Isolating the light dependence of carbon isotope fractionation in Gephyrocapsa (Emiliania) huxleyi

Estimates of past atmospheric carbon dioxide content ( p CO 2 ) derived from geochemical archives are critical to the advancement of climate science. One of the most frequently used marine proxies is the biological discrimination of stable carbon isotopes (ε p ) recorded in long-chain ketones of the haptophyte algae, a method known as alkenone paleobarometry. In its conventional application, the fractionation of carbon isotopes is assumed to reflect the kinetic isotope effect (KIE) of the carbon-fixing enzyme Rubisco, and the cellular carbon budget is assumed to be governed by the ratio of demand by carbon fixation to diffusive CO 2 supply (demand:supply). Changes in this budget cause variation in the expression of ε p , with the smallest ε p values reflecting the highest ratio of demand:supply, i.e., the lowest relative CO 2 concentrations. However, prior work indicates that photosynthetic irradiance also influences expression of ε p . To isolate the influence of the photosynthetic effect, we cultivated a calcifying strain of Gephyrocapsa ( Emiliania ) huxleyi in nitrate-limited chemostat culture under conditions of constant growth rate and CO 2 supply, but six different levels of irradiance. The resulting values of ε p varied between 16 ‰ at photon flux density (PFD) < 50 μmol m −2 s −1 to 20 ‰ at PFD > 150 μmol m −2 s −1 . The low-light cultures exhibited slightly decreased carbon uptake at steady-state, and as such, the smallest values of ε p were recorded under the highest in-situ CO 2 concentrations, opposite to ε p vs. demand:supply expectations. The data are consistent with prior batch-culture studies of G. huxleyi and other Gephyrocapsa strains, suggesting that while those experiments used different cultivation approaches, a light-dependent effect on cell carbon budgets may be universal, perhaps through direct influence on the efficiency of carbon concentrating mechanisms (CCMs). These results point toward future strategies to improve the physiological accuracy of ε p models and refine p CO 2 reconstructions.