The Biogeography of Life at its Limits: Regional Distributions of Extremely Halophilic Archaea

The development of novel, quantitative frameworks to delineate biogeographical regions has stimulated work in this classic area of ecology and biogeographical regionalisation has been demonstrated in a variety of conspicuous higher taxa. Microbial communities are known to differ in composition over regional-continental scales, however, whether these differences are consistent with the concept of biogeographic regionalisation has not been tested. Therefore, using high-throughput sequencing, we characterised halite-associated archaeal communities from 17 locations, spanning two continents and three, a priori defined, biogeographic regions. We tested for the presence of biogeographic clustering in these communities by clustering them into differing numbers of clusters, and evaluating the statistical support and spatial coherence of these clusters. We also examined whether the abundance of individual genera may be indicative of a community’s biogeographic context using a machine learning algorithm, random forest classification. When we clustered communities into a number of clusters comparable to that expected for higher organisms, we found very little statistical support, indicating a greater number of clusters may be present. However, when we grouped our communities into an increased number of clusters, these clusters lacked spatial coherence, providing evidence that such regions may not exist in extremophilic microbial communities. Additionally, we identified several genera whose presence and abundance in halite-associated communities was indicative of the biogeographic context of each community suggesting that individual taxa, rather than entire communities, may be more limited to specific regions.