HEAT-INDUCED ALTERATIONS IN THE PHOTOSYNTHETIC ELECTRON-TRANSPORT AND EMISSION PROPERTIES OF THE CYANOBACTERIUM SPIRULINA-PLATENSIS

Heat-stress-induced photosynthetic electron transport and emission properties were studied in the cyanobacterium Spirulina platensis. Heat treatment of intact cells up to 50-degrees-C did not cause major changes in the absorption and emission properties of both chlorophyll a and phycocyanin. However, above 50-degrees-C, there was a specific bleaching of phycobiliproteins and an uncoupling of energy transfer in phycobilisomes. Heat stress also reduced the extent and slowed down the decay kinetics of light-induced quenching of the long wavelength emission band which has been shown to be associated with the redox state of P 700, the primary donor of photosystem I. Electron transport activities measured in intact cells showed a decline in the photosystem II mediated Hill activity and an increase in the photosystem I activity with increasing temperatures. However, isolated thylakoid membranes did not exhibit heat-induced stimulation in photosystem I activity. This indicates that the enhancement of photosystem I activity in intact cells is mostly due to increased permeability of cells for the entry of acceptors and donors upon heat treatment. However, mild heat treatments induced damage at the plastoquinone pool, as indicated by the inhibition in the durohydroquinone to methylviologen intersystem electron flow. These results suggest that unlike higher plants, the thylakoid membranes of the cyanobacterium Spirulina platensis do not show heat-induced stimulation in photosystem I activity. We argue that the lack of heat-induced stimulation in photosystem I activity in this cyanobacterium may arise as a result of the absence of light harvesting chlorophyll a/b complex and also variations in the membrane lipid organizations.