Fig. 3.

Model structure and comparison between simulations and laboratory cultures. (A) Structure of the hydrogen fractionation model in this work. There are two simulated compartments: the chloroplast and cytoplasm. The hydrogen isotope ratios of NADPH in chloroplast (NADPH_x), intracellular water (H₂O_x and H₂O_c), production of small C3 carbon metabolites, fatty acid precursors from chloroplast (C16), and alkenones from cytoplasm (C37) are simulated. In different colors, the six fluxes affecting the hydrogen isotope ratios of alkenones are highlighted: in red, fluxes using NADPH as a substrate; in green, fluxes using carbon molecules, and in blue, fluxes with water. The ferredoxin-NADP reductase (FNR) introduces a δ²H in NADPH_x of −600‰ relative to the stromal water (H₂O_x) (flux F₁). This ²H-depleted H from NADPH_x exchanges with the ²H-enriched H₂O_x via flavoproteins such as glutathione reductase (GR) (fluxes F₂). NADPH_x is mainly consumed by the CBB cycle and the FAS complex (fluxes F₃ and F₄), whose competition with flavoproteins (GR) determines NADPH residence time (τ_NADPx). The CBB produces C3 compounds that can generate acetyl-CoA for fatty acid production by the FAS. These fatty acids can be exported to the cytoplasm for further elongation to produce alkenones. The different H₂O channels and pumps influence the water δ²H by exchanging water (WE) between the chloroplast and cytoplasm (fluxes F₅). (B) Comparison between simulation and laboratory culture results on CO₂(aq) and light effect at 18 °C. The lines are simulation results at fixed 18 °C temperature at varying CO₂(aq). The circles are measurements in laboratory culture. Gray bars represent the SD of hydrogen isotope fractionations calculated from C37:3, C37:2, and C38:2Et alkenones (α_alkenone). Only the cultures carried out at 18 °C are presented in this panel to remove the temperature effect. The color of circles and lines represents the light intensity in µE and shares the same colormap.