A core of rhizosphere bacterial taxa associates with two of the world’s most isolated plant congeners

Understanding the contributions of abiotic and biotic conditions to soil microbial diversity, structure, and function, remains a central focus in soil biology and biogeochemistry. Here we aim to determine how geography and host plant identity influence these different components of rhizosphere bacterial communities and endosymbionts associated with Acacia heterophylla on Reunion island (Mascarene archipelago, Indian Ocean) and A. koa in the Hawaiian Islands (Hawaiian archipelago, Pacific Ocean). These two tree species are remarkable: they are each other’s closest living relatives despite their habitats being more than 16 000 km apart. Using 16S rRNA amplicon next-generation sequencing data we show that the structure of rhizosphere communities of these two acacias is largely driven by dispersal limitation between sites and local soil chemical conditions within sites. Despite high taxonomic turnover in soils collected from different sites, we found their predicted functions to be largely similar, suggestive of functional redundancy. We also identify a core of rhizosphere taxa associated with both Acacia species in both archipelagos, which included potential nitrogen-fixing mutualists. Isolation and characterisation of rhizobia from root nodules of both acacias further supported strong selection by these plants for the same Bradyrhizobium endosymbionts. Overall, our data suggest that phylogenetically-closely related plants may show remarkably similar selectivity for bacterial mutualists over vast geographic distances.