First evidence of tolerance to vanadium in the solitary tunicate Ciona robusta: inflammatory and oxidative stress responses following sodium orthovanadate exposure

Vanadium is naturally present in seawater mainly as vanadate, with typical dissolved concentrations of approximately 30-35 nM (1.5-1.8 ug V L-1). Tunicates are known to bioaccumulate vanadium up to 10,000-fold compared with surrounding seawater. In recent decades, anthropogenic sources have progressively increased vanadium concentrations in marine environments, raising concerns about potential toxic effects on marine organisms. This study investigated the effects to sodium orthovanadate exposure on the solitary tunicate Ciona robusta by evaluating inflammatory- and oxidative stress-related biomarkers. Animals were exposed for 7 and 14 days to two nominal concentrations of sodium orthovanadate, 100 and 1000 ug L-1, corresponding to approximately 0.54 and 5.44 uM of dissolved V, respectively. The lower exposure concentration falls within the upper range reported for vanadium-contaminated seawater, whereas the higher one was selected as an experimental challenge condition. Indirect inflammatory-related biomarkers, including phenoloxidase, arylsulfatase, acid phosphatase, and alkaline phosphatase, were evaluated. Activity of antioxidant (catalase, superoxide dismutase) and detoxification (glutathione S-transferase) enzymes, as well as acetylcholinesterase activity and oxidative damage endpoints, including protein carbonyl content and lipid peroxidation, were also measured. Responses of most biomarkers in treated organisms did not differ significantly from controls, suggesting a remarkable capacity of C. robusta to withstand vanadium exposure without triggering broad inflammatory and oxidative or metabolic stress. Only alkaline phosphatase activity was inhibited at 1000 ug L-1 after 7 days, likely reflecting direct interference of vanadate with phosphate metabolism, though enzymatic activity recovered after 14 days. Superoxide dismutase activity increased significantly after 14 days at the highest concentration, suggesting activation of defense line against oxidative stress. Overall, these results support the hypothesis that C. robusta possesses considerable biochemical mechanisms that enhance its tolerance towards vanadium, highlighting its value as a biological system for better investigating the strategies that allow tunicates to tolerate and manage vanadium exposure.