Analysis of PSI and PSII driven electron transportduring steady-state photosynthesis in the cyanobacteriumSynechococcus PCC7942

Abstract

Photosynthetic electron transport is required for the transport of CO2 and HCO3- in Synechococcus PCC 7942. This transport supports the CO2 concentrating mechanism in cyanobacteria. The roles that linear and cyclic electron flow play in the energization of these processes is unclear. This study describes a system for measuring the quantum yields of photochemistry in both photosystems and, thus, the extent of electron flow, during steady-state cyanobacterial photosynthesis. These measurements were made in a custom-built cuvette coupled by a membrane inlet to a mass spectrometer enabling the concomitant measurement of photosynthetic gas exchange. In wild-type cells, the relationship between f PSI and f PSII was found to be linear between the CO2 compensation point and to approximately ¼ Vmax. The ratio of f PSI/f PSII was always greater than 1 and increased as net photosynthesis approached zero. This was suggestive of significant cyclic electron transport around PSI which became more pronounced as the rate of Ci supply decreased. Whether CO2 or HCO3- was used to drive photosynthesis the relationship between f PSI and f PSII was the same indicative of similar rates of linear and cyclic electron transport. Since we found no greater engagement of f PSI in the presence of enhanced CO2 supply we could not provide evidence for a model of CO2 transport coupled exclusively to cyclic electron transport. A double mutant (ndhD3-/ndhD4-) of Synechococcus PCC7942 in which CO2 transport is absent showed an alteration in electron transport properties relative to wild-type cells.