Acclimation of the filamentous cyanobacterium Plectonema boryanum UTEX 485 to temperature and irradiance

Changes in environmental conditions such as temperature and irradiance may result in an imbalance between energy absorbed through photochemistry and energy utilized through metabolism. Photosynthetic organisms have developed a variety of mechanisms to maintain a balance in energy budget and avoid photodamage of the photosynthetic apparatus. The objective of this thesis was to investigate the strategies utilized by cyanobacteria for photosynthetic acclimation to growth under either low temperature- or high light-induced imbalances in energy budget. The filamentous cyanobacterium Plectonema boryanum UTEX 485 responds to changes in energy balance by adjusting the capacity for light absorption through a decrease in cellular levels of light harvesting pigments and a concomitant accumulation of myxoxanthophyll in the cell envelope. Myxoxanthophyll may screen the photosynthetic apparatus from the excess radiation and protect it against photoinhibition. Alterations in the efficiency of the excitation energy transfer from phycobilisome to PSII reaction centres and in the distribution of excitation energy between the two photosystems due to changes in photosystem stoichiometry and antenna size appear to play an important role in the maintenance of energy balance. In addition, alternative electron transport pathways, such as NADH dehydrogenase-mediated electron supply from cytosolic sources and cyclic electron transport around photosystem I may be essential for the acclimation of P. boryanum to imbalances in energy budget. A sensing/signalling mechanism for these acclimatory responses appears to involve the photosynthetic electron transport components downstream of the plastoquinone pool.