Hydrogen isotope fractionation is controlled by CO2 in coccolithophore lipids

Hydrogen isotope ratios (d2H) 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, for the first time, CO2(aq) in continuous cultures of the haptophyte Gephyrocapsa oceanica. We analyze the hydrogen isotope fractionation in alkenones (aalkenone), a class of acyl lipids specific to this species and other haptophyte algae. We find a strong decrease in the aalkenone with increasing CO2(aq), and confirm aalkenone correlates with temperature and light. Based on the known biosynthesis pathways, we develop a new cellular model of the d2H 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 greater exchange of NADPH with 2H-richer intracellular water, increasing aalkenone. Higher chloroplast CO2(aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of aalkenone 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 aalkenone 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, aalkenone may be a powerful tool to elucidate carbon limitation of photosynthesis.