Controls on carbon isotope fractionation by diatoms in the Peru upwelling region

Abstract

The fractionation of carbon isotopes during photosynthesis by phytoplankton is quantified for samples of suspended material collected along two transects across the Peru continental margin in 1992. The magnitude of fractionation is estimated using the δ 13C of 24-methylcholesta-5,24(28)-dien-3β-ol (diatoms) and compared to that of C 37.2 alkenone (haptophytes). Isotopic fractionation by diatoms exhibits a wide range and large scatter when plotted against the reciprocal of the concentration Of CO 2(aq), while a strong correlation is observed for fractionation by alkenone-bearing haptophytes. Diatom growth rates, calculated from silicate concentrations and assuming Monod growth kinetics, normalized to [CO2 (aq)] are well correlated to diatom fractionation factors. These results support the concept that growth rates, in addition to CO 2 concentrations, impose a control on the fractionation of carbon isotopes by both taxonomic groups of algae. In addition, the very small fractionation factors for diatoms indicate that species in the Peru upwelling region employed mechanisms which actively transport inorganic carbon into cells. A size dependence is observed for the δ 13C of the diatom sterol: 24-methylcholesta-5,24(28)-dien-3β-ol is enriched in 13C in samples of suspended material > 20 μm relative to the <20-μm fraction. This suggests that surface-area-to-volume ratios also impose a control on the fractionation of carbon isotopes by diatoms, a proposition that is supported by detailed cell geometry and isotopic data for two larger size fractions from one sample.