NADPH production in dark stages is critical for cyanobacterial photocurrent generation: a study using mutants deficient in oxidative pentose phosphate pathway

Live cyanobacteria and algae integrated onto an extracellular electrode can generate a light-induced current (i.e., a photocurrent). Although the photocurrent is expected to be correlated with the redox environment of the photosynthetic cells, the relationship between the photocurrent and the cellular redox state is poorly understood. Here, we investigated the effect of the reduced nicotinamide adenine dinucleotide phosphate [NADP(H)] redox level of cyanobacterial cells (before light exposure) on the photocurrent using several mutants (Delta zwf, Delta gnd, and Delta glgP) deficient in the oxidative pentose phosphate (OPP) pathway, which is the metabolic pathway that produces NADPH in darkness. The NAD(P)H redox level and photocurrent in the cyanobacterium Synechocystis sp. PCC 6803 were measured noninvasively. Dysfunction of the OPP pathway led to oxidation of the photosynthetic NADPH pool in darkness. In addition, photocurrent induction was retarded and the current density was lower in Delta zwf, Delta gnd, and Delta glgP than in wild-type cells. Exogenously added glucose compensated the phenotype of Delta glgP and drove the OPP pathway in the mutant, resulting in an increase in the photocurrent. The results indicated that NADPH accumulated by the OPP pathway before illumination is a key factor for the generation of a photocurrent. In addition, measuring the photocurrent can be a non-invasive approach to estimate the cellular redox level related to NADP(H) pool in cyanobacteria.

Automated summary

In this study, the effect of the reduced nicotinamide adenine dinucleotide phosphate (NADP(H)) redox level on the photocurrent was investigated using mutants deficient in the oxidative pentose phosphate (OPP) pathway. The results showed that dysfunction of the OPP pathway led to oxidation of the photosynthetic NADPH pool in darkness, resulting in a decrease in photocurrent induction and current density. Exogenously added glucose compensated the phenotype of the mutants and increased the photocurrent. Therefore, measuring the photocurrent can be a non-invasive approach to estimate the cellular redox level related to NADP(H) pool in cyanobacteria.