Distribution of Inorganic Carbon Among its Component Species in Cyanobacteria: Do Cyanobacteria in fact Actively Accumulate Inorganic Carbon?
✍ Scribed by Yehouda Marcus
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 373 KB
- Volume
- 185
- Category
- Article
- ISSN
- 0022-5193
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✦ Synopsis
A mathematical model is presented, which describes the distribution of inorganic carbon (Ci) between the species CaCO3, CaHCO + 3 , CO 2 - 3 , HCO - 3 and CO2 in the cytosol of a high CO2-requiring mutant of the cyanobacterium Synechocystis PCC 6803, which lacks carboxysomes. The model assumes that entry and efflux of Ci occurs via a CO 2 - 3 transport system and diffusion of CO2. HCO - 3 is considered as impermeable. Intracellular Ci is distributed among the species according to the cytoplasmic pH and the calcium concentration. Former models considered entry of HCO - 3 , and not CO 2 - 3 , and the intracellular interactions of calcium with CO 2 - 3 and their implications for the Ci accumulating mechanism were not considered.
The model predicts that in the presence of 10 -3 -10 -2 M calcium, at low and moderate external inorganic carbon concentrations, CaCO3 is the main cytoplasmic Ci species, whereas in its absence HCO - 3 is the main Ci species. Due to sequesteration of CO 2 - 3 by calcium and low permeability of the cells to CO2, the CO2 concentration is low and no net photosynthesis is observed. Photosynthesis is predicted to occur only at external Ci concentrations higher than 1 mM, where the CO 2 - 3 transport is saturated and the main source of Ci is diffusion of CO2. This converts the mutant to a CO2 user with a high photosynthetic Km(CO2), unlike the wild type which utilizes external CO 2 - 3 as a substrate for photosynthesis at low external concentration.
The sequesteration of CO 2 - 3 by Ca 2 + efficiently reduces the CO 2 - 3 and the CO2 accumulation ratio in comparison with that of total Ci resulting in an inward CO2 gradient and reduced CO 2 - 3 gradient over a wide range of external Ci concentrations. This raises doubts as to whether the transport of Ci as such is an active transport.
The simulation predicts that in cases where the affinity of the uptake mechanism for CO 2 - 3 is higher than that measured for this mutant and the accumulation ratio for Ci is consequently higher, the predicted accumulation ratio for the transported species CO 2 - 3 remains low. Taking the permeability of the cells to gases to be as low as previously measured, the simulation predicts that O2 may be accumulated in the cytoplasm up to saturation at CO2 saturated photosynthesis rates. This results in a high rate of photorespiration and competitive inhibition of CO2 fixation.
Entry of CO 2 - 3 , accumulation of charged Ci species, Ci utilization and efflux of carbon stimulates ions movements required to compensate the charges of CO 2 - 3 , HCO - 3 and CaHCO + 3 . The model predicts accumulation of cations up to 50 mEq × l -1 or exclusion of the same amount of anions required to electroneutralize the accumulation of charged Ci species in the absence of calcium and up to 28 mEq × liter -1 in the presence of 20 mM calcium. The steady-state CO 2 - 3 dependent flux of electroneutralizing ions was found to be equal to the steady-state rate of Ci utilization and efflux (photosynthesis, photorespiratory CO2 evolution and CO2 leakage) multiplied by the valency of CO 2 - 3 . The interconversion of Ci species resulting from Ci utilization and dissipation of Ci involves H + liberation in the hydration reaction and the dissociation of HCO - 3 to CO 2 - 3 or H + consumption in the conversion of CO 2 - 3 to HCO - 3 and dehydration of the latter. The H + production and the CO 2 - 3 dependent transport of positive ions are interrelated and are non-monotonously altered in anti-parallel fashion as a function of the external Ci concentration. The presence of calcium reduces the magnitude of both these fluxes.