Spatial Patterns of Microbial Diversity, Composition and Community Structure in Fe-Mn Deposits and Associated Sediments in the Atlantic and Pacific Oceans

Mining of deep-sea Fe-Mn deposits will remove crusts and nodules in large areas from the seafloor. The growth of a few millimeters of these minerals by Fe and Mn oxides precipitation takes millions of years, and yet little is known about their microbiome. Besides being key elements of the biogeochemical cycles and essential links of food and energy to deep-sea trophic webs, microbes have been identified to affect manganese oxide formation. Hence, polymetallic crusts and nodules may present unique habitats that deserve better understanding. In this study, we determined the composition and diversity of Bacteria and Archaea in deep-sea Fe-Mn crusts, nodules, and associated sediments from two oceanic elevations in the Atlantic Ocean, the Tropic Seamount in the northeast and the Rio Grande Rise (RGR) in the southwest. Sequencing of the 16S rRNA gene was performed using the Illumina MiSeq platform and statistical analyses using environmental data were performed in R. Additionally, we included public domain environmental DNA data of Fe-Mn crusts, nodules, and associated sediments from Clarion-Clipperton Zone and Takuyo-Daigo Seamount in the Pacific Ocean to compare microbial diversity in Fe-Mn deposits from different ocean basins. Our results indicated that Atlantic seamounts harbor an unusual and unknown Fe-Mn deposit microbiome with lower diversity and richness compared to deposits from Pacific areas. Crusts and nodules from Atlantic seamounts revealed the presence of unique taxa (Alteromonadales, Nitrospira, and Magnetospiraceae) and a higher relative abundance of sequences related to potential metal-cycling bacteria, such as Betaproteobacteriales and Pseudomonadales. The microbial beta-diversity from Atlantic seamounts was clearly grouped into microhabitats according to crusts, nodules, and sediments geochemical composition. Furthermore, community structure analysis using principal coordinate analysis also showed that the microbial communities of all seamounts were significantly divided into ocean basins and sampling areas. Despite the time scale of million years for these deposits to grow, a combination of environmental settings (temperature, salinity, depth, substrate geochemistry, nutrient, and organic matter availability) played a significant role in shaping the crusts and nodules microbiome, which was distinct between the Atlantic and Pacific Fe-Mn deposits. Our results suggest that the microbial community inhabiting Fe-Mn deposits participate in biogeochemical reactions indispensable to deep-sea ecosystems, which implies that understanding the microbial community is of utmost importance for any baseline environmental study in areas of potential deep-sea mining. ### Competing Interest Statement The authors have declared no competing interest.