Valorization of nutrients and CO2 from a recirculating aquaculture system into high-value astaxanthin: A circular bioeconomy approach with Haematococcus pluvialis

Upcycling CO2-rich exhaust air and nutrient-rich effluent water from recirculating aquaculture systems (RAS) into high-value bioproducts can enhance aquaculture sustainability and promote a circular bioeconomy. This study investigated the effects of RAS-derived CO2 supplementation on the growth and astaxanthin production of the microalga Haematococcus pluvialis cultivated in RAS water. Cultures were grown in 1.5-L photobioreactors and aerated with either ambient room air (~ 400 ppm CO2) or RAS exhaust air (RAS CO2; 1900–2500 ppm) at ~ 18 °C, representative of Nordic RAS conditions. During the stationary phase, room-air cultures were subdivided and aerated with either room air or RAS CO2, while RAS-CO2 cultures continued under the same aeration. All cultures were then subjected to high light (200–400 µmol photons m−2s−1) to induce astaxanthin synthesis. Compared with room air, RAS CO2 increased H. pluvialis growth rate by 15%, cell density by 57%, and dry weight (DW) by 76%. The removal of total N, NO3-N, and PO4-P exceeded 95% in all treatments. Regardless of initial growth conditions, RAS CO2 during the stress phase increased astaxanthin content by 17–19%, reaching up to 1.12% of dry weight, and increased astaxanthin concentration by 92–119%, reaching up to 23.26 mg L−1, relative to room air. These findings indicate that CO2 stripped directly from a RAS is sufficient to promote both growth and astaxanthin production in H. pluvialis relative to ambient room air. Overall, this study demonstrates RAS exhaust CO2 as a viable carbon source for high-value astaxanthin production under realistic aquaculture operating conditions.