Inorganic carbon fixation by deep prokaryotes as an unaccounted-for CO2 sink in Antarctic waters

The Ross Sea is one of the most productive regions of the Ocean and is a major site of deep-water formation, ultimately affecting the global thermohaline circulation. As such, it plays a pivotal role in regulating atmospheric CO2 through the biological and the solubility carbon pumps. Despite the recognised importance of this area, the knowledge of the carbon fluxes mediated by microbes in its interior remains limited. Here we describe the outcomes of 62 incubation experiments aimed at understanding the processing of inorganic and organic carbon pools between 200 and 2000 m. Results showed that in these waters DIC fixation occurs at rates between 0.03 and 3.12 nmol C L−1 d−1 and represents a relevant CO2 sink, especially under high particulate organic carbon conditions. Both Archaea (Nitrosopumilaceae) and Bacteria (SUP05 and SAR324) were identified as putative major DIC fixers in High Salinity Shelf Water, Circumpolar Deep Water, and Antarctic Bottom Water. Microbial carbon fixation in deep waters below the illuminated water column accounts for ~5% of the annual dissolved inorganic carbon sink in the Ross Sea, according to incubation experiments replicating Antarctic waters between 200 m and 2000 m depth.