Does oligotrophy favor chemoautotrophy over photoautotrophy?

• The study investigates photosynthesis and dark carbon fixation (DCF) in the photic layer of the eastern Mediterranean Sea (EMS), focusing on oligotrophic conditions. • Results show that DCF contributes more to primary productivity (PP) offshore (∼10 %) and during oligotrophic summer (∼10 %), while coastal and winter contributions are lower (∼2–4 %). • Chemoautotrophic carbon fixation (DCF) is significant in ultra-oligotrophic waters, suggesting it should be included in global primary productivity estimates, especially in expanding oligotrophic regions like subtropical oceans. • he study highlights that chemoautotrophs have competitive advantages under nutrient-limited conditions due to their small size and ability to utilize both organic and inorganic nutrients, which may explain higher DCF in summer. • Annual integrated DCF in EMS is estimated at ∼ 5 g C m −2 y −1 , accounting for about 3.5–11 % of total primary production, emphasizing the ecological importance of chemoautotrophy in oligotrophic marine ecosystems. 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.