The carbon isotope fractionation in algal organic matter (E_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 E_p to changes in cellular carbon demand relative to diffusive CO₂ supply, with larger E_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 pCO₂ 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 E_p as carbon demand relative to supply decreases, consistent with DIC manipulations. We compile existing Noelaerhabdaceae E_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 E_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 E_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 E_p.

G. oceanica RCC1303 was cultivated in triplicate batch culture at five different pCO₂ levels. CO₂ modification was achieved by continuously aerating the headspace of the culture flask. Alkenones were extracted using accelerated solvent extraction and carbon isotope ratios were measured by GC-IRMS. CO₂ concentrations were calculated from measurements of pH, total alkalinity, temperature, and salinity. Cell sizes were measured by flow cytometry. Existing data were compiled from the literature. Cell sizes and carbon content were standardized to the middle of the photoperiod, where applicable.

Missing values are noted with “NaN.” Data sources are listed in the references and explanatory columns (e.g. “Source of cell radius data”). Information in columns titled “Note on XYZ” (e.g. “Note on d13C_POC uncertainty”) reflect the source of information in the original publication.