Current estimates point to a mismatch of particulate organic carbon supply derived from the surface ocean and the microbial organic carbon demand in the meso- and bathypelagic realm. Based on recent findings that chemoautotrophic Crenarchaeota are abundant in the mesopelagic zone, we quantified dissolved inorganic carbon (DIC) fixation in the meso- and bathypelagic North Atlantic and compared it with heterotrophic microbial activity. Measuring 14C-bicarbonate fixation and 3H-leucine incorporation revealed that microbial DIC fixation is substantial in the mesopelagic water masses, ranging from 0.1 to 56.7 μmol C m -3 d -1, and is within the same order of magnitude as heterotrophic microbial activity. Integrated over the dark ocean's water column, DIC fixation ranged from 1-2.5 mmol C m -2 d -1, indicating that chemoautotrophy in the dark ocean represents a significant source of autochthonously produced 'new organic carbon' in the ocean's interior amounting to about 15-53% of the phytoplankton export production. Hence, chemoautotrophic DIC fixation in the oxygenated meso- and bathypelagic water column of the North Atlantic might substantially contribute to the organic carbon demand of the deep-water microbial food web.