INFLUENCE OF CHANGES IN CO2 CONCENTRATION AND TEMPERATURE ON MARINE-PHYTOPLANKTON C-13/C-12 RATIOS - AN ANALYSIS OF POSSIBLE MECHANISMS

The C-13/C-12 fractionation associated with net transport fluxes and chemical conversions, and with equilibria, associated with inorganic C assimilation processes in marine phytoplankton are quite well understood, though some gaps remain. These values are used in models of overall C-13/C-12 fractionation in inorganic C assimilation involving the two major mechanisms involved in inorganic C entry, i.e. diffusion of CO2 and active transport of CO2 and/or HCO3-. The CO2 diffusion model predicts the observed decrease in the C-13/C-12 of plankton organic C relative to source CO2 when CO2 concentration increases and/or temperature decreases. The inorganic C active transport model is complicated by repression of the active transport mechanism at high inorganic C levels, but this model also predicts the observed effect on cell C-13/C-12 of changes in CO2 Partial pressure or temperature for cell growth. More refined modelling and more input data are needed for both transport processes. Operation of either of the alternative mechanisms for inorganic C entry can be consistent with growth rate not being limited by inorganic C supply even when the photosynthetic rate is inorganic C-limited.