Salinity preference of wetland microbial communities is phylogenetically clustered

Background/Question/Methods

Understanding if and how the evolutionary history of a species relates to its ecology is a fundamental question for biologists. With macroorganisms, phylogeny is often an ecologically meaningful way to classify organisms, as closely related taxa frequently have similar ecological characteristics. In contrast, phylogeny has long been considered to be an unreliable indicator of ecology for microorganisms because of high rates of horizontal gene transfer and rapid evolution. Challenging this historical assumption, several recent studies have demonstrated that phylogenetically-clustered taxa of microbes exhibit a substantial degree of ecological similarity, even at high levels of taxonomic organization. However, it is still unclear to what extent this phylogenetic conservation is driven by key environmental variables.

Salinity is a major driver of bacterial community composition across the globe, but it is not yet known whether these patterns reflect ecological coherence in the salinity preferences of phylogenetic groups.  We used a reciprocal transplant experiment to address this question for soil microbial communities from tidal wetlands, and simultaneously examined the communities’ functional response to salinity increases.  An understanding of how salinity regulates microbial communities is particularly needed in tidal wetland soils as climate change associated sea-level rise is predicted to cause widespread saltwater intrusion.

Results/Conclusions

The salinity of both the origin and host environments affected bacterial community composition (16S rRNA gene sequences) and activity (e.g., extracellular enzyme activity, and CO₂ and CH₄ production). Variation in bacterial community composition was highly correlated with variation in activity, suggesting the effect of environment on function could be mediated, at least in part, by the microbial community composition. Phylotypes were categorized as exhibiting a preference for freshwater, saltwater, or having no salinity preference by comparing presence/absence patterns across the treatments. Using phylogenetic analyses, a significant influence of evolutionary history was seen in all preference categories. This phylogenetic signature was corroborated by differences in the salinity preferences of high-level taxonomic groups. For instance, the majority of alpha-proteobacteria and gamma-proteobacteria phylotypes preferred saltwater, while the phylotypes of beta-proteobacteria were more likely to prefer freshwater. Overall, our results indicate that salinity’s influence on bacterial community composition results from phylogenetically-clustered salinity preferences. This research provides evidence for ecological coherence with regard to the salinity preferences of bacterial phylogenetic groups, and suggests that global patterns in microbial biodiversity arise from divergent evolution deeply rooted in the phylogenetic tree.