Chemoautotrophic and photoautotrophic inorganic carbon fixation rates in marine oligotrophic systems; The Mediterranean and Red seas as case studies

Dark inorganic carbon fixation (DCF) by chemoautotrophs is thought to play a significant role in marine systems, especially in oligotrophic marine ecosystems where photosynthesis is typically low. We investigated DCF and its contribution to the total primary productivity (PP) in the ultra-oligotrophic eastern Mediterranean Sea (2021-2024) and the meso/oligotrophic northern Red Sea (2010-2023). Our results show that DCF is indeed substantial, and corresponds to ~25-40% of the annual primary productivity rates in both areas. The contribution of DCF to PP was high during the summer and increased from the coast to the offshore water. During wintertime, the contribution of DCF to PP was typically low, without clear spatial or vertical trends. Additionally, aphotic DCF rates were similar to those found in the photic zone. Lastly, our results show that organic nutrient amendments significantly elevate dark inorganic carbon fixation, whereas the addition of inorganic nutrients elevates photosynthesis and to a lesser extent DCF. These results suggest that DCF may be an important biochemical process throughout the water column of oligotrophic seas, and thus should be incorporated into oceanic carbon production estimates.