Phosphate perils in marine aquaculture: Systematic toxicity assessment in turbot juveniles in cultivation.

Recirculating Aquaculture Systems (RAS) aim to minimize water usage while maintaining optimal conditions for aquatic organisms, which is increasingly recognized as a crucial strategy for advancing sustainable aquaculture practices and enhancing global food security. However, many current marine RAS lack efficient treatments for removing dissolved wastes like phosphates (PO43--P), impacting water quality and potentially affecting the health of cultured organisms. Here, juveniles of turbot (Scophthalmus maximus), a promising and growing marine culture species, were exposed to PO43--P at different concentrations for 60 days to evaluate systematic responses and toxicity mechanisms. The juveniles adapted to 60 mg/L PO43--P by enhancing tissue repair activity in gill tissue via upregulation of Hedgehog and Wnt signaling pathways. Histological and molecular data indicated physiological damages and cell apoptosis in the gill tissue exposed to 120 mg/L PO43--P. Liver and spleen tissue damages were also observed in the 120 mg/L PO43--P group, and elevated levels of plasma alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase suggested potential impairments in both hepatic and immune functions. The oxidative stress responses, including upregulation of antioxidant genes and reduced enzyme activity, could be responsible for the damage of multiple tissues and systematic toxicity. These findings contribute new insights into the toxicity of PO43--P in fish and provide a theoretical basis for the necessity of PO43--P monitoring and removal technology research in marine aquaculture systems.