Global biogeography of Alnus-associated Frankia actinobacteria

Macroecological patterns of microbes have received relatively little attention until recently. This study aimed to disentangle the determinants of the global biogeographic community of Alnus-associated actinobacteria belonging to the Frankia alni complex. By determining a global sequence similarity threshold for the nitrogenase reductase (nifH) gene, we separated Frankia into operational taxonomic units (OTUs) and tested the relative effects of Alnus phylogeny, geographic relatedness, and climatic and edaphic variables on community composition at the global scale. Based on the optimal nifH gene sequence similarity threshold of 99.3%, we distinguished 43 Frankia OTUs from root systems of 22 Alnus species on four continents. Host phylogeny was the main determinant of Frankia OTU-based community composition, but there was no effect on the phylogenetic structure of Frankia. Biogeographic analyses revealed the strongest cross-continental links over the Beringian land bridge. Despite the facultative symbiotic nature of Frankia, phylogenetic relations among Alnus species play a prominent role in structuring root-associated Frankia communities and their biogeographic patterns. Our results suggest that Alnus species exert strong phylogenetically determined selection pressure on compatible Actinobacteria.