Aquaculture has become one of the fastest growing food production systems on the planet, but its success has come with an uncomfortable side effect: nutrient-dense wastewater that threatens rivers, coastal waters and the very ecosystems the industry depends on. A new study published in the journal Blue Biotechnology suggests an elegant way out of this dilemma, showing that two common microalgae can transform fish farm effluent into high-quality feed ingredients while simultaneously scrubbing the water clean. The research, led by Wong Ryan Lieng Song and colleagues at Xiamen University Malaysia together with Universiti Putra Malaysia, evaluated the freshwater green alga Chlorella vulgaris and the marine microalga Nannochloropsis oculata grown in synthetic aquaculture wastewater, with results that could reshape how fish farms manage both their waste and their feed supply chains.

The problem the researchers set out to tackle is fundamentally chemical. Intensive fish farming generates effluents loaded with nitrogen and phosphorus compounds, chiefly ammonium from fish excretion and uneaten feed. When ammonium-rich water is discharged into natural water bodies it drives eutrophication, shifts pH, increases toxicity and depletes dissolved oxygen, endangering aquatic life. Conventional treatment technologies can remove these nutrients, but they are expensive, maintenance-heavy and often environmentally risky in their own right. Microalgae offer an alternative that sounds almost too good to be true: these photosynthetic organisms consume ammonium and phosphate as fertilizer, converting a pollutant into biomass that is itself rich in protein, carbohydrates, lipids and bioactive compounds suitable for aquafeed.

What has been missing, the authors argue, is attention to the quality of the biomass produced. Most previous studies focused narrowly on nutrient removal percentages, treating the algae merely as a cleaning agent. Yet the form of nitrogen in the water, whether nitrate, nitrite or ammonium, shapes the metabolic pathways of the algae and therefore the biochemical composition of the resulting cells. Some earlier work found troubling trade-offs: Chlorella sorokiniana grown in tilapia wastewater removed ammonia efficiently but produced biomass with only modest protein, while Chlorella vulgaris cultured in trout farm effluent yielded just 17.93 percent protein despite removal efficiencies above 90 percent. If the biomass is nutritionally poor, the promise of simultaneous remediation and feed production collapses.

To test whether better outcomes were possible, the team cultured both species in a synthetic aquaculture wastewater formulated to mirror nutrient profiles reported from Malaysian fish farms, containing 3 milligrams per liter of ammonium, 2 milligrams per liter of nitrite and 2 milligrams per liter of phosphate. A standard algal growth medium, F/2, served as the control. Cultures began at an initial density of 1 million cells per milliliter and were maintained at 23 degrees Celsius under 60 micromoles per square meter per second of light on a 12-hour light-dark cycle. Over seven days the researchers tracked optical density, cell counts and specific growth rates, then measured total ammonia nitrogen removal and analyzed protein, carbohydrate and lipid content of the freeze-dried biomass using established colorimetric methods.

The growth results delivered a surprise. While Chlorella vulgaris achieved higher optical density, a measure of light-absorbing biomass in the culture, it was Nannochloropsis oculata that dominated in raw cell numbers. By day seven, N. oculata grown in wastewater had reached 17.6 million cells per milliliter, the highest density recorded in the entire experiment and significantly above the 13.3 million cells per milliliter it achieved in the nutrient-optimized F/2 control. Chlorella, by contrast, reached only about 2.1 million cells per milliliter in wastewater. The specific growth rate told the same story: N. oculata in wastewater posted 0.409 per day, the highest of all treatments, while C. vulgaris managed just 0.107 per day in the same medium. The researchers note that optical density alone can mislead, since cell size, morphology and extracellular materials affect light scattering, underscoring the value of combining measurements.

Nutrient removal was strong across the board. After seven days, C. vulgaris stripped 83.7 percent of total ammonia nitrogen from the wastewater, with 78.0 percent removal in F/2, while N. oculata achieved 71.3 percent in wastewater and 72.3 percent in F/2. Both figures comfortably exceed the 50 percent threshold generally considered effective for algal nutrient removal. The chemistry behind this efficiency favors ammonium: unlike nitrate, which must be reduced through energy-intensive enzymatic steps involving nitrate and nitrite reductase, ammonium enters algal cells through specific transporters and is incorporated directly into amino acids via the glutamine synthetase-glutamate synthase pathway. This preference explains why both species performed well in ammonium-dominated wastewater, and why the authors suggest that longer culture periods and higher initial inoculum densities could push removal even higher.

