Reusing bioreactor effluent as a circular bioresource strategy for cultivating marine microalgae and producing Omega-3 polyunsaturated fatty acids.

The rapid growth of the aquaculture industry underscores the need for sustainable alternatives for traditional fishmeal. We investigated the potential of reusing treated effluent as a circular bioresource strategy. Effluent from two bioreactors producing single-cell protein (SCP), fed with soybean-processing wastewater, was used to cultivate omega-3 polyunsaturated fatty acid (PUFA)-producing marine microalgae. Marine microalgae strains were cultivated in marine growth medium amended with varying percentages of autoclaved bioreactor effluent (BE). The effects of lowered salinity, due to the addition of effluent, and temperature on chlorophyll a, a proxy for biomass, and omega-3 PUFAs content were assessed. We found that Dunaliella tertiolecta and Nannochloropsis oculata can grow in marine medium containing 85 % autoclaved BE (corresponding to 5.3 ppt salinity); Tisochrysis lutea grows in marine medium containing 50 % BE (15.3 ppt salinity); and a Singaporean strain of Nannochloropsis oceanica grows in 10 % BE (25.5 ppt salinity). The PUFAs alpha-linolenic acid (ALA), docosahexaenoic acid (DHA), and/or eicosapentaenoic acid (EPA) were also synthesized by the strains when grown in their respective optimized media formulations. Lower growth temperatures (15 °C and/or 18 °C) led to higher production of omega-3 PUFAs in all four strains. This proof-of-concept study demonstrates that some microalgal strains accumulate DHA, EPA and/or ALA when grown in medium containing autoclaved effluent from an SCP-producing bioreactor and may potentially be used as a supplement to aquafeed. This approach promotes resource efficiency, supports a circular bioeconomy, and integrates algal biotechnology with microbial protein systems for sustainable feed production and wastewater reuse.