Glacier-fed streams (GFSs) serve as headwaters to many of the world’s largest river networks. Although being characterized by extreme environmental conditions (i.e., low water temperatures, oligotrophy) GFSs host an underappreciated microbial biodiversity, especially within benthic biofilms which play pivotal roles in downstream biogeochemical cycles. Yet, we still lack a global overview of the GFS biofilm microbiome. In addition, little is known on how environmental conditions shape bacterial diversity, and how these relationships drive global distribution patterns. This is particularly important as mountain glaciers are currently vanishing at a rapid pace due to global warming. Here, we used 16S rRNA gene sequencing data from the Vanishing Glaciers project to conduct a first comprehensive analysis of the benthic microbiome from 148 GFSs across 11 mountain ranges. Our analyses revealed marked biogeographic patterns in the GFS microbiome, mainly driven by the replacement of phylogenetically closely related taxa. Strikingly, the GFS microbiome was characterized by pronounced level of endemism, with >58% of the Amplicon Sequence Variants (ASVs) being specific to one mountain range. Consistent with the marked dissimilarities across mountain ranges, we found a very small taxonomic core including only 200 ASVs, yet accounting for >25% of the total relative abundance of the ASVs. Finally, we found that spatial effects such as dispersal limitation, isolation and spatially autocorrelated environmental conditions overwhelmed the effect of the environment by itself on benthic biofilm beta diversity. Our findings shed light on the previously unresolved global diversity and biogeography of the GFS microbiome now at risk across the world’s major mountain ranges because of rapidly shrinking glaciers.
