Geology shifts gut microbial community composition in wild Mountainsnails (genus: Oreohelix)

Background. Species which require soil mineral macronutrients for survival may depend on soil microbiome communities to aid in nutrient processing. Land snails, which utilize environmental minerals to synthesize a shell of calcium carbonate (CaCO3), may rely on or possess distinct gut microbiome communities depending on soil mineral characteristics. Here, we investigate whether the occurrence of calcareous vs. non-calcareous soils shifts the composition of the gut microbiome of species from the calciphilous and highly diverse land snail genus Oreohelix from the Western United States.Methods. We collected snail and soil samples from nine sites in central Idaho, five near calcium-rich geology, and four that were not. We sequenced the V4 region of the 16S rRNA gene to assess the gut microbiome compositions of Oreohelix land snails on and off calcium-rich substrates. After data clean-up and filtering we had 68 snail and 25 soil samples.Results. We found that snail gut microbiome differed greatly from the surface soil microbiome, with many amplicon sequence variants being unique and ubiquitous in the snails. We also found small, but significant, differences between snails on and off calcium-rich rocks. Our findings indicate that the gut microbial community assembly process of land snails is complex and does not reflect a simple relationship with the underlying soil microbiome. While we find a pattern of differences based on calcium-rich geology, the microbiomes of these snails are forming based on a variety of other factors, likely including diet and host filtering. Furthermore, we found multiple taxa that were ubiquitous in the snails and rare in the substrate microbiomes. Future work should focus on disentangling the role of habitat and the functional importance (or lack thereof) of the microbial taxa that are common to almost every sampled snail.