Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Photosynthetic acclimation to low temperature has been studied extensively. However, since low temperature causes a preferential decrease in the rates of biochemical as compared to photochemical reactions, some alterations brought about by acclimation to low temperature may be the result of the light-harvesting apparatus absorbing more light energy than can be readily dissipated through photosynthesis. This possibility was investigated by using two species of green algae, Chlorella vulgaris and Dunaliella salina and the techniques of chlorophyll a fluorescence, O{dollar}\sb2{dollar}-evolution, protein electrophoresis and Western blotting, high performance liquid chromatography, RNA blot hybridization, and enzyme and metabolite assays. Compared to algae grown at high temperature and moderate irradiance, acclimation to low temperature resulted in a 5-8 fold lower chlorophyll content, a 2-3 fold higher ratio of chlorophyll a/b, increased carotenoid content, lower abundance of light-harvesting polypeptides and cab mRNA, and an acquired resistance to photoinhibition. However, these alterations were found not to be due to low temperature per se. In fact, adjustment in the structure and function of the photosynthetic apparatus upon growth of cells under a low temperature/low light regime mimicked growth under a high temperature/moderate light regime. Similarly, growth of cells under a low temperature/moderate light regime mimicked growth at high temperature and high light.;These results were rationalized in terms of changes in photosystem II excitation pressure which reflects modulation of photosynthetic redox poise. It was shown that growth of cells at either low temperature/moderate light or high temperature/high light caused an increase in the proportion of Q{dollar}\rm \sb{lcub}A{rcub},{dollar} the first stable electron acceptor of PSII, in the reduced state (high excitation pressure) compared to algae growth at either moderate temperature/moderate irradiance or low temperature/low irradiance (low excitation pressure). Increased reduction of PSII was shown to be, in part, due to feedback limitation at the level of photosynthetic carbon metabolism.;These results indicate that functional and structural alterations previously ascribed to changes in either temperature or irradiance are actually the result of altered photosynthetic redox poise. Photosynthetic redox poise represents a central component of the signal transduction pathway which regulates the structure and function of the photosynthetic apparatus.



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