Biochemical responses of the solitary tunicate Ciona robusta to vanadium exposure: is vanadium tolerance a biochemical superpower of sessile marine invertebrates?

In seawater, vanadium occurs naturally as a trace metal at dissolved concentrations of 30-35 nM, yet increasing anthropogenic inputs - including fossil fuel combustion and industrial discharges - are elevating its environmental burden in coastal and transitional ecosystems. Despite its ecotoxicological relevance, vanadium remains a neglected contaminant compared to other metals of concern. Sessile marine invertebrates face disproportionate exposure risks due to their inability to escape contaminated habitats; paradoxically, certain taxa - most notably tunicates - exhibit extraordinary vanadium bioaccumulation capacity, up to 10,000-fold above ambient levels, implying the evolution of highly effective tolerance mechanisms. This study investigated the ecotoxicological effects of sodium orthovanadate on the solitary tunicate Ciona robusta, a cosmopolitan and ecologically relevant species, following 7- and 14-day exposures to environmentally relevant (100 ug L-1) and challenge (1000 ug L-1) concentrations. A multivariate biomarker approach was applied, targeting inflammation-related enzymes (phenoloxidase, arylsulfatase, acid and alkaline phosphatase), antioxidant and detoxification responses (catalase, superoxide dismutase, glutathione S-transferase), neurotoxicity (acetylcholinesterase), and oxidative damage endpoints (protein carbonylation, lipid peroxidation). Remarkably, most biomarkers remained unaffected across treatments, underscoring the capacity of C. robusta to withstand vanadium stress without eliciting broad physiological disruption. Alkaline phosphatase inhibition at the highest concentration after 7 days -consistent with vanadate interference with phosphate metabolism - was transient and fully recovered by day 14. A significant increase in superoxide dismutase activity at 14 days suggests a delayed but effective antioxidant mobilization. These findings highlight C. robusta as a valuable ecotoxicological model, not only for understanding vanadium dynamics in tunicates, but more broadly for investigating biochemical resilience strategies in sessile marine fauna exposed to metal contamination in coastal and transitional ecosystems.