Changes in the microbiome and associated host tissue structure in the blue mussel (<i>Mytilus edulis</i>) following exposure to polystyrene microparticles

Marine life is increasingly exposed to microplastics, which can be ingested and disrupt the relationship between host tissues and their microbiomes. We investigated the effects of microplastics (5 µm polystyrene beads) on the microbial community and host tissue structure in organs at high risk of exposure (digestive gland and gills) in blue mussels ( Mytilus edulis Linnaeus, 1758). We exposed mussels to concentrations of microplastic consistent with levels found in local coastal waters. High exposures (1000 particles per m 3 per mussel) decreased the alpha and beta diversity in the microbiome of the digestive gland, with an increase in relative abundance of Polaribacter and a decrease in other species in the Flavobacteriaceae. Both low (10 particles per m 3 per mussel) and high exposures to polystyrene also changed tissue structure in the hosts, with an increase in immune cells (hemocytes) and reactive lysosomes in the gills, and in the digestive gland, a loss of cell specialization in digestive cells and an increase in cell breakdown products. Thus, exposure to particles of polystyrene in concentrations consistent with levels detected in local coastal zones reduces microbial biodiversity of the digestive gland and disrupts host tissues, which may indicate a loss of the host–symbiont interactions that support tissue homeostasis.