Salinity-dependent effects of ZnO nanoparticles on bioenergetics and intermediate metabolite homeostasis in a euryhaline marine bivalve, Mytilus edulis

Highlights: • Combined effects of nZnO and salinity stress have been investigated in mussels. • Exposure to nZnO had weak effects on energy or intermediate metabolite homeostasis. • Zn²⁺ was metabolically more damaging to the Baltic Sea mussels than nZnO. • Fluctuating salinity (5–15) was bioenergetically less stressful than low salinity (5). • The biological effects of nZnO and Zn²⁺ became less detectable under salinity stress. Engineered nanoparticles including ZnO nanoparticles (nZnO) are important emerging pollutants in aquatic ecosystems creating potential risks to coastal ecosystems and associated biota. The toxicity of nanoparticles and its interaction with the important environmental stressors (such as salinity variation) are not well understood in coastal organisms and require further investigation. Here, we examined the interactive effects of 100 μg l⁻¹ nZnO or dissolved Zn (as a positive control for Zn²⁺ release) and salinity (normal 15, low 5, and fluctuating 5–15) on bioenergetics and intermediate metabolite homeostasis of a keystone marine bivalve, the blue mussel Mytilus edulis from the Baltic Sea. nZnO exposures did not lead to strong disturbances in energy or intermediate metabolite homeostasis regardless of the salinity regime. Dissolved Zn exposures suppressed the mitochondrial ATP synthesis capacity and coupling as well as anaerobic metabolism and modified the free amino acid profiles in the mussels indicating that dissolved Zn is metabolically more damaging than nZnO. The environmental salinity regime strongly affected metabolic homeostasis and altered physiological and biochemical responses to nZnO or dissolved Zn in the mussels. Exposure to low (5) or fluctuating (5–15) salinity affected the physiological condition, energy metabolism and homeostasis, as well as amino acid metabolism in M. edulis. Generally, fluctuating salinity (5–15) appeared bioenergetically less stressful than constantly hypoosmotic stress (salinity 5) in M. edulis indicating that even short (24 h) periods of recovery might be sufficient to restore the metabolic homeostasis in this euryhaline species. Notably, the biological effects of nZnO and dissolved Zn became progressively less detectable as the salinity stress increased. These findings demonstrate that habitat salinity must be considered in the biomarker-based assessment of the toxic effects of nanopollutants on coastal organisms.