Simulation of C1fluxes in cyanobacteria: comparison between the carboxysome hypothesis and the equilibrium proposal

Abstract

Inorganic carbon fluxes were simulated by a mathematical model using an equilibrium hypothesis for a wide range of conditions in a closed system composed of air-grown cells of Synechococcus UTEX 625 in a reaction vessel connected to a mass spectrometer. The metabolic scheme took into account the input fluxes of CO2and HCO3−transport into the cells, the output fluxes of CO2and HCO3−efflux, the diversion of Q toward the formation of the internal C2pool, and photosynthetic CO2fixation. The equations expressed the variation in concentration of each inorganic species outside and inside the cell as a function of time. The input fluxes were previously characterized by their kinetic constants (K1/2and Vm) both during initial uptake occurring upon illumination of the cells and under steady-state photosynthesis conditions. The efflux rates of the various Cispecies from the cells were investigated under a wide variety of experimental conditions. Using these efflux rates, the permeability coefficients of the cell for CO2and HCO3−were calculated previously. Using the kinetic constants for CO2and HCO3−transport, the permeability coefficients of the cell for CO2and HCO3−and the geometrical characteristics of the cells, the model simulated precisely the [HCO3−]/[CO2] ratio and the [CO2] and [O2] changes in the extracellular medium as well as the rate of filling of the internal Cipool under various conditions. Accurate fitting of experimental data with calculated values were possible only when the intracellular Cispecies were assumed to be in equilibrium throughout the entire cell volume. Results are discussed and compared with those given by previous hypotheses. Key words: Synechococcus UTEX 625, blue green algae, cyanobacteria, mathematical model, active CO2transport, active HCO3−transport, steady state, photosynthesis, Ciconcentrating mechanism.