Valorisation of seafood wastewater using halotolerant purple bacteria with high antioxidant content as aquafeed

The rapid expansion of aquaculture has intensified the demand for fishmeal and fish oil, increasing pressure on marine ecosystems and generating large volumes of saline wastewater. Halotolerant purple phototrophic bacteria (PPB) offer a dual solution by enabling resource recovery from seafood processing effluents while producing high-value compounds for potential aquafeed applications. In this study, a mixed PPB culture was enriched from estuarine sediments and operated in a photobioreactor under saline conditions (30 g NaCl L⁻¹), achieving removal efficiencies up to 75.5% and 82.3% for nitrogen and phosphorus and 88.0% for organic carbon at an HRT of 4 days. Biomass contained on average 0.51 ± 0.05 g protein g^-1 VSS with essential amino acid profiles comparable to fishmeal. In a second step, three PPB species present in the mixed consortium, Rhodobacter capsulatus, Cereibacter sphaeroides, and Afifella marina, were evaluated in batch cultures to determine their antioxidant potential across a salinity gradient (1 to 70 g NaCl L⁻¹). Increasing salinity caused a clear decline in total antioxidant capacity; however, C. sphaeroides and A. marina maintained relatively stable pigment levels at moderate salinities. C. sphaeroides reached the highest coenzyme Q10 content (up to 6.14 mg g⁻¹ dry weight), although productivities decreased markedly above 35 g NaCl L⁻¹. Overall, the results indicate that halotolerant PPB can valorise saline aquaculture waste streams at salinities up to ∼35 g NaCl L⁻¹, producing biomass enriched in proteins, pigments, and antioxidants with potential for circular bioeconomy and sustainable aquafeed ingredient applications.