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

The study, led by Dishti Dabeedass of the University of Mauritius together with colleagues at Hokkaido University in Japan and Jeju National University in the Republic of Korea, focused on the floating-cage facility at Pointe-aux-Feuilles, operational since 2002 and producing roughly 3,000 tons of fish annually. Fish farming releases feed residues and metabolic waste into the surrounding water, creating a nutrient-enriched micro-ecosystem in the lagoon. These conditions frequently trigger substantial proliferation of opportunistic macroalgae that cling naturally to aquaculture cages, ropes and nets. Rather than treating this growth as a nuisance, the research team asked a different question: what are these seaweeds made of, and could they be worth something?

To find out, the divers hand-collected approximately one kilogram of fresh biomass from each of seven seaweed taxa at depths of no more than two metres, carefully picking from multiple individuals to ensure representative sampling while minimising environmental disturbance. The samples, spanning green, brown and red macroalgal functional groups, were sealed in sterile bags, transported on ice and stored at minus 80 degrees Celsius to preserve their biochemical integrity. Because many tropical macroalgal genera are notoriously plastic in form and include cryptic species that defy visual identification, the team went beyond morphology, extracting genomic DNA and sequencing chloroplast and mitochondrial marker genes, including tufA, psbA, cox3 and rbcL, confirming species identities through nucleotide BLAST searches and depositing the sequences in GenBank.

The molecular work resolved the seven colonisers into a taxonomically diverse line-up: the green seaweeds Ulva torta in its tubular form, Ulva lactuca in its blade form and Caulerpa racemosa; the brown alga Padina sanctae-crucis lineage number two; and the red seaweeds Dasya corymbifera, Hypnea cornuta lineage number three and Gracilaria rangiferina. Each pooled sample was then subjected to a battery of validated Association of Official Analytical Chemists protocols and spectrophotometric assays, quantifying ascorbic acid, total phenolics, total flavonoids, beta-carotene, chlorophyll a and monomeric anthocyanins, with every measurement performed in triplicate.

The headline result came from the green seaweeds. Ulva torta exhibited the highest ascorbic acid concentration at 640 plus or minus 25.6 milligrams per kilogram of fresh weight, with Ulva lactuca and Hypnea cornuta close behind. Green macroalgae, with their chlorophyll-dominated photosynthetic machinery and rapid growth dynamics, are physiologically primed for ascorbate biosynthesis, which serves photoprotection and redox regulation. The authors note that these vitamin C levels are nutritionally significant and comparable to values reported in selected terrestrial plant sources, although direct comparisons warrant caution because of methodological differences. What makes the finding striking is that the biomass was growing passively within a fish farm, without targeted fertilisation or controlled cultivation, demonstrating that nutritionally valuable seaweed can occur naturally within lagoonal environments suited to Integrated Multi-Trophic Aquaculture, or IMTA.

The brown and red algae told a different chemical story. Padina sanctae-crucis dominated the secondary-metabolite league tables, posting the highest total phenolic content at 22.25 plus or minus 5.74 milligrams per kilogram fresh weight and the highest flavonoid content at 398.8 plus or minus 14.6 milligrams per kilogram. Brown algae synthesise phlorotannins, a unique class of polyphenols produced via the acetate-malonate pathway and absent from other algal lineages, compounds that serve antioxidant defence, ultraviolet screening and anti-herbivore protection in shallow, high-irradiance lagoons. The red seaweeds, meanwhile, excelled at carotenoid and pigment chemistry: Dasya corymbifera recorded the highest beta-carotene concentration at 28.18 plus or minus 2.22 milligrams per kilogram fresh weight and the highest chlorophyll a content at 24.00 plus or minus 0.05 milligrams per kilogram, while Dasya corymbifera and Gracilaria rangiferina were the only species with detectable monomeric anthocyanins, at 0.301 and 0.167 milligrams per kilogram respectively.

Statistical analysis confirmed that these differences were far from random. Analysis of variance revealed highly significant species effects across every antioxidant and pigment parameter measured, with effect sizes approaching the theoretical maximum; for chlorophyll a and anthocyanins, species identity explained essentially all of the observed variation. The authors stress an important interpretive caveat, however. Each species was represented by a single composite sample pooled from multiple individuals, so the triplicate assays reflect analytical precision rather than natural within-species variability, and the statistical comparisons describe differences among composite samples rather than estimates of biological variation across populations.

The team is equally candid about the environmental dimension of the work. Although fish farms are commonly associated with localised nutrient enrichment, dissolved nutrient concentrations were not measured during sampling, and no reference populations from non-aquaculture sites were included. The study therefore cannot establish a direct causal link between aquaculture-derived nutrients and the observed antioxidant profiles. Seaweed biochemistry is known to be strongly governed by nutrient availability: under nutrient-replete conditions, macroalgae tend to prioritise growth and accumulate proteins, chlorophylls and water-soluble vitamins such as ascorbic acid, whereas nutrient limitation shifts metabolism toward carbon-rich secondary metabolites like phenolics and flavonoids. Disentangling these nutrient-driven responses will require seasonal sampling, water-quality monitoring and comparative studies across aquaculture and non-aquaculture habitats, priorities the authors lay out for future research.

Even with those caveats, the implications for Mauritius and similar small island developing states are considerable. The global seaweed sector is worth more than eight billion US dollars annually, dominated by Asian producers, yet Mauritius, despite documenting 435 seaweed species and commanding sovereign rights over an Exclusive Economic Zone of nearly 1.9 million square kilometres, harvests seaweed only at limited artisanal scale. In an IMTA framework, seaweeds act as extractive organisms that assimilate dissolved nutrients and improve water quality while generating marketable biomass, and the new biochemical profiles provide a baseline for choosing which species to pair with finfish. Fast-growing green species such as Ulva could supply vitamin C-rich functional food ingredients, brown species like Padina could feed nutraceutical and cosmeceutical pipelines with phlorotannin-rich extracts, and red species such as Dasya corymbifera could serve as marine sources of carotenoids prized for their stability and bioactivity. The research aligns with Sustainable Development Goals 12 and 14 on responsible consumption and marine stewardship, and the authors argue that further work on seasonal variability, nutrient flux dynamics and bioavailability will be needed to translate baseline chemistry into commercial opportunity. For now, the message from the Mauritian lagoon is clear: the structures built to farm fish are also quietly growing some of the ocean’s most biochemically interesting food, and nobody is harvesting it.

Subject of Research: Antioxidant profiles of edible seaweeds colonising lagoonal aquaculture structures in Mauritius

Article Title: Lagoonal aquaculture structures support antioxidant rich edible seaweeds with potential for integrated multitrophic aquaculture in Mauritius

Article References: Dabeedass, D., Akita, S., Vieira, C., Tojo, N., Yao, F., & Nazurally, N. (2026). Lagoonal aquaculture structures support antioxidant rich edible seaweeds with potential for integrated multitrophic aquaculture in Mauritius. Discover Oceans, 3(1), Article 45. https://doi.org/10.1007/s44289-026-00157-x

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00157-x

Keywords: seaweed, aquaculture, Mauritius, antioxidants, vitamin C, IMTA, phlorotannins, beta-carotene, Ulva, blue economy, lagoon, nutraceuticals