Inorganic Carbon Uptake during Photosynthesis

Equations have been developed which quantitatively predict the theoretical time-course of photosynthetic (14)C incorporation when CO(2) or HCO(3) (-) serves as the sole source of exogenous inorganic carbon taken up for fixation by cells during steady state photosynthesis. Comparison between the shape of theoretical (CO(2) or HCO(3) (-)) and experimentally derived time-courses of (14)C incorporation permits the identification of the major species of inorganic carbon which crosses the plasmalemma of photosynthetic cells and facilitates the detection of any combined contribution of CO(2) and HCO(3) (-) transport to the supply of intracellular inorganic carbon. The ability to discriminate between CO(2) or HCO(3) (-) uptake relies upon monitoring changes in the intracellular specific activity (by (14)C fixation) which occur when the inorganic carbon, present in the suspending medium, is in a state of isotopic disequilibrium (JT Lehman 1978 J Phycol 14: 33-42). The presence of intracellular carbonic anhydrase or some other catalyst of the CO(2)-HCO(3) (-) interconversion reaction is required for quantitatively accurate predictions. Analysis of equations describing the rate of (14)C incorporation provides two methods by which any contribution of HCO(3) (-) ions to net photosynthetic carbon uptake can be estimated.