Integrated Cultivation of Thalassiosira sp. Using Nitrified Recirculating Aquaculture System Effluent: Nutrient Recovery, CO2 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, CO2 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 CO2 fixation rate of 152.89 mg CO2 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.