A MODEL OF PHOTOSYNTHETIC C-13 FRACTIONATION BY MARINE-PHYTOPLANKTON BASED ON DIFFUSIVE MOLECULAR CO2 UPTAKE

A predictive model of carbon isotope fractionation (epsilon(p)) and ab undance (delta(13)C(phyto)) is presented under circumstances where pho tosynthesis is strictly based on CO2(aq) that passively diffuses into marine phytoplankton cells. Similar to other recent models, the one pr esented here is based on a formulation where the expression of intrace llular enzymatic isotope fractionation relative to that imposed by CO2 (aq) transport is scaled by the ratio of intracellular to external [CO 2(aq)], c(i)/c(e). Unlike previous models, an explicit calculation of c(i) is made that is dependent on c(e) as well as cell radius, cell gr owth rate, cell membrane permeability to CO2(aq), temperature, and, to a limited extent, pH and salinity. This allows direct scaling of c(i) /c(e) to each of these factors, and thus a direct prediction of epsilo n(p) and delta(13)C(phyto) responses to changes in each of these varia bles. These responses are described, and, where possible, compared to recent experimental and previous modeling results.