Cyclic electrontransfer pathways in SYNECHOCOCCUS Sp. PCC 7002 cyanobacteria during photosynthesis at high light intensity

Electron transport;Photosynthesis;Cyanobacteria

Marathe, AnuradhaKallas, Toivo ;

University of Wisconsin

关键词: Electron transport;    Photosynthesis;    Cyanobacteria;   

Others  :  https://minds.wisconsin.edu/bitstream/handle/1793/60985/Marathe%2c_Anuradha.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: University of Wisconsin

【 摘 要 】

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 eukaryoticalgae. Cyanobacteria and microalgae can thus potentially produce biofuels in aneconomical and environmentally sustainable manner at rates sufficient to replace asubstantial fraction of fossil fuels. To employ cyanobacteria for biofuels, a detailedknowledge of photosynthetic electron pathways is required. Linear electron flow fromphotosystem II (PSII) via the plastoquinone (PQ) pool, cytochrome (Cyt) bf complex, andphotosystem I (PSI) generates ATP and NADPH. Cyclic electron flow around PSI andthe Cyt bf complex generates ATP only, provides the ;;extra;; ATP for efficient CO2fixation, and is implicated in defenses against photodamage. Cyclic electron flowmediated by the NAD(P)H dehydrogenase (NDH-1) complex is the major, known cyclicpathway in cyanobacteria. In plant chloroplasts, a PSI-Cyt bf supercomplex catalyzescyclic flow. Such a supercomplex has not been identified in cyanobacteria and thecontributions of linear and cyclic electron flow under different environmental conditionsremain poorly understood. In this thesis, the fast-growing, high-light tolerant, marinecyanobacterium, Synechococcus sp. PCC 7002 and two mutants, NdhF (lacking theNDH-1 complex) and PetB-R214H (impaired electron flow in the Cyt bf complex) wereinvestigated with respect to cyclic electron transfer pathways under optimal and high,full-sunlight conditions. PSI and Cyt bf kinetics were studied with pump-probe, kineticsspectrophotometer (Biologic JT-10) that can monitor light-induced redox changes in thephotosynthetic apparatus of living cells. The NDH-I route accounted for most of thecyclic flow (~10% of the total) under optimal light as observed previously. At high lightintensity, PSI content decreased but cyclic electron flow increased dramatically in boththe wild type and NdhF mutant. Most interestingly, in the NdhF mutant at high lightintensity, cyclic electron flow accounted for 50% or more of total electron flow. Thesedata 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. PCC7002 cyanobacteria at extreme, high-light intensities.