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13Seamounts are often covered with Fe and Mn oxides, known as ferromanganese (Fe-Mn) crusts. Future mining of 14 these crusts is predicted to have significant effects on biodiversity in mined areas. Although microorganisms have 15 been reported on Fe-Mn crusts, little is known about the role of crusts in shaping microbial communities. Here, 16we investigated microbial community based on 16S rRNA gene sequences retrieved from Fe-Mn crusts, coral 17 skeleton, calcarenite and biofilm at crusts of the Rio Grande Rise (RGR). RGR is a prominent topographic feature 18 in the deep southwestern Atlantic Ocean with Fe-Mn crusts. Our results revealed that crust field of the RGR harbors 19 a usual deep-sea microbiome. We observed differences of microbial structure according to the sampling location 20 and depth, suggesting an influence of water circulation and availability of particulate organic matter. Bacterial and 21 archaeal groups related to oxidation of nitrogen compounds, such as Nitrospirae, Nitrospinae phyla, 22Nitrosopumilus within Thaumarchaeota group were present on those substrates. Additionally, we detected 23 abundant assemblages belonging to methane oxidation, i. e. Ca. Methylomirabilales (NC10) and SAR324 24 (Deltaproteobacteria). The chemolithoautotrophs associated with ammonia-oxidizing archaea and nitrite-oxidizing 25 bacteria potentially play an important role as primary producers in the Fe-Mn substrates from RGR. These results 26 provide the first insights into the microbial diversity and potential ecological processes in Fe-Mn substrates from 27 the Atlantic Ocean. This may also support draft regulations for deep-sea mining in the region.28 29

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