Studies on the metabolism of a sulfuroxidizing bacterium


1. The cells of Thiobacillus thiooxidans, which had been in contact with sulfur or sulfide in air (or COn-free air), could fix added COa very rapidly after replacing air with nitrogen. This fixation is designated as the postoxidative fixation. 2. " Preoxidation " of the sulfur compounds is mandatory for the occurrence of the postoxidative fixation. The cells which had preliminarily oxidized sulfide could not show the COa-fixation, when they were placed under an anaerobic con-dition in the absence of the sulfur compound. 3. These results indicate that sulfur compounds may have an important role as the electron donor for the reduction of CO=, besides their role as the substrate of respiration to secure energy for the fixation of CO2. In 1942, VOGLER and UMBREIT (1) studied the mechanism of CO2-fixation by an autotrophic bacterium, Thiobacillus thiooxidans, and reported that the cells which incorporated inorganic phosphate from the medium during the sulfur oxidation in COj-free air, could fix a noticeable amount of CO2 under the anaerobic atmosphere with a release of phosphate into the medium. They concluded that the capacity for fixing carbon dioxide could be kept in the cells after the sulfur oxida-tion was over. This fixation was named "delayed fixation " by them. The delayed fixation, however, has been questioned by BAALSRUD and BAALSRUD (2) who reported that T. thiooxidans fixed carbon dioxide during the thiosulfate oxidation, but no persisting fixation was shown to occur when the thiosulfate oxidation was over. According to NEW-BURGH (3), the sulfur oxidation was not coupled with phosphorylation in COz-free air, and the delayed fixation was not detected, but the cells, Abbreviation: TCA, trichloroacetic acid

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