Red Seaweeds Turn Shrimp Farm Waste Into Clean Water in Just Days

Shrimp farming has become one of the world’s fastest-growing food industries, and India now ranks as the second-largest shrimp farming nation on the planet. But behind every plate of farmed shrimp lies a hidden cost: effluent water loaded with ammonium, nitrate, and phosphate that flows into coastal ecosystems, triggering algal blooms, oxygen depletion, and mass die-offs of marine life. Now, a team of researchers at CSIR-Central Salt and Marine Chemicals Research Institute in Bhavnagar, India, has shown that humble intertidal red seaweeds could be the answer, scrubbing these pollutants out of shrimp farm wastewater with remarkable efficiency, sometimes in a matter of hours.

The new study, published in the journal Blue Biotechnology, put five native red seaweed species to the test: Sarconema filiforme, Gracilaria corticata, Scinaia carnosa, Gracilaria dura, and Acanthophora sp. All were collected from the intertidal rocky shore at Veraval on Gujarat’s coast, while the effluent came from a shrimp farm at Mithi Virdi in Bhavnagar. The researchers cultivated the seaweeds in controlled laboratory tanks at 24 degrees Celsius with a 12:12 light-dark cycle and constant aeration, then transferred precisely weighed two-gram samples into flasks containing 400 milliliters of effluent water. Over the following days, they sampled the water at intervals ranging from fifteen minutes to 96 hours, measuring how quickly each species stripped nutrients from the polluted medium.

The analytical approach was rigorous. Phosphate concentrations were determined using the ascorbic acid method, with the resulting blue complex measured at 880 nanometers on a UV-Vis spectrophotometer. Nitrate was quantified through the vanadium chloride method, detected as a pink complex at 543 nanometers, while ammonium was estimated via the phenate method, which produces indophenol blue measured at 640 nanometers. Calibration curves with R-squared values above 0.99 were required for valid quantification, and all analyses were run in duplicate alongside blanks and standards to guard against contamination and instrument drift. This colorimetric toolkit allowed the team to track the disappearance of each nutrient with high precision across the entire time course.

The results were striking. Sarconema filiforme emerged as the overall champion, achieving the highest removal efficiencies across the board: roughly 85.77 percent of phosphate, 100 percent of nitrate, and 73.80 percent of ammonium within the experimental window. When uptake was normalized to biomass, S. filiforme again led the pack, absorbing nitrate at 1.18 micromoles per gram of wet weight per hour and ammonium at 2.16 micromoles per gram of wet weight per hour. Phosphate removal was also strong across all species, with Scinaia carnosa, Gracilaria corticata, and Gracilaria dura each removing more than 81 percent, while Acanthophora sp. trailed at about 63 percent but with high variability.

Nitrate proved to be the nutrient the seaweeds craved most. All five species achieved near-complete nitrate removal, and Acanthophora sp. accomplished something remarkable: complete nitrate uptake in just 48 hours, the fastest assimilation pattern recorded in the study. The overall preference followed a clear sequence of nitrate greater than phosphate greater than ammonium, a pattern the researchers attribute to red algae’s protein-rich pigments and high nitrogen quotas, which make them especially effective at assimilating nitrogen compounds. Ammonium, by contrast, was taken up more slowly, likely because high concentrations are toxic and most seaweeds preferentially use nitrate as their nitrogen source when both are available.

The physiological mechanisms behind this cleanup are well understood. Seaweeds absorb dissolved inorganic nutrients through a combination of passive diffusion, facilitated diffusion, and active transport via carrier proteins embedded in their cell membranes. Ammonium is the energetically cheapest nitrogen source, requiring no chemical reduction before incorporation into amino acids, while nitrate must be converted to nitrite and then ammonium inside the cell before it can be used to build proteins. Phosphate enters through dedicated transporters and fuels energy transfer via ATP, nucleic acid synthesis, and membrane formation. This dual passive-and-active uptake machinery allows seaweeds to keep functioning even when ambient nutrient concentrations are low, and to surge when nutrients are abundant, exactly the conditions found in aquaculture effluent.

But the study also delivered a sobering caveat: the best removers are not necessarily the best survivors. Despite its stellar uptake numbers, Sarconema filiforme showed signs of physiological stress under prolonged exposure and eventually died in the high-ammonium effluent. Acanthophora sp., the nitrate sprinter, likewise suffered high mortality after its rapid initial performance. High ammonium is known to suppress the light-saturated photosynthetic electron transport rate and maximum quantum yield in other seaweed species, inhibiting photosynthesis to the point of death, and the researchers believe a similar mechanism may have doomed their top performer. Only Gracilaria corticata remained viable after extended exposure, staying healthy even after 20 days in effluent water, by which point the water’s color had visibly faded, a sign that the seaweed had absorbed most of the dissolved nutrients.

This survival difference carries a crucial lesson for designing real-world treatment systems: nutrient removal efficiency alone is not enough. Longevity and tolerance under high nutrient loads matter just as much. The effluent itself was punishingly concentrated, containing 14.56 milligrams per liter of phosphate, 18.34 milligrams per liter of nitrate, 57.38 milligrams per liter of ammonium, and a salinity of 18.56 grams per liter. The authors therefore propose a sequential bioremediation strategy that deploys multiple species in stages, matching each seaweed’s uptake profile and environmental tolerance to the conditions it will face. Fast but fragile species like Acanthophora sp. could handle the initial nitrate pulse, while hardy generalists like Gracilaria corticata provide sustained, long-term polishing.

The broader context makes this work especially timely. The concept of Integrated Multi-Trophic Aquaculture, or IMTA, pairs fed species like shrimp with extractive species like seaweeds and filter-feeding invertebrates, so that the waste of one becomes the resource of another. Seaweeds in such systems do more than clean the water; they also yield valuable biomass that can be harvested for food, agar and carrageenan production, fertilizers, medicines, and biofuels, turning a pollution problem into a secondary crop. Gujarat’s coastline alone hosts around 285 seaweed species, and the researchers emphasize that using indigenous, locally adapted species avoids the ecological risks of introducing invasive aliens while ensuring compatibility with local temperature and salinity regimes.

For a coastal industry that generates both export income and employment across India, the prospect of a low-cost, nature-based solution to effluent pollution is compelling. The study demonstrates that five common intertidal red seaweeds can strip the three principal eutrophying nutrients from shrimp farm wastewater within 96 hours, with one species achieving complete nitrate removal in half that time. The challenge now is scaling from laboratory flasks to farm-scale treatment channels, where light, temperature, and nutrient loads fluctuate far more than they did under controlled conditions. If the sequential approach holds up in the field, shrimp farms of the future may grow their own cleanup crews, harvesting both protein and clean water from the same system.

Subject of Research: Nutrient uptake efficiency of intertidal red seaweeds for bioremediation of shrimp farm effluent

Article Title: Time-course nutrient uptake efficiency of intertidal red seaweeds in shrimp farm effluent: a sustainable bioremediation approach

Article References: Jaiswar, S., Raval, D., Patel, P., & Bhagiya, B. K. (2025). Time-course nutrient uptake efficiency of intertidal red seaweeds in shrimp farm effluent: a sustainable bioremediation approach. Blue Biotechnology, 2(1), Article 23. https://doi.org/10.1186/s44315-025-00044-8

Image Credits: AI Generated

DOI: 10.1186/s44315-025-00044-8

Keywords: bioremediation, seaweed, shrimp aquaculture, nutrient uptake, eutrophication, red algae, Gracilaria corticata, Sarconema filiforme, IMTA, coastal pollution, sustainable aquaculture, wastewater treatment