development and nutrient absorption, which is consistent with the higher yields observed for these species in the present study.

Love et al. (2015) [25] provided an overview of the mean productivity of microgreens of a number of different species in aquaponic systems, reporting yields of 1500–2000 g/m² for Brassica species under optimal conditions. This range is comparable to the yields recorded in the present study, with Brassica oleracea reaching 2180.69 g/m². The overall consistency of these findings highlights the effectiveness of higher seeding densities and the maintenance of optimal water quality parameters in order to achieve maximum productivity.

The variation in productivity observed between different species, as recorded in the present study, is consistent with the available data. Schmautz et al. (2016) [36], for example, reported that species-specific nutrient requirements and growth rates can lead to significant differences in yields. The much lower yields recorded here for Amaranthus paniculatus (374 g/m²) likely reflect its distinct nutrient and environmental needs, in comparison with the less nutrient-demanding Beta vulgaris and cruciferous species.

The results of the present study indicate that microgreens grown on coconut fiber in a floating aquaponic system have a high nutritional potential and can be recommended for daily consumption in a healthy diet [37]. Amaranthus paniculatus microgreens are rich in ascorbic acid, total phenols, and total flavonoids, as well as essential minerals such as copper, manganese, iron, potassium, and zinc. Beta vulgaris microgreens are high in dietary fiber and essential minerals, such as iron, sodium, and potassium, and vitamins A and B. Euruca sativa microgreens are rich in vitamins, minerals, and bioactive compounds, including vitamin E, carotenoids, ascorbic acid, calcium, iron, potassium, zinc, phenolic compounds, and isothiocyanates. Brassica oleracea var. capitata microgreens also provide valuable macroelements and microelements. Overall, then, all these microgreens are excellent additions to a balanced diet.

In the present study, the aquaculture system achieved an adequate zootechnical performance for the cultivation of M. amazonicum, with significant growth and high survival rates. At the beginning of the study period, the prawns had a mean length of 56.00 mm and mean weight of 1.75 g, and at the end of the 13-day period, the mean length had increased to 63.39 mm, and their weight had risen to 2.12 g, with a survival rate of 90%. These results are consistent with the findings of previous studies of the performance of M. amazonicum in aquaponic systems. Sterzelecki et al. (2021) [14] reported that prawns raised in integrated aquaculture systems presented substantial growth and high survival rates, which they attributed to the optimal quality of the water and the availability of nutrients. Similarly, Faria Lima et al. (2019) [38] observed that the growth rates of prawns in aquaponic systems may match or even surpass those of traditional aquaculture systems, due to the continuous flow of nutrients from the integrated hydroponic systems, which enhances the overall health and growth potential of the prawns.

The results of the present study are also consistent with the available data, which highlight the importance of the maintenance of water quality, such as dissolved oxygen concentrations, pH, and temperature, to ensure the health and growth of prawns in aquaponic systems. The high prawn survival rate recorded here (90%) highlights the effectiveness of the controlled aquaponic environment for the robust growth of prawns, due to the stable environmental conditions and adequate nutrient cycling of the system. Overall, comparisons with the available data demonstrate clearly that integrated aquaponic systems can provide a conducive environment for the cultivation of M. amazonicum, supporting high growth and survival rates, which are essential for the commercial viability and sustainability of aquaponic systems.

5. Conclusions

The present study demonstrated that an aquaponic system combining the raising of freshwater prawns with the cultivation of microgreens can provide an effective environment for the cultivation of both types of organism, in terms of water quality, as well as enhancing the yields of both the prawns and the plants. The water in the aquaculture tanks had higher electrical conductivity, and oxygen and nutrient concentrations, which are all favorable to the growth of the organisms. The prawns presented considerable growth, with a high survival rate (90%), while Brassica oleracea and Beta vulgaris achieved higher yields in the prawn wastewater, in comparison with plain dechlorinated water.

These findings reinforce the results of previous studies on the potential benefits of integrated aquaponic systems, highlighting their potential for efficient and sustainable agricultural production. The integration of prawn cultivation with microgreen production not only enhances overall productivity but also proves to be a viable method for producing high-quality, nutrient-rich vegetables alongside native species. This innovative model offers a promising approach to implementing a bioeconomy in the Amazon, promoting sustainable agriculture while leveraging local biodiversity. However, additional studies are needed to verify the model’s effectiveness in situ.