Abstract
The anaerobic bacteriumChlorobium assimilates carbon dioxide in the light with various sulfur compounds as electron donors. The well-known metabolic pathway proceeds from the oxidation of sulfide via sulfur to sulfate. In the dark the reaction is partially reversed when sulfur is reduced to hydrogen sulfide. The fermenting cells thereby release an excess of reductant. We have now found a hydrogen sulfide production from sulfur, which is light-dependent. It is more than ten times faster than the dark reaction. This appears in experiments where the cell suspension is illuminated in absence of CO2 and flushed continuously with H2 or Ar. The H2S is trapped with ZnCl2 and the S2- titrated with iodine. The total amount of H2S evolved in the light increases proportionally with the amount of sulfur added, and about one-half of the added sulfur is converted to H2S. Another part of the metabolized sulfur appears at the same time as sulfate, but all the sulfur oxidized to sulfate does not account for the larger amount of sulfur reduced to hydrogen sulfide. Very likely other unanalyzed oxidized sulfur compounds must also have been produced.
Use of H2 instead of Ar as the anaerobic gas phase does not increase the amount of H2S produced, nor does the addition of thiosulfate; sulfur itself is the preferred electron donor for the sulfur reduction. Up to a light intensity of 10000 ergs cm-2sec-1 CO2 does not affect H2S production. Without CO2, saturation of the light-dependent evolution of H2S is reached at about 40000 ergs cm-2sec-1. In contrast, presence of CO2 at this light intensity makes the sulfide production disappear completely. On application of mass spectrometry to the gas exchange upon illumination, at high light intensity a H2S gush is found during the first 3 min. This is followed by CO2 fixation, while simultaneously the reductant H2S is now taken up.
WithRhodospirillum rubrum, the addition of sulfur leads to a moderate evolution of H2S. In contrast toChlorobium this reaction inR. rubrum is not light-sensitive, nor does it produce detectable amounts of sulfate. After addition of malate the rate of H2S evolution does increase in the light, since the cells use malate as an electron donor during their photochemical metabolism.
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Paschinger, H., Paschinger, J. & Gaffron, H. Photochemical disproportionation of sulfur into sulfide and sulfate byChlorobium limicola formathiosulfatophilum . Arch. Microbiol. 96, 341–351 (1974). https://doi.org/10.1007/BF00590189
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DOI: https://doi.org/10.1007/BF00590189