Redox changes accompanying inorganic carbon limitation in Synechocystis sp PCC 6803

Inorganic carbon (C-i) is the major sink for photosynthetic reductant in organisms capable of oxygenic photosynthesis. In the absence of abundant C-i, the cyanobacterium Synechocystis sp. strain PCC6803 expresses a high affinity C-i acquisition system, the CO2-concentrating mechanisms (CCM), controlled by the transcriptional regulator CcmR and the metabolites NADP(+) and alpha-ketoglutarate, which act as co-repressors of CcmR by modulating its DNA binding. The CCM thus responds to internal cellular redox changes during the transition from C-i-replete to C-i-limited conditions. However, the actual changes in the metabolic state of the NADPH/NADP(+) system that occur during the transition to C-i-limited conditions remain ill-defined. Analysis of changes in the redox state of cells experiencing C-i limitation reveals systematic changes associated with physiological adjustments and a trend towards the quinone and NADP pools becoming highly reduced. A rapid and persistent increase in F-0 was observed in cells reaching the C-i-limited state, as was the induction of photoprotective fluorescence quenching. Systematic changes in the fluorescence induction transients were also observed. As with Chl fluorescence, a transient reduction of the NADPH pool ('M' peak), is assigned to State 2-->State 1 transition associated with increased electron flow to NADP(+). This was followed by a characteristic decline, which was abolished by C-i limitation or inhibition of the Calvin-Benson-Bassham (CBB) cycle and is thus assigned to the activation of the CBB cycle. The results are consistent with the proposed regulation of the CCM and provide new information on the nature of the Chl and NADPH fluorescence induction curves. (C) 2014 Elsevier B.V. All rights reserved.