Microplastic and an associated metal contaminant (Palladium) impair the immune response against pathogenic bacteria of the marine bivalve Mytilus galloprovincialis

Abstract

Microplastics (MPs) are widely distributed in marine environments and have been reported to cause harmful physiological effects in marine bivalves including immune modulation. While Mytilus galloprovincialis is a model species in environmental monitoring studies, little is known regarding the effects of MPs and palladium (Pd), an emerging contaminant, metal on the immune functioning of this species. In this study, gills and gland samples of specimens of M.ytilus galloprovincialis which were exposed to the same particle concentration (2.5 x 10 66 μm3μm /mL) of Control (microalgae), MP (virgin microplastics) and MP-Pd (Pd spiked microplastics) once an hour during 4 hours. Thus, mussels were exposed to 6095 ng of Pd/mussel. Samples analyzed were collected after 4 and 24 hours of exposure and after 24, 48 and 144 hours of depuration. Several innate immune activities (lysozymes, peroxidase, protease, antiprotease, and bactericidal activities) were analyzed. Our data demonstrated that after 24 hours of exposure to MP, a decrease in lysozymes and peroxidase activities occurred, but not in the bactericidal activity which increased. However, when MP-Pd was used, an increase in all these activities was observed compared to MP levels group. These data suggest that when MP are spiked with Pd, the latter compound might induce an inflammatory process that will results in higher levels of most of all the immune activities analyzed. In the gland, however, most of the activity levels were decreased upon MP-Pd treatment compared to MP levels at different time point of exposure depending on the activity. Interestingly, after 24 hours of removing the pollutants from the water, most of the activity levels in both tissues, gills and gland, were recovered to control levels, but not the bactericidal activity. Considering this impairment of the bactericidal activity against possible pathogenic bacteria, a potential threat to mussels population in a polluted scenario is highly plausible