Geology correlates with gut microbial community composition in the Mountainsnails (Oreohelicidae: Oreohelix).

Background: Species that require soil mineral macronutrients for survival may depend on specific 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 is associated with shifts the composition of the gut microbiome of 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, and four away from calcium-rich geology. We sequenced the V4 region of the 16S rRNA gene of these samples 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 microbiome samples.

Results: We found that snail gut microbiomes differed significantly 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 associated with the proximity of calcium-rich geology, the snail microbiome communities are likely forming based on a variety of other factors, including diet and host filtering. Furthermore, we found multiple microbial taxa that were ubiquitous in the snails and rare in the nearby 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.