Phytoplanktonic carbon isotope fractionation: Equations accounting for CO2-concentrating mechanisms

Abstract

A model of carbon isotope discrimination by phytoplankton was developed which took into account the occurrence of a carbon-concentrating mechanism (CCM). A simple equation was obtained for the model involving CO2 active transport. In the case of HCO3- active transport, another equation was developed based on a series of approximations. The former equation was used to analyse reported and newly obtained data from culture experiments and field observations in both freshwater and marine environments. In most cases, a linear relationship between a combined parameter, (1-f)Ci, which was made up of the relative contribution of active CO2 uptake to total carbon uptake (f) and the intracellular CO2 concentration (Ci), and CO2 concentration in bulk solution (Ce) was obtained as (1-f)Ci = ace-b, with a high correlation coefficient (r2 > 0.9). The slope a is suggested as a measure of the ratio of diffusive to total (diffusive + active) CO2 transport, while b/a represents CO2 demand.