Abstract
Core sets of sox genes were detected in several genome sequenced members of the environmental important OM60/NOR5 clade of marine gammaproteobacteria. However, emendation of media with thiosulfate did not result in stimulation of growth in two of these strains and cultures of Congregibacter litoralis DSM 17192T did not oxidize thiosulfate to sulfate in concentrations of one mmol L−1 or above. On the other hand, a significant production of sulfate was detected upon growth with the organic sulfur compounds, cysteine and glutathione. It was found that degradation of glutathione resulted in the formation of submillimolar amounts of thiosulfate in the closely related sox-negative strain Chromatocurvus halotolerans DSM 23344T. It is proposed that the Sox multienzyme complex in Congregibacter litoralis and related members of the OM60/NOR5 clade is adapted to the oxidation of submillimolar amounts of thiosulfate and nonfunctional at higher concentrations of reduced inorganic sulfur compounds. Pelagic bacteria thriving in the oxic zones of marine environments may rarely encounter amounts of thiosulfate, which would allow its utilization as electron donor for lithoautotrophic or mixotrophic growth. Consequently, in evolution the Sox multienzyme complex in some of these bacteria may have been optimized for the effective utilization of trace amounts of thiosulfate generated from the degradation of organic sulfur compounds.
Highlights
Aerobic marine gammaproteobacteria affiliated to the OM60/NOR5 clade are widespread in saline environments and of ecological importance in several euphotic coastal environments [1]
Aerobic marine bacteria may benefit from sox genes in several ways that are independent of the well-known lithotrophic oxidation of thiosulfate to sulfate
In a previous study it was assumed that the absence of growth stimulation by thiosulfate in cultures of C. litoralis could be due to the missing of putative essential sox genes, like, Table 1: Growth response of several type strains of the OM60/NOR5 clade in batch culture with and without thiosulfate
Summary
Aerobic marine gammaproteobacteria affiliated to the OM60/NOR5 clade are widespread in saline environments and of ecological importance in several euphotic coastal environments [1]. Several pathways for the oxidation of reduced sulfur compounds to sulfate are known in bacteria, but most knowledge exists about a thiosulfate oxidizing multienzyme complex, which is encoded by a set of sulfur oxidizing (sox) genes [3]. It turned out that sox genes are present mainly in members of the OM60/NOR5 clade that encode genes enabling aerobic anoxygenic photoheterotrophy, like Congregibacter litoralis (C. litoralis) DSM 17192T, Congregibacter sp. It was ISRN Microbiology shown that in C. litoralis emendation of cultivation media with thiosulfate did not stimulate growth [8] This was unexpected, because in marine members of the Roseobacter clade that encode a complete set of sox genes mixotrophic growth with thiosulfate as additional energy source could be demonstrated [9]. To get a clue about a yet unknown function of sox genes in aerobic marine gammaproteobacteria a study was initiated in which the sulfur metabolism in C. litoralis was analyzed in detail and compared with closely related species lacking a Sox complex
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