Mobilization of sulfur by green sulfur bacteria : physiological and molecular studies on Chlorobaculum parvum DSM 263

Abstract

Green sulfur bacteria are photolithotrophs that use inorganic sulfur compounds as electron donors for photosynthesis. Elemental, solid sulfur is one of the electron donors used. Sulfur is produced by green sulfur bacteria during the oxidation of sulfide to sulfate, and during the oxidation of thiosulfate to sulfur and sulfate. Green sulfur bacteria have been known for long, and the genomes of 12 strains have been sequenced. Yet, it is not clear how green sulfur bacteria can access elemental sulfur, which is practically insoluble at the temperatures at which most of them grow. The present work has been done using pure cultures of the mesophilic strain Chlorobaculum parvum DSM 263. By studying the dynamics of inorganic sulfur compounds in growing cultures of Cla. parvum DSM 263, it was shown that the sulfur produced during thiosulfate oxidative disproportionation can be either oxidized immediately to sulfate, or released outside the cells. Extracellular sulfur oxidation needs a dedicated system, which is not synthesised in the presence of thiosulfate. Experiments conducted at low light intensities revealed that thiosulfate is not consumed exclusively via the Sox multienzymatic system, known thus far as the only thiosulfate oxidizing system present in green sulfur bacteria. It is not clear yet if this additional pathway for thiosulfate consumption allows strain DSM 263 to obtain electrons for thiosulfate, or if it is used by bacteria to consume the reducing power that cells might have in excess. Physiological studies conducted on Cla. parvum DSM 263 fed with sulfur furnished evidence that the oxidation of elemental sulfur to sulfate is accompanied by the formation of yet unidentified sulfur compound(s). Similarly to what commented for thiosulfate consumption, these compounds could be intermediates in the oxidation of sulfur to sulfate, or could be side products of sulfur consumption. Essential elements for the utilisation of elemental sulfur were shown to be present in the membranes of Cla. parvum DSM 263. Differential membrane proteomic studies were thus performed on cells of Cla. parvum DSM 263 grown on sulfur or on sulfide. Seven proteins were found to be overexpressed in sulfuric conditions, revealing that bacteria do possess different proteomic equipment for oxidizing the insoluble sulfur rather than the soluble sulfide. Identification of these overexpressed proteins and of their interaction partners will probably help revealing the cellular apparatus of extracellular solid sulfur mobilization.