Microbiome diversity relates to host species diversity in aquatic filer-feeders (Unionidae)

Freshwater ecosystems are increasingly threatened and facing the loss of aquatic animal biodiversity. One consequence of this loss of animal biodiversity is the loss of the bacterial communities (microbiomes) associated with those animals, as well as the loss of ecosystem services that the animals or their microbiomes perform. Freshwater mussels (Unionidae) are among the most threatened aquatic animal groups and commonly occur in mixed species, densely populated assemblages. As filter-feeders, the gut microbiome of freshwater mussels is functionally important within their ecosystem, although the composition, functions, and drivers of the mussel gut microbiome are still not well described. Sampling 810 individual freshwater mussels, representing >30 species, we explored the links between host species diversity and density, and the diversity and structure of the gut microbiome in a hotspot of mussel biodiversity, the southeastern United States. We demonstrate that the gut microbiome of freshwater mussels is shaped by abiotic (dissolved oxygen, phosphate, temperature, pH) and biotic (mussel species richness, density) conditions. Higher mussel biodiversity (species richness) was correlated with greater dissimilarity between gut microbiomes, and to a lesser extent, with greater phylogenetic diversity in the gut microbiome of individual mussels. The consequence is that sites with more mussel hosts and/or more mussel species should be expected to host a greater microbial diversity associated with the mussel community.