Contribution of dark inorganic carbon fixation to bacterial carbon demand in the oligotrophic Southeastern Mediterranean Sea

Photosynthetically derived organic matter sinking to depth from 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 eastern Mediterranean Sea where the annual primary production is one of the lowest in the world's oceans. Yet its aphotic zone is considered a hotspot for microbial activity. Here, we investigated the role of photic and aphotic dark inorganic carbon fixation rates (DCF) and their contribution to bacterial carbon demand in the southeastern 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 the same order of magnitude as photosynthesis (34 vs. 45 g C m-2 yr-1, respectively). Using a carbon mass balance that considers photosynthesis, DCF and bacterial production, 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.