Integrated Cultivation of Thalassiosira sp. Using Nitrified Recirculating Aquaculture System Effluent: Nutrient Recovery, CO 2 Fixation, and Fucoxanthin-Rich Biomass Production

The integration of diatom cultivation with aquaculture systems offers a promising strategy to simultaneously address nutrient-rich effluent discharge and the high costs of synthetic media. This study evaluates the growth performance, nutrient removal, CO 2 fixation, and fucoxanthin production of the marine diatom Thalassiosira sp. cultivated in three media: nitrified effluent from a recirculating aquaculture system (RAS; denoted as Aqua), synthetic F/2 medium, and a mixed medium (F/2 + Aqua, 1:1 v / v ). The mixed medium demonstrated the best overall performance, indicating a synergistic effect between aquaculture-derived nutrients and targeted supplementation. After 8 days, biomass concentration reached 655 mg L −1 , representing a 30% and 317% increase compared with F/2 and Aqua, respectively, with a CO 2 fixation rate of 152.89 mg CO 2 L −1 d −1 . This medium also achieved high nutrient removal efficiencies (93.67% nitrate and 97.94% phosphate) and enhanced fucoxanthin production (4.15 mg L −1 ). In addition, biomass contained essential fatty acids, including arachidonic acid (7.12% of total fatty acid (TFA)) and eicosapentaenoic acid (7.58% TFA), supporting its suitability for aquaculture. Importantly, partial substitution of synthetic nutrients with RAS effluent reduced medium-input costs by approximately 62% while maintaining high productivity. Overall, this study demonstrates a resource-efficient, cost-effective, and sustainable approach for integrating wastewater treatment with high-value diatom biomass production, supporting circular aquaculture systems.