The quantum efficiency of photosynthesis by the green sulfur bacterium, Chlorobium thiosulfatophilum, has been determined in systems in which thiosulfate, tetrathionate, and molecular hydrogen served as electron donors. It was found that about 10 ± 1 quanta are used for the assimilation of 1 molecule of CO2, and that the quantum number is independent of the nature of the electron donor. These results are considered as support for the view that also in the bacterial photosyntheses the primary photochemical reaction consists in the photolysis of H2O, and that the chemical energy released during the oxidation of the electron donor is not utilized for CO2 assimilation. Hence the photosynthetic processes of the green sulfur bacteria are thermodynamically less efficient than is green plant photosynthesis.

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