Cyclic electron transfer pathways in SYNECHOCOCCUS Sp. PCC 7002 cyanobacteria during photosynthesis at high light intensity
File(s)
Date
2012-01Author
Marathe, Anuradha
Advisor(s)
Kallas, Toivo
Metadata
Show full item recordAbstract
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.
Subject
Electron transport
Photosynthesis
Cyanobacteria
Permanent Link
http://digital.library.wisc.edu/1793/60985Description
A Thesis Submitted In Partial Fulfillment of the Requirements For the Degree of Master of Science-Biology Microbiology