Antioxidant and immune responses in bivalves under toxicant stress: insights for biomedical and environmental research

Antioxidant defenses are widely used in bivalves as biomarkers of environmental pollution, however, the control mechanisms of such responses for this group areunderstudied. We were the first to reveal the activation of the Nrf2-Keap1 pathway in a marine invertebrate (oyster Crassostrea gigas) by classic Nrf2 inducers, such ashydroquinones and curcuminoids. A clear induction of GSH-related antioxidant defenses was observed, which was confirmed by gene expression, enzymatic activity, andprotein content, supporting the idea of a conserved and functional Nrf2/Keap1 pathway in bivalves.Because several classes of environmental pollutants are frequently reported as modulators of redox responses, we next focused on understanding the consequences of such modulations for oyster immune health. We exposed oysters to specific antioxidant system disruptors (CDNB and BSO) and found that oyster immune cell (hemocyte) function under control conditions was not sensitive to antioxidant inhibition, however partial GSH depletion altered oyster susceptibility to bacterial challenges.Our previous studies demonstrate the importance of refining the basic understanding of bivalve redox-related mechanisms for a better application of bivalves inenvironmental research. Now, we are focusing on the role of physiological oxygen levels on oyster immune cell function.We performed real-time oxygen level measurements in the blood (hemolymph) of live oysters which revealed levels ranging from ~10 to 0 %. Anoxic conditions were reachedvery rapidly, within about 5-7 min after the oyster shells were completely closed. These preliminary results reveal that oyster cells are very tolerant to extreme and rapidphysiological oxygen variations, demonstrating their potential as a novel biological model to study redox mechanisms supporting elevated oxygen stress tolerance.Altogether, our results provide important insights for environmental, aquaculture and biomedical research