The Coastal Seafloor Microbiota Is Structured by Local Selection of Cosmopolitan Taxa.

Understanding the assembly processes of the coastal seafloor microbiota is crucial for gaining insights into how ocean ecosystems work. In our study, we addressed the question about how local selection affects the global distribution of coastal seafloor microorganisms. We identified two main clusters of samples by examining the geographical distribution of 356 high-quality prokaryote metagenome-assembled genomes (MAGs) from 94 coastal samples collected along the Norwegian and Icelandic coasts. There was no identifiable correlation between the abundance of MAGs and the geographic distance between them central to the identified clusters (no distance decay). In contrast, noncentral MAGs demonstrate a pronounced distance decay. We also observed significant functional differences between the two sample clusters. One cluster showed enrichment in functions such as dissimilatory nitrate reduction to ammonium (DNRA), acetoclastic methanogenesis, thiosulphate conversion and acetate and butyrate metabolism. The other cluster was enriched in propionate metabolism, nitrite oxidation to nitrate and cobalamin-dependent carbon fixation. These results suggest that localised environmental selection acts on cosmopolitan taxa to shape seafloor microbiota. Our findings therefore profoundly impact the understanding of seafloor ecological processes and their management.