COS 56-3
Dispersal limitation and environmental filtering of bacterial communities along a long-term glacial chronosequence
Soil bacteria are abundant, diverse, and of great functional importance; however, we have a nascent understanding of their biogeographic patterns and underlying causes. Here, we investigated the relative effects of dispersal limitation and environmental filtering on the phylogenetic structure of saprotrophic soil bacteria along a 4000-year glacial chronosequence in the Great Lakes region of North America. To disentangle the effects of environmental filtering from possible effects of dispersal limitation, we included 20 years of chemical and physical environmental monitoring in a distance-based linear model (DistLM). Among Bacteria, it is possible that dispersal limitation, short generation times and large population size lead to genetic divergence over time, inasmuch, dissimilarity between bacterial communities would be correlated with geographic distance (a proxy for time). Conversely, it is possible that nearly unlimited microbial dispersal may overwhelm forces of genetic drift, in which case, dissimilarity between bacterial communities may be correlated to environmental changes across sites. To investigate the relative contribution of dispersal limitation and environmental filtering on bacterial phylogenetic structure, we analyzed bacterial 16S rRNA gene composition from forest floor using Pacific Biosciences RSII high-throughput sequencing technology.
Results/Conclusions
Operational taxonomic units (OTUs) were selected at 95% sequence similarity to examine trends among taxonomically broad bacterial assemblages. Bacterial phylogenetic structure was different across sites (P < 0.01), as indicated by abundance-weighted UniFrac distance. A significant (P < 0.05) relationship between geographic distance (proxy for time) and bacterial phylogenetic dissimilarity suggests dispersal limitation acts to structure saprotrophic bacterial communities on a time scale of centuries to millennia. However, significant correlations between bacterial phylogenetic distance and biogeochemical and physical differences across sites, suggests that environmental filtering also acts as significant force in bacterial community assembly. In addition to geographic distance, soil pH, litter C:N, litter mass, and soil temperature and moisture accounted for significant portions of bacterial phylogenetic structure. In sequential DistLM model building, geographic distance (+15%) and environmental factors (+59%) accounted for significant and unique portions of phylogenetic dissimilarity across the chronosequence. Together, our study provides evidence that both dispersal limitation and habitat filtering act in unique ways to structure present-day bacterial communities.