Abstract
After termination of longer-illumination (more than 30 s), the wild type of Synechocystis PCC 6803 showed the oscillation kinetics of post-illumination increase in Chl fluorescence: a fast phase followed by one or two slow phases. Unlike the wild type, ndh-B defective mutant M55 did not show any post-illumination increase under the same conditions, indicating that not only the fast phase, but also the slow phases were related to the NDH-mediated cyclic electron flow around photosystem I (PS I) to plastoquinone (PQ). The fast phase was stimulated by dark incubation or in the presence of Calvin cycle inhibitor, iodoacetamide (IA) or cyclic photophosphorylation cofactor, phenazine methosulphate (PMS), implying the redox changes of PQ by electrons generated at PS I reduced side, probably NAD(P)H or ferredoxin (Fd). In contrast, the slow phases disappeared after dark starvation or in the presence of IA or PMS, and reappeared by longer re-illumination, suggesting that they are related to the redox changes of PQ by the electrons from the photoreductants produced in carbon assimilation process. Both the fast phase and slow phases were stimulated at high temperature and the slow phase was promoted by response to high concentration of NaCl. The mutant M55 without both phases could not survive under the stressed conditions.
Highlights
Similar to eukaryotic phototrophs, cyanobacteria carry out oxygen photosynthesis with two distinguishable photosystems, photosystem I (PS I) and photosystem II (PSII; Scherer, 1990)
The phenomenon was explained as the reduction of plastoquinone (PQ) by the electrons from photoreductants accumulated in the stroma or cytosol during illumination, and may reflect cyclic electron transport around PS I mediated by NDH-1 (Mi et al, 1995) and by plastid NADH dehydrogenase-like complex (NDH; Sazanov et al, 1998; Shikanai et al, 1998)
NdhB defective mutant M55 did not show any post-illumination increase under the same conditions (Figure 1), indicating that fast phase, and slow phases related to the reduction of PQ by the NDH-mediated cyclic electron flow
Summary
Cyanobacteria carry out oxygen photosynthesis with two distinguishable photosystems, photosystem I (PS I) and photosystem II (PSII; Scherer, 1990). Cyclic electron flow around PS I is considered physiological important for its providing extra ATP for carbon assimilation (Schurman et al, 1972; Mills et al, 1978; Slovacek et al, 1978), and for its adjusting the production ratio of ATP to NADPH for developmental stages (Hatch, 1987), environmental stress (Mi et al, 2001; Wang et al, 2006), and Oscillation of Chl Fluorescence physiological requirements (Xu et al, 2014) Since lacking organelles such as chloroplasts and mitochondria, respiratory electron transport chain of cyanobacteria couples with photosynthetic intersystem chain by sharing some components (Jones and Myers, 1963). The respiratory electron transport which can produce ATP in darkness and remove oxygen plays an important role in nitrogen fixation (Scherer et al, 1988)
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