Affiliation:
1. Climate Geology, Department of Earth Sciences, ETH Zürich, Zurich 8092, Switzerland
Abstract
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.
Funder
Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung
Eidgenössische Technische Hochschule Zürich
Publisher
Proceedings of the National Academy of Sciences