¹³C Fractionation in Emiliania huxleyi: Stirred versus Unstirred Dilute Batch Cultures

Abstract

It has been shown that δ13C of phytoplankton (δ13CP) varies with [CO2(aq)] and rate of algal growth. It is possible that, by using δ13CP from sediments, [CO2(aq)] can be inferred. However, other factors can complicate the relationship between δ13CP and [CO2(aq)]. Discrepancies in results acquired by different culturing methods (chemostat and dilute batch cultures) is an important issue in understanding carbon isotope fractionation in phytoplankton that must be resolved before paleo-CO2 can be inferred from sediment cores. In this study, dilute batch cultures of Emiliania huxleyi were grown to observe possible differences in carbon isotopic fractionation (εP) in stirred and unstirred conditions. Three sets of experiments were performed and, in each set of cultures, stirred cultures exhibit greater fractionation than their unstirred counterparts. We postulate that phytoplankton in stirred cultures have greater εP because the boundary layer immediately surrounding the cells is constantly agitated and does not become enriched in 12C over time as carbon dioxide is assimilated into the cell. Stirred cultures displayed εP values 1.73‰, 6.71‰, and 2.34‰ greater than unstirred cultures of the same sets (using mean values of εP when applicable). However, this does not completely account for the magnitude of differences in and trends of fractionation observed in the chemostat and dilute batch cultures. Factors that may contribute to these discrepancies are discussed.