Coupling microalgal bioremediation of recirculating aquaculture effluents with photobiological hydrogen production

Recirculating aquaculture system (RAS) effluents contain substantial nitrate and phosphate loads, posing a significant eutrophication risk to aquatic ecosystems if left untreated. While microalgal bioremediation is a promising strategy, expanding opportunities for valorisation is crucial for its successful implementation. This study couples the cultivation of nitrate-utilizing Chlamydomonas reinhardtii strain CC-1690 in RAS effluent with photobiological hydrogen (H 2 ) production to achieve nutrient removal, bioenergy production, and high-quality biomass. Within 70 h, CC-1690 achieved near-complete depletion of nitrate and phosphate from the effluent. Subsequently, H 2 production was induced via carbon limitation and anoxia, yielding a cumulative 49 µmol H 2 mg −1 Chl over 72 h. Although decreased chlorophyll content and F v /F m indicated physiological stress, key photosynthetic subunits (PsbA, PSBO, CP47, PetB and PsaA) remained largely stable. Importantly, the process preserved biomass quality; total lipid content increased slightly, enriched in palmitic (16:0) and α-linolenic (18:3ω-3) fatty acids while protein content was unaffected. These results demonstrate that integrating H 2 production with RAS effluent remediation offers a robust circular economy approach, valorising wasted nutrients into bioenergy and high-quality biomass with potential downstream applications.