Hydrogen isotope fractionation is controlled by CO 2 in coccolithophore lipids.

Hydrogen isotope ratios (δ ² H) 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 ² H/ ¹ H fractionation in algal lipids by systematically manipulating temperature, light, and CO ₂ (aq) in continuous cultures of the haptophyte Gephyrocapsa oceanica . 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 CO ₂ (aq) and confirm α _alkenone correlates with temperature and light. Based on the known biosynthesis pathways, we develop a cellular model of the δ ² H 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 ² H-richer intracellular water, increasing α _alkenone . Higher chloroplast CO ₂ (aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of α _alkenone to CO ₂ (aq) in our cultures suggests that carbon concentrating mechanisms (CCM) do not achieve a constant saturation of CO ₂ at the Rubisco site, but rather that chloroplast CO ₂ varies with external CO ₂ (aq). The pervasive inverse correlation of α _alkenone with CO ₂ (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.
Competing Interests: Competing interests statement:The authors declare no competing interest.