Light-dependent carbon isotope fractionation in the coccolithophorid Emiliania huxleyi

The carbon isotopic composition of marine phytoplankton varies significantly with growth conditions. Aqueous CO2 concentration [CO2] and algal growth rate (m) have been suggested to be important factors determining isotope fractionation («p). Here we examine «p of the coccolithophorid Emiliania huxleyi in relation to CO2 concentration and light conditions in dilute batch cultures. Cells were incubated at different irradiance cycles, photon flux densities (PFDs), and [CO2]. Isotope fractionation varied between 6.7 and 12.3‰ under 16 : 8 h light : dark cycle (L : D) and between 14.7 and 17.8‰ at continuous light. «p was largely independent of ambient [CO2], varying generally by less than 2‰ over a range of [CO2] from 5 to 34 mmol L21. Instantaneous carbon-specific growth rates (mC) and PFDs, ranging from 15 to 150 mmol m22 s21, positively correlated with «p. This result is inconsistent with theoretical considerations and experimental results obtained under constant light conditions, suggesting an inverse relationship between «p and m. In the present study the effect of PFDs on «p was stronger than that of m and thus resulted in a positive relationship between m and «p. In addition, the L : D cycle of 16 : 8 h resulted in significantly lower «p values compared to continuous light. Since the observed offset of about 8‰ could not be related to daylengthdependent changes in mC, this implies a direct influence of the irradiance cycle on «p. These findings are best explained by invoking active carbon uptake in E. huxleyi. If representative for the natural environment, these results complicate the interpretation of carbon isotope data in geochemical and paleoceanographic applications. Photosynthetic carbon fixation discriminates against the heavier CO2, causing the isotopic composition of organic material to be depleted in 13C compared to the inorganic carbon source. Most of this isotope fractionation occurs during enzymatic CO2 fixation by ribulose-1,5-bisphosphatecarboxylase/oxygenase (RubisCO). Several studies demonstrated that isotope fractionation of phytoplankton varies over a wide range as a function of the environmental conditions and physiological characteristics of the algal species. Degens et al. (1968) were the first to provide experimental evidence for a positive correlation between aqueous CO2 concentration [CO2] and carbon isotope fractionation («p) in marine phytoplankton. More recently isotope fractionation was shown to be inversely correlated with growth rate (m) (Fry and Wainright 1991). To account for the combined effects of m and [CO2], various authors used the «p versus m/[CO2] relationship for interpreting isotope data. In case of entirely diffusive CO2 uptake, an inverse linear correlation between «p and m/[CO2] would be expected (Francois et al. 1993; Laws et al. 1995; Rau et al. 1996). Any deviation from linearity strongly suggests that processes other than uncatalyzed diffusive CO2 influx are involved in carbon acquisition (Laws et al. 1997; Burkhardt et al. 1999a). The relationship between [CO2] and isotope fractionation 1 Corresponding author ([email protected]).