Characterisation of CO2 and HCO3- uptake in the cyanobacterium Synechocystis sp PCC6803

The availability of a complete genome database for the cyanobacterium Synechocystis sp. PCC6803 (glucose-tolerant strain) has raised expectations that this organism would become a reference strain for work aimed at understanding the CO2-concentrating mechanism (CCM) in cyanobacteria. However, the amount of physiological data available has been relatively limited. In this report we provide data on the relative contributions of net HCO3- uptake and CO2 uptake under steady state photosynthetic conditions. Cells were compared after growth at high CO2 (2% v/v in air) or limiting CO2 conditions (20 ppm CO2). Synechocystis has a very high dependence on net HCO3- uptake at low to medium concentrations of inorganic carbon (Ci). At high Ci concentrations net CO2 uptake became more important but did not contribute more than 40% to the rate of photosynthetic O-2 evolution. The data also confirm that high Ci cells of Synechocystis sp. PCC6803 possess a strong capacity for net HCO3- uptake under steady state photosynthetic conditions. Time course experiments show that induction of maximal Ci uptake capacity on a shift from high CO2 to low CO2 conditions was near completion by four hours. By contrast, relaxation of the induced state on return of cells to high CO2, takes in excess of 230 h. Experiments were conducted to determine if Synechocystis sp. PCC6803 is able to exhibit a 'fast induction' response under severe Ci limitation and whether glucose was capable of causing a rapid inactivation in Ci uptake capacity. Clear evidence for either response was not found.