Light-dependent carbon isotope fractionation in the coccolithophorid Emiliania huxleyi
Limnol. Oceanogr., 47(1), 2002, 120-128 | DOI: 10.4319/lo.2002.47.1.0120
ABSTRACT: The carbon isotopic composition of marine phytoplankton varies significantly with growth conditions. Aqueous CO2 concentration [CO2] and algal growth rate (µ) have been suggested to be important factors determining isotope fractionation (ep). Here we examine ep 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. ep was largely independent of ambient [CO2], varying generally by less than 2‰ over a range of [CO2] from 5 to 34 mmol L-1. Instantaneous carbon-specific growth rates (µC) and PFDs, ranging from 15 to 150 mmol m-2 s-1, positively correlated with ep. This result is inconsistent with theoretical considerations and experimental results obtained under constant light conditions, suggesting an inverse relationship between ep and µ. In the present study the effect of PFDs on ep was stronger than that of mand thus resulted in a positive relationship between µ and ep. In addition, the L:D cycle of 16 : 8 h resulted in significantly lower ep values compared to continuous light. Since the observed offset of about 8‰ could not be related to daylength dependent changes in µC, this implies a direct influence of the irradiance cycle on ep. 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.