Cyclic electron transfer pathways in SYNECHOCOCCUS Sp. PCC 7002 cyanobacteria during photosynthesis at high light intensity
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With current global warming, there is growing interest in coupling carbon dioxide (CO2) capture to chemical synthesis via photosynthesis. Cyanobacteria convert up to 10% of the sun's energy into biomass compared to 1% by energy crops and 5% by eukaryotic algae. Cyanobacteria and microalgae can thus potentially produce biofuels in an economical and environmentally sustainable manner at rates sufficient to replace a substantial fraction of fossil fuels. To employ cyanobacteria for biofuels, a detailed knowledge of photosynthetic electron pathways is required. Linear electron flow from photosystem II (PSII) via the plastoquinone (PQ) pool, cytochrome (Cyt) bf complex, and photosystem I (PSI) generates ATP and NADPH. Cyclic electron flow around PSI and the Cyt bf complex generates ATP only, provides the 'extra' ATP for efficient CO2 fixation, and is implicated in defenses against photodamage. Cyclic electron flow mediated by the NAD(P)H dehydrogenase (NDH-1) complex is the major, known cyclic pathway in cyanobacteria. In plant chloroplasts, a PSI-Cyt bf supercomplex catalyzes cyclic flow. Such a supercomplex has not been identified in cyanobacteria and the contributions of linear and cyclic electron flow under different environmental conditions remain poorly understood. In this thesis, the fast-growing, high-light tolerant, marine cyanobacterium, Synechococcus sp. PCC 7002 and two mutants, NdhF (lacking the NDH-1 complex) and PetB-R214H (impaired electron flow in the Cyt bf complex) were investigated with respect to cyclic electron transfer pathways under optimal and high, full-sunlight conditions. PSI and Cyt bf kinetics were studied with pump-probe, kinetics spectrophotometer (Biologic JT-10) that can monitor light-induced redox changes in the photosynthetic apparatus of living cells. The NDH-I route accounted for most of the cyclic flow (~10% of the total) under optimal light as observed previously. At high light intensity, PSI content decreased but cyclic electron flow increased dramatically in both the wild type and NdhF mutant. Most interestingly, in the NdhF mutant at high light intensity, cyclic electron flow accounted for 50% or more of total electron flow. These data suggest that this efficient cyclic electron flow is catalyzed by the formation of a PSI-Cyt bf supercomplex required for adaptation and growth of Synechococcus sp. PCC 7002 cyanobacteria at extreme, high-light intensities.