biostudies-literatureTorres-Romero IEidgenössische Technische Hochschule Zürich (ETH)Eidgenössische Technische Hochschule ZürichSwiss National Science Foundatione2318570121https://www.ebi.ac.uk/biostudies/studies/S-EPMC11214045biostudies-literatureUnknown121(26)Hydrogen isotope ratios (δ²H) represent an important natural tracer of metabolic processes, but quantitative models of processes controlling H-fractionation in aquatic photosynthetic organisms are lacking. Here, we elucidate the underlying physiological controls of ²H/¹H fractionation in algal lipids by systematically manipulating temperature, light, and CO₂(aq) in continuous cultures of the haptophyte <i>Gephyrocapsa oceanica</i>. We analyze the hydrogen isotope fractionation in alkenones (α_alkenone), a class of acyl lipids specific to this species and other haptophyte algae. We find a strong decrease in the α_alkenone with increasing CO₂(aq) and confirm α_alkenone correlates with temperature and light. Based on the known biosynthesis pathways, we develop a cellular model of the δ²H of algal acyl lipids to evaluate processes contributing to these controls on fractionation. Simulations show that longer residence times of NADPH in the chloroplast favor a greater exchange of NADPH with ²H-richer intracellular water, increasing α_alkenone. Higher chloroplast CO₂(aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of α_alkenone to CO₂(aq) in our cultures suggests that carbon concentrating mechanisms (CCM) do not achieve a constant saturation of CO₂ at the Rubisco site, but rather that chloroplast CO₂ varies with external CO₂(aq). The pervasive inverse correlation of α_alkenone with CO₂(aq) in the modern and preindustrial ocean also suggests that natural populations may not attain a constant saturation of Rubisco with the CCM. Rather than reconstructing growth water, α_alkenone may be a powerful tool to elucidate the carbon limitation of photosynthesis.Proceedings of the National Academy of Sciences of the United States of AmericaHydrogen isotope fractionation is controlled by CO₂ in coccolithophore lipids.PMC11214045200021SEED-17 21-2182070Core funding200021_182070Jaggi MClark AJStoll HMZhang HTorres-Romero IMcLeod REWijker RSfalseHydrogen isotope fractionation is controlled by CO₂ in coccolithophore lipids.Hydrogen isotope ratios (δ²H) represent an important natural tracer of metabolic processes, but quantitative models of processes controlling H-fractionation in aquatic photosynthetic organisms are lacking. Here, we elucidate the underlying physiological controls of ²H/¹H fractionation in algal lipids by systematically manipulating temperature, light, and CO₂(aq) in continuous cultures of the haptophyte <i>Gephyrocapsa oceanica</i>. We analyze the hydrogen isotope fractionation in alkenones (α_alkenone), a class of acyl lipids specific to this species and other haptophyte algae. We find a strong decrease in the α_alkenone with increasing CO₂(aq) and confirm α_alkenone correlates with temperature and light. Based on the known biosynthesis pathways, we develop a cellular model of the δ²H of algal acyl lipids to evaluate processes contributing to these controls on fractionation. Simulations show that longer residence times of NADPH in the chloroplast favor a greater exchange of NADPH with ²H-richer intracellular water, increasing α_alkenone. Higher chloroplast CO₂(aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of α_alkenone to CO₂(aq) in our cultures suggests that carbon concentrating mechanisms (CCM) do not achieve a constant saturation of CO₂ at the Rubisco site, but rather that chloroplast CO₂ varies with external CO₂(aq). The pervasive inverse correlation of α_alkenone with CO₂(aq) in the modern and preindustrial ocean also suggests that natural populations may not attain a constant saturation of Rubisco with the CCM. Rather than reconstructing growth water, α_alkenone may be a powerful tool to elucidate the carbon limitation of photosynthesis.2024-01-01T00:00:00Z2024 Jun2026-06-01T17:38:45.974Z2025-04-04T21:45:09.26ZS-EPMC112140453890523810.1073/pnas.2318570121