Dark inorganic carbon fixation contributes to bacterial organic carbon demand in the oligotrophic Southeastern Mediterranean Sea

Abstract. Settling organic matter derived from photosynthesis at the illuminated layers is often not sufficient to meet the energy demands of microbes in the dark ocean. This ‘mismatch’ is especially notable in the warm and oligotrophic Mediterranean Sea where the annual photosynthesis is one of the lowest in the world’s oceans yet its aphotic zone is considered a hotspot for microbial activity and biomass. Here, we investigated the role of photic and aphotic dark inorganic carbon fixation rates (DCF) in supporting bacterial carbon demand at the offshore south-eastern Mediterranean Sea during the mixed and stratified periods. Our results demonstrate that DCF rates are measurable throughout the water column (0–1750 m) and are on the same order of magnitude as photosynthesis (34 vs. 45 g C m^-2 y^-1, respectively). Using a carbon mass balance that considers photosynthesis, DCF and bacterial production (and hence respiration) we show that chemoautotrophy provides ~35 % of the ‘missing carbon’ supply needed for microbial growth and activity in the aphotic layer, while other sources of dissolved organic carbon remain to be elucidated. These findings underscore the need for further research into the factors affecting DCF, its role in global carbon budgets, and its potential to enhance atmospheric inorganic carbon sequestration.