Hydrogen isotope fractionation is controlled by CO in coccolithophore lipids.

Hydrogen isotope ratios (δH) 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 H/H fractionation in algal lipids by systematically manipulating temperature, light, and CO(aq) in continuous cultures of the haptophyte . We analyze the hydrogen isotope fractionation in alkenones (α), a class of acyl lipids specific to this species and other haptophyte algae. We find a strong decrease in the α with increasing CO(aq) and confirm α correlates with temperature and light. Based on the known biosynthesis pathways, we develop a cellular model of the δH 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 H-richer intracellular water, increasing α. Higher chloroplast CO(aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of α to CO(aq) in our cultures suggests that carbon concentrating mechanisms (CCM) do not achieve a constant saturation of CO at the Rubisco site, but rather that chloroplast CO varies with external CO(aq). The pervasive inverse correlation of α with CO(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 may be a powerful tool to elucidate the carbon limitation of photosynthesis.