Does oligotrophy favor chemoautotrophy over photoautotrophy?

We investigated the spatiotemporal variability of photosynthesis and dark carbon fixation (DCF) in the photic layer of the eastern Mediterranean Sea (EMS). Our results show that the contribution of DCF to primary productivity (PP) was generally low in the coastal area (typically ∼ 2–4 %) and higher offshore (often ≥ 10 %). We show that the contribution of DCF to PP was higher during the thermally-stratified summer period (low nutrient availability) when heterotrophic microbial metabolism prevailed compared to the thermally-mixed winter period (higher nutrient availability) when photoautotrophy predominated. Depth-integrated DCF contributed ∼ 5 g C m⁻² y⁻¹ at both offshore and coastal waters, which was ∼ 3.5–11 % of the annual photic zone PP. Our results substantiate previous studies indicating that inorganic carbon fixation by chemoautotrophs should be considered and included in production estimates, especially in low production areas and in systems where oligotrophy is expanding, such as the subtropical and tropical oceans.