Ferromanganese (Fe-Mn) sedimentary layers and nodules occur at different depths within sediments at deep basins and ridges of Lake Baikal. Researchers studied Fe-Mn nodules and host sediments recovered at the slope of Bolshoy Ushkany Island. Layer-by-layer ²³⁰Th/U dating analysis determined the initial age of the Fe-Mn nodule formation scattered in the sediments as 96 ± 5–131 ± 8 Ka. The distribution profiles of the main ions in the pore waters of the studied sediment are similar to those observed in the deep-sea areas of Lake Baikal, while the chemical composition of Fe-Mn nodules indicates their diagenetic formation with hydrothermal influence. Among the bacteria in microbial communities of sediments, members of organoheterotrophic Gammaproteobacteria, Chloroflexi, Actinobacteriota, Acidobacteriota, among them Archaea—chemolithoautotrophic ammonia-oxidizing archaea Nitrososphaeria, dominated. About 13% of the bacterial 16S rRNA gene sequences in Fe-Mn layers belonged to Methylomirabilota representatives which use nitrite ions as electron acceptors for the anaerobic oxidation of methane (AOM). Nitrospirota comprised up to 9% of the layers of Bolshoy Ushkany Island. In bacterial communities of Fe-Mn nodule, a large percentage of sequences were attributed to Alphaproteobacteria, Actinobacteriota and Firmicutes, as well as a variety of OTUs with a small number of sequences characteristic of hydrothermal ecosystems.