A chloroplast pump model for the COSubscript2/S

Authors: Brian M Hopkinson

Publish Date: 2013/11/29

Volume: 121, Issue: 2-3, Pages: 223-233

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Abstract

Prior analysis of inorganic carbon Ci fluxes in the diatom Phaeodactylum tricornutum has indicated that transport of Ci into the chloroplast from the cytoplasm is the major Ci flux in the cell and the primary driving force for the CO2 concentrating mechanism CCM This flux drives the accumulation of Ci in the chloroplast stroma and generates a CO2 deficit in the cytoplasm inducing CO2 influx into the cell Here the “chloroplast pump” model of the CCM in P tricornutum is formalized and its consistency with data on CO2 and HCO3 − uptake rates carbonic anhydrase CA activity intracellular Ci concentration intracellular pH and RubisCO characteristics is assessed The chloroplast pump model can account for the major features of the data Analysis of photosynthetic and Ci uptake rates as a function of external Ci concentration shows that the model has the most difficulty obtaining sufficiently low cytoplasmic CO2 concentrations to support observed CO2 uptake rates at low external Ci concentrations and achieving high rates of photosynthesis There are multiple ways in which model parameters can be varied within a plausible range to match measured rates of photosynthesis and CO2 uptake To increase CO2 uptake rates CA activity can be increased kinetic characteristics of the putative chloroplast pump can be enhanced to increase HCO3 − export or the cytoplasmic pH can be raised To increase the photosynthetic rate the permeability of the pyrenoid to CO2 can be reduced or RubisCO content can be increasedThis work was supported by grants from the National Science Foundation EF 1041023 and MCB 1129326 to BH J Losh and FMM Morel Princeton University are thanked for providing data on total protein and RubisCO content in P tricornutum Comments from two anonymous reviewers improved the work

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