CONTROLS ON CARBON ISOTOPE FRACTIONATION IN PHYTOPLANKTON: FIELD EVIDENCE

Knowledge of carbon isotope fractionation in marine phytoplankton is critical to many geologic carbon cycle models. Results of early studies of carbon isotopic compositions of marine particulate and sedimentary organic matter including biomarker compounds suggested carbon isotope variations were mainly due to changes in the concentration of aqueous carbon dioxide (CO₂(aq)). However, recent results obtained on phytoplankton grown under controlled laboratory conditions demonstrate that carbon isotope fractionation depends on algal growth rate and the cellular surface area-to-carbon ratio in addition to the concentrations of CO₂(aq). Although data from field studies support this hypothesis, there has been no rigorous field test of the laboratory-derived relationships because of difficulty determining carbon isotope fractionation and growth rate of the same algae. Consequently we developed a method to evaluate the effect of growth rate on carbon isotopic fractionation in natural populations of the alkenone-producing algae so that laboratory-based microalgal fractionation hypotheses may be field-tested. In field studies, cell geometry can be quantitatively constrained only when the source of the phytoplankton carbon analyzed is known. Isotopic analyses of alkenones provide a way to constrain the size and shape of the source organism in the open ocean because alkenones are known to be produced in these environments by only Emiliania huxleyi and the closely related Gephyrocapsa oceanica both of which are reasonably similar in size and shape. We compare growth rate estimated from laboratory culture experiments of E. huxleyi with measured in situ growth rates of the alkenone-producing algae from experiments in the subtropical North Pacific gyre, the subarctic Pacific, the Bering Sea and the Sea of Cortez to determine the effect of algal growth rate on carbon isotope fractionation. Results of these experiments provide the first rigorous test of laboratory derived fractionation hypotheses in the ocean and show that current laboratory based fractionation models do not adequately describe variations in natural microalgal populations.