Chronic exposure to environmental concentrations of harmful algal bloom-forming dinoflagellates induces oxidative stress and reduces immune and hepatic functions in red seabream.

Harmful algal blooms (HABs) caused by dinoflagellates can be detrimental to aquaculture and fisheries. However, little is known regarding their ichthyotoxic effects on fish, particularly after chronic exposure to sublethal levels. In this study, significant modulations in physiology, immunity, antioxidant components, and hepatic indicators owing to non-toxin-producing dinoflagellate strains (Alexandrium affine and Cochlodinium polykrikoides) were analyzed in juvenile red seabream, Pagrus major, exposed to sublethal concentrations (0, 1, and 100 cells mL) for 60 days. At 60 days, higher mortality was induced by A. affine than by C. polykrikoides. Significant increases in respiration rate and plasma cortisol were observed in red seabream exposed to 100 cells mL of the two dinoflagellates. Intracellular reactive oxygen species and malondialdehyde levels were significantly elevated in the gill and liver tissues in response to 100 cells mL of either dinoflagellate. Immunity parameters such as alternative complement activity, lysozyme activity, and total immunoglobulin content were significantly decreased during exposure to 100 cells mL of the two dinoflagellates. Although no significant change was observed in the gonadosomatic index, the hepatosomatic index was significantly decreased by exposure to 100 cells mL of the two dinoflagellates on day 60. The significant decrease in enzymatic activities of ethoxyresorufin-O-deethylase, alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase upon exposure to 100 cells mL of either dinoflagellate suggested impaired hepatic function through prolonged exposure. Our results suggest that consistent exposure to sublethal concentrations of HAB-forming dinoflagellates is detrimental to fish physiology and biochemical defenses.