A model of photosynthetic 13C fractionation by marine phytoplankton based on diffusive molecular CO2 uptake

A predictive model of carbon isotope fractionation (E,) and abundance (613Cphglo) is presented under circumstances where photosynthesis is strictly based on C02(aq) that passively diffuses into marine phytoplankton cells. Similar to other recent models, the one presented here is based on a formulation where the expression of intracellular enzymatic isotope fractionation relative to that imposed by CO,(aq) transport is scaled by the ratio of intracellular to external [ C 0 2 ( a q ) J , c,/c, Unlike previous models, an explicit calculation of c, is made that is dependent on c, as well as cell radius, cell growth rate, cell membrane permeability to C02(aq), temperature, and, to a limited extent, pH and salinity. This allows direct scaling of ci/c, to each of these factors, and thus a direct prediction of E, and 6'3Cvhs,o responses to changes in each of these variables. These responses are described, and, where possible, compared to recent experimental and previous modellng results.