Photosynthetic electron transfer and membrane energization in spheroplasts and membrane vesicles of the thermophilic cyanobacterium Synechoccus 6716.

The higher the incubation temperature, the higher the light intensity that membrane vesicles of the thermophilic cyanobacterium Synechococcus 6716 require for the saturation of O2-production. If membrane vesicles are incubated at temperatures at which intact cells are growing optimally, photosynthetic O2-production and membrane energization decrease rapidly, suggesting that the thermophilic properties are rapidly lost. If membrane integrity is maintained (spheroplasts) the harmful effect of higher temperatures is much less. The effects of 2,5-dibromo-3-methyl-6-isopropyl-p-benzo-quinone (DBMIB), 5-chloro-3-t-butyl-2'-chloro-4'-nitrosalicylanilide (S-13), 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and N,N'-dicyclohexylcarbodiimide (DCCD) are the same as in chloroplasts, be it that DCCD acts as an electron transfer inhibitor at higher concentrations. The supposed alternative site of DCMU inhibition in cyanobacteria is rejected.Spheroplasts show a reversible energy-dependent fluorescence quenching of 9-amino-6-chloro-2-methoxyacridine (ACMA) caused by illumination. ATP hydrolysis only give rise to fluorescence quenching in membrane vesicles. Long incubation at higher temperatures reduces the fluorescence quenching of membrane vesicles and spheroplasts, the latter being more stable than the former.