Dataset S1 - Noelaerhabdaceae organic carbon isotope culture data compilation

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.