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 CO ₂ (2% v/v in air) or limiting CO₂ conditions (20 ppm CO₂). Synechocystis has a very high dependence on net HCO ₃^- 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 O₂ 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.