Investigation into the CO2 concentrating step rates within the carbon concentrating mechanism of Synechocystis sp. PCC6803 at various pH and light intensities reveal novel mechanistic properties

Synechocystis sp. PCC6803 actively uptake inorganic carbon to elevate the carbon dioxide concentration around their Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) enzyme. The mechanism is comprised of a set of enzymatic complexes which act in an orchestrated manner to transfer extracellular dissolved inorganic carbon into the cell and concentrate it within a specialized compartment. NADPH dehydrogenase-1 is the first operational super-complex in light conditions and, therefore, allows a direct estimation of carbon flux into the cell, albeit before photosynthesis and the concentrating mechanism reach a chemical steady-state condition. We calculated the uptake rate during this period by fitting a nonlinear function to the sharp decline in the CO2 gas exchange profile recorded with membrane inlet mass spectrometry, considering the constant change in rate during this step. We estimate the carbon dioxide uptake rates and pool sizes, showing that these values correspond to previously reported steady state rates in the literature. During the investigation, we reveal new properties of the uptake mechanism. At pH lower than six, the enzymatic complex responsible for the CO2 uptake becomes inactive. Its activity recovers within minutes when attenuating the pH within the cell or elevating it back at or above pH 6.0 outside the cell, a characteristic which seems to be conserved in activity in Symbiodinium microadriaticum, Isochrysis galbana, Phaeodactylum tricornutum and Emiliania huxleyi. We, therefore, suggest that within the carbon concentrating mechanism, CO2 insertion processes can be deactivated when the organism faces pH 5.7.