| Summary: | Microorganisms are essential for the functioning of marine ecosystems. Determining both the distribution and drivers of diversity are of utmost importance. Through interactions with hosts, bacteriophages (phages) affect the structure and diversity of marine microbial communities. Phages are also polyvalent and therefore represent a mobile genetic pool that ensues significant implications for the evolutionary dynamics of microbes on a global scale. Current community-level experimentation is lacking and what remains unknown is how phages, both local and foreign, drive the functional fates of hosts. Additionally, it is unresolved if taxonomic complexity directly contributes to the success of a microbial ecosystem. Functional redundancy may explain discrepancies between the two, but both the level and generality of redundancy is underexplored in marine microbial systems. Here, we implement a Multi-phenotype Assay Plates approach and utilize host metabolism as a functional proxy to determine the microbial biogeography of natural marine microbial communities (MMCs) and explore two potential mechanisms behind the structuring of function: phages and community complexity. We first demonstrate that functional richness of MMCs did not follow a global latitudinal diversity gradient. Whole phage-host community interactions revealed that local phages significantly determined the total functional richness in Arctic MMCs and significantly manipulated all MMCs’ metabolic functions, most often via phage control. Foreign phages did not significantly affect the total functional richness of MMCs, however there was a high incidence of community gain-of-function phenotypes and compositional changes in the metabolic fingerprint due to the viability of foreign phages. The source of phage was extremely significant over a large spatial scale and either drastically less or nonsignificant for the small spatial scale, suggesting that marine phages are locally adapted to their hosts. Dilution-to-extinction experiments revealed that the ...
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