Electrochemical investigation of cyanobacteria Synechococcus sp. PCC7942-catalyzed photoreduction of exogenous quinones and photoelectrochemical oxidation of water

The electron transfer from the photosynthetic system in cyanobacteria, Synechococcus sp. PPC7942 to exogenous electron acceptors was examined using several electrochemical techniques. 1,4-Benzoquinone (BQ) and 2,6-dimethyl-1,4-benzoquinone (DMBQ) were found to function as good exogenous electron acceptors for the photosystem. Kinetic analysis with rotating disk amperometry revealed that the photoreduction of these quinones proceeds in Michaelis–Menten type kinetics for the concentration of the quinones and the light intensity. The electron transfer rate of the BQ reduction was as high as 68% compared with that of the photosynthetic oxygen evolution. Synechococcus sp. cell-entrapped and DMBQ-embedded carbon paste electrodes provided steady-state current ascribed to the photoelectrochemical oxidation of water. Although several inhibitors against the photosynthetic system suppressed the photoelectrochemical response, phenylmercury acetate, which inhibits ferredoxin and ferredoxin–NADP oxidoreductase, was found to enhance the photocurrent. Some electrochemical aspects of this system are discussed.