The multiple responses of Mytilus galloprovincialis in the multi-stressor scenario: Impacts of low pH, low dissolved oxygen, and microplastics

Along with high temperatures, acidification, deoxygenation, and microplastics (MPs) pollution represent key drivers in coastal marine ecosystems. Sessile invertebrates living in coastal habitats are primarily exposed to the combination of these drivers, often at severe levels. Here, we investigated the individual and combined impacts of low pH (pH: 7.35), low dissolved oxygen (DO) (1.91 mg L), and MP (26 μg L) in the Mediterranean mussel Mytilus galloprovincialis by measuring organismal and cellular parameters after a 15-day exposure period. Organismal parameters (respiration rate, ammonia excretion rate, absorption efficiency) as well as electron transport system (ETS) activity were not impacted by the stressors, either individually or combined. At the cellular level, however, we observed significant effects of these stressors individually and interactively on the hemocyte count, hemocyte viability, genotoxicity (comet assay), and gill metabolite profiles. In addition, we observed partial effects on the hemolymph phagocytic activity (PA) and granulocyte/hyalinocyte (G/H) ratio, and mantle lipid peroxidation (LPO). Metabolomics results manifested that the gill of mussels can serve as a valuable indicator of metabolic status under the stress of low pH, low DO and MP. Metabolites involved in osmoregulation, membrane stability, oxidative stress, energy, amino acid and nitrogen metabolism were significantly affected by the stressors, with low DO being the main driver of metabolic changes. We suggest that the individual and variable interactions of these stressors negatively impact M. galloprovincialis, except for the organismal and, to some extent, biochemical parameters.