Novel bacterial group potentially dominates sulfur cycling in the dark ocean

Abstract The dark ocean (>200 m depth) is the largest habitat on Earth. Dissolved inorganic carbon (DIC) fixation in the oxygenated waters of the dark ocean is in the same order of magnitude as heterotrophic microbial biomass production 1,2. Recent evidence suggests sulfur oxidation could be a major energy source for deep ocean microbes 3. However, the global relevance and the identity of the major players in sulfur oxidation in the oxygenated deep-water column remain elusive. Here we combined single-cell genomics, community metagenomics, metatranscriptomics and single-cell activity measurements to present a novel, ubiquitous mixotrophic bacterial group (UBA868) dominating the total expression of RuBisCO genes (up to 40%) and of key sulfur oxidation (soxB and rdsrA) genes (up to 100%) in the global dark ocean. Our study also underscores the unrecognized importance of mixotrophic microbes, such as UBA868, in the biogeochemical cycles of the deep ocean.