Synergistic impacts of nanopollutants (nZnO) and hypoxia on bioenergetics and metabolic homeostasis in a marine bivalve Mytilus edulis

Coastal ecosystems face increasing threats from anthropogenic pollution and environmental stressors like hypoxia and nanoparticle exposure. The Baltic Sea exemplifies these challenges due to nutrient pollution and hypoxia. We investigated the combined effects of zinc oxide nanoparticles (nZnO), which possess unique properties such as high reactivity and bioavailability, and hypoxia on bioenergetics and metabolite homeostasis of the blue mussel Mytilus edulis from the Baltic Sea. Mussels were first exposed to environmentally relevant concentrations of nZnO (100 µg Zn L-1) and subsequently subjected to short-term (24 h) or long-term (7 d) hypoxia (<0.1% air saturation) followed by recovery periods (1 h and 24 h). Our findings reveal complex effects of nZnO on mussel metabolism under normoxic and hypoxic conditions. Under normoxic conditions, nZnO alters mussel metabolism without causing energy deficit. Prolonged severe hypoxia induces anaerobic metabolism and glycogen depletion. Under hypoxic conditions, nZnO disrupts mussels' metabolic response to anaerobic conditions, threatening their anaerobic survival capacity. Control mussels swiftly recover metabolic homeostasis upon reoxygenation, whereas nZnO-exposed mussels show delayed recovery, with ongoing energy disturbances. Overall, these findings underscore the metabolic impacts of nZnO and hypoxia in keystone marine mussels and emphasize the importance of considering oxygen levels in assessments of nanoparticle toxicity in coastal ecosystems.