Carbon Isotope Fractionation in Noelaerhabdaceae Algae in Culture and a Critical Evaluation of the Alkenone Paleobarometer

The carbon isotope fractionation in algal organic matter (epsilon(p)), including the long-chain alkenones produced by the coccolithophorid family Noelaerhabdaceae, is used to reconstruct past atmospheric CO2 levels. The conventional proxy linearly relates epsilon(p) to changes in cellular carbon demand relative to diffusive CO2 supply, with larger epsilon(p) values occurring at lower carbon demand relative to supply (i.e., abundant CO2). However, the response of Gephyrocapsa oceanica, one of the dominant alkenone producers of the last few million years, has not been studied closely. Here, we subject G. oceanica to various CO2 levels by increasing pCO(2) in the culture headspace, as opposed to increasing dissolved inorganic carbon (DIC) and alkalinity concentrations at constant pH. We note no substantial change in physiology, but observe an increase in epsilon(p) as carbon demand relative to supply decreases, consistent with DIC manipulations. We compile existing Noelaerhabdaceae epsilon(p) data and show that the diffusive model poorly describes the data. A meta-analysis of individual treatments (unique combinations of lab, strain, and light conditions) shows that the slope of the epsilon(p) response depends on the light conditions and range of carbon demand relative to CO2 supply in the treatment, which is incompatible with the diffusive model. We model epsilon(p) as a multilinear function of key physiological and environmental variables and find that both photoperiod duration and light intensity are critical parameters, in addition to CO2 and cell size. While alkenone carbon isotope ratios indeed record CO2 information, irradiance and other factors are also necessary to properly describe alkenone epsilon(p).

Automated summary

The study investigates the response of Gephyrocapsa oceanica to varying CO2 levels and finds that epsilon(p) correlates with carbon demand relative to supply and is consistent with DIC manipulations. It also shows that the response of epsilon(p) depends on light conditions and the range of carbon demand relative to CO2 supply, which is incompatible with the traditional diffusive model.