Temporal variability and functional redundancy in the gut microbiomes of freshwater mussels (Unionidae)

Freshwater mussels perform essential ecosystem functions, yet we have no information on how their microbiomes may fluctuate over time. In this study we examined how the microbiome of six freshwater mussel species (Lampsilis ornata, Obovaria unicolor, Elliptio arca, Fusconaia cerina, Cyclonaias asperata and Tritogonia verrucosa) varied from 2016 to 2019 in the same river. We examined the taxonomic, phylogenetic and inferred functional (from 16S rRNA sequences) facets of microbiome diversity. Significant temporal differences were identified in five of the six species sampled between both years. However, not all species that exhibited a temporally variable microbiome were functionally distinct, indicating a high degree of functional redundancy within the mussel gut microbiome. Inferred biosynthesis pathways showed more temporal variation than pathways involved in degradation, while pathways involved in cellular metabolism were stable. There was no evidence for phylosymbiosis across any facet of microbiome biodiversity. These results indicate that temporal variation is an important factor in assembly of the gut microbiomes of freshwater mussels and provides further support that the mussel gut microbiome is involved in host digestion.