Light-induced and osmotically-induced changes in chlorophyll a fluorescence in two Synechocystis sp PCC 6803 strains that differ in membrane lipid unsaturation

Membranes of wild-type (WT) cells of the cyanobacterium Synechocystis sp. P CC 6803 are abundant in polyunsaturated fatty acids in membrane lipids and thus more fluid than membranes of desA(-)/desD(-) mutant cells which contai n no polyunsaturated fatty acids. Using intact cells we examined the effect s of normal and chilling temperatures on membrane fluidity-dependent proper ties. We probed the thylakoid membranes by inducing light/dark acclimative changes in chlorophyll a (Chl a) fluorescence; and we probed the plasma mem branes either by suppressing the Chl a fluorescence of light-acclimated cel ls under hyper-osmotic conditions, or by measuring the electric conductivit y of cell suspensions. Thylakoid membranes of mutant cells undergo reversib le thermotropic transition between 19 degrees C and 22 degrees C (midpoint at 20.5 degrees C). No analogous transition was detected in the thylakoid m embranes of WT cells in the temperature range from 2 to 34 degrees C. Plasm a membranes of both WT and mutant cells did not experience thermotropic tra nsition in the temperature range from 2 degrees C to 34 degrees C as detect ed either fluorimetrically or by means of electric conductivity. Hyper-osmo tic conditions caused fast transient fluorescence quenching in WT cells at 34 degrees C, but not at 14 OC, and not in mutant cells at either 34 degree s C or 14 degrees C. This transient quenching sensed probably the higher fl uidity of the plasma membranes of WT cells. Hyper-osmotic media and dark ac climation had similar effects on the 77 K fluorescence of Synechocystis cel ls: they suppressed the ratio of photosystem II fluorescence to photosystem I fluorescence.