Valorization of Aquaculture Residues by Heterotrophic Cultivation of the Extremophilic Microalga Galdieria Sulphuraria for Protein- and Phycocyanin-Rich Biomass Production

Abstract This study evaluated a bioprocess using the thermo-acidophilic red microalga Galdieria sulphuraria to convert shrimp- and sludge-derived residues into biomass and pigments. Shrimp residues contained 57% (w/w) protein and 11% (w/w) lipids, whereas sludge contained 28–32% (w/w) protein and 13% (w/w) carbohydrates. Enzymatic and acid-assisted hydrolyses were tested for nutrient release. Free amino nitrogen (FAN) from sludge hydrolysates remained low (1–2 mg g − 1 ), while shrimp residues produced 10-15-fold higher FAN. In flask cultures, G. sulphuraria grew in 25–50% (v/v) sludge hydrolysate, reaching a maximum growth rate of 1.3 d − 1 at 50% (v/v) with 2.5 g L − 1 glucose. Higher sludge (≥ 75%, v/v) or shrimp hydrolysate (≥ 25%, v/v) inhibited growth. In fed-batch, blending 15% (v/v) sludge and 10% (v/v) shrimp hydrolysates with 10 g L − 1 glucose increased FAN and supported 1.2 d − 1 initial growth; phycocyanin reached 8 mg g − 1 . Feeding 70% sludge and 30% shrimp hydrolysate plus 22.5 g L − 1 glucose yielded 9–10 g L − 1 biomass with 47–62% (w/w) protein. Amino-acid screening showed strong specificity: ammonium sulfate supported 0.95 d − 1 , with alanine (0.87 d − 1 ) and proline (0.80 d − 1 ) as alternatives. These results define constraints and control points for decentralized residue-to-biomass conversion in a circular bioeconomy.