CONTROLS ON CARBON-ISOTOPE FRACTIONATION BY DIATOMS IN THE PERU UPWELLING REGION

The fractionation of carbon isotopes during photosynthesis by phytopla nkton is quantified for samples of suspended material collected along two transects across the Peru continental margin in 1992. The magnitud e of fractionation is estimated using the delta(13)C of 24-methylchole sta-5,24(28)-dien-3 beta-ol (diatoms) and compared to that of C-37:2 a lkenone (haptophytes). Isotopic fractionation by diatoms exhibits a wi de range and large scatter when plotted against the reciprocal of the concentration of CO2(aq), while a strong correlation is observed for f ractionation by alkenone-bearing haptophytes. Diatom growth rates, cal culated from silicate concentrations and assuming Monod growth kinetic s, normalized to [CO2((aq))] are well correlated to diatom fractionati on factors. These results support the concept that growth rates, in ad dition to CO2 concentrations, impose a control on the fractionation of carbon isotopes by both taxonomic groups of algae. In addition, the v ery small fractionation factors for diatoms indicate that species in t he Peru upwelling region employed mechanisms which actively transport inorganic carbon into cells. A size dependence is observed for the del ta(13)C of the diatom sterol: 24-methylcholesta-5,24(28)-dien-3 beta-o l is enriched in C-13 in samples of suspended material > 20 mu m relat ive to the <20-mu m fraction. This suggests that surface area-to-volum e 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. Copy right (C) 1997 Elsevier Science Ltd.