The nutritional analysis is where the study breaks new ground. N. oculata grown in wastewater produced biomass containing 46.3 percent protein and 40.9 percent carbohydrate by dry weight, with the carbohydrate figure the highest of any treatment. The protein level is particularly striking because it exceeds most previously reported values for microalgae cultivated in aquaculture wastewater. For comparison, other studies have reported protein contents of only 41 to 42 percent for Chlorella sorokiniana and Scenedesmus across 37 different wastewaters, 37.11 percent for co-cultivated Chlorella and Phaeodactylum, and a mere 17.93 percent for C. vulgaris in trout farm effluent. Since aquafeed formulations generally target protein above 30 percent and carbohydrates above 10 percent, the N. oculata biomass comfortably clears both bars.

The metabolic story behind these numbers is instructive. C. vulgaris grown in wastewater saw its protein drop to 35.9 percent from 53.0 percent in F/2, consistent with the known effect of nitrogen limitation, which forces algae to redirect carbon away from protein synthesis and into carbohydrate storage. N. oculata responded differently, apparently tolerating the wastewater conditions while still accumulating substantial protein and boosting carbohydrate reserves. Lipid content remained low in all treatments, between roughly 0.9 and 3.9 percent of dry weight, and contrary to the common expectation that nutrient stress triggers lipid accumulation, C. vulgaris actually accumulated more lipid in the nutrient-rich control medium. The authors attribute this to strain-specific characteristics and caution that the interplay between wastewater composition and lipid metabolism remains poorly understood.

The implications extend beyond the laboratory. If N. oculata can sustain high growth, remove over 71 percent of ammonia nitrogen within a week, and simultaneously produce feed-grade protein and carbohydrate, fish farms could in principle close their own loops: effluent nutrients feed the algae, the algal biomass feeds the fish, and the discharge load shrinks. Because microalgae also supply omega-3 and omega-6 fatty acids, carotenoids, vitamins and antioxidants, such biomass could improve growth, immunity and survival in farmed species while reducing dependence on wild-caught fishmeal, a mounting concern as marine fish stocks decline. The economics are equally compelling, since conventional nitrogen removal infrastructure is costly and the algae approach converts a waste stream into a saleable product.

The researchers are careful to note that their experiment used synthetic wastewater under controlled conditions, and that integrating algal cultures into real recirculating aquaculture systems will require further work on scale, salinity matching, contamination and harvesting. Still, the study provides a clear proof of concept with a species-specific recommendation: Nannochloropsis oculata stands out as the leading candidate for wastewater-based aquafeed production, while Chlorella vulgaris remains a strong option where maximum ammonia removal is the priority. As the aquaculture industry faces tightening environmental regulations and rising feed costs, the idea of microscopic algae quietly converting pollution into protein may prove one of the most practical sustainability tools the sector has seen.

Subject of Research: Cultivation of microalgae in aquaculture wastewater for simultaneous nutrient removal and aquafeed biomass production

Article Title: Potential of culturing microalgae Chlorella vulgaris and Nannochloropsis oculata with aquaculture wastewater for simultaneous aquafeed production and wastewater remediation

Article References: Song, W. R. L., Keong, Y. S., Yusoff, F. M., Ping, T. J., & Rahman, N. A. (2024). Potential of culturing microalgae Chlorella vulgaris and Nannochloropsis oculata with aquaculture wastewater for simultaneous aquafeed production and wastewater remediation. Blue Biotechnology, 1(1), Article 19. https://doi.org/10.1186/s44315-024-00020-8

Image Credits: AI Generated

DOI: 10.1186/s44315-024-00020-8

Keywords: microalgae, aquaculture, wastewater remediation, Chlorella vulgaris, Nannochloropsis oculata, aquafeed, ammonia nitrogen removal, protein content, eutrophication, sustainable aquaculture, biomass, nutrient recycling

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