Circular valorization of acid-hydrolyzed fishery wastewater: integrated remediation and enhanced C-phycocyanin production by Spirulina sp.

The valorization of fishery-derived wastes through microalgal biotechnology provides a practical route to integrate wastewater remediation with high-value bioproduct generation. This study demonstrates an integrated process in which solid fish waste was converted into an algal growth medium via sulfuric acid hydrolysis (3% H2SO4; 1:8, w/v) combined with autoclave-assisted pretreatment (121 °C, 20 min), and subsequently utilized for the mixotrophic cultivation of Spirulina sp. in bench-scale photobioreactors. The resulting hydrolysate contained a chemical oxygen demand (COD) of 2,897 ± 28 mg L-1, NO3−–N of 284.89 ± 11.04 mg L-1, and PO43-–P of 130.23 ± 0.47 mg L-1 at an adjusted pH of 9.10 ± 0.05. Process optimization identified a 75% (v/v) hydrolysate concentration and an irradiance of 180 μmol photons m-2 s-1 as the most effective operating condition. Under this optimized regime, the culture reached a maximum biomass concentration of 2.10 ± 0.03 g L-1, representing an 11.1-fold increase relative to the autotrophic BG-11 control. Simultaneously, robust nutrient polishing was achieved, with NO3−–N and PO43-–P removal efficiencies of 96.6% and 93.2%, respectively. Concurrently, pigment synthesis was significantly enhanced, delivering an intracellular C-phycocyanin content of 66.00 ± 0.85 mg g-1 DW (a 14.7-fold increase over the control) and a volumetric productivity of 10.66 ± 0.20 mg L-1 d-1. Lipid accumulation also increased in the hydrolysate-grown biomass (12.25%) versus the control (5.87%). Overall, the proposed circular bioprocess confirms that acid-hydrolyzed fishery waste serves as a highly effective substrate for simultaneous bioremediation and high-yield C-phycocyanin production, establishing a scalable resource recovery strategy for the fishery industry.