Abstract
Green sulfur bacteria (GSB) constitute a closely related group of photoautotrophic and thiotrophic bacteria with limited phenotypic variation. They typically oxidize sulfide and thiosulfate to sulfate with sulfur globules as an intermediate. Based on genome sequence information from 15 strains, the distribution and phylogeny of enzymes involved in their oxidative sulfur metabolism was investigated. At least one homolog of sulfide:quinone oxidoreductase (SQR) is present in all strains. In all sulfur-oxidizing GSB strains except the earliest diverging Chloroherpeton thalassium, the sulfide oxidation product is further oxidized to sulfite by the dissimilatory sulfite reductase (DSR) system. This system consists of components horizontally acquired partly from sulfide-oxidizing and partly from sulfate-reducing bacteria. Depending on the strain, the sulfite is probably oxidized to sulfate by one of two different mechanisms that have different evolutionary origins: adenosine-5′-phosphosulfate reductase or polysulfide reductase-like complex 3. Thiosulfate utilization by the SOX system in GSB has apparently been acquired horizontally from Proteobacteria. SoxCD does not occur in GSB, and its function in sulfate formation in other bacteria has been replaced by the DSR system in GSB. Sequence analyses suggested that the conserved soxJXYZAKBW gene cluster was horizontally acquired by Chlorobium phaeovibrioides DSM 265 from the Chlorobaculum lineage and that this acquisition was mediated by a mobile genetic element. Thus, the last common ancestor of currently known GSB was probably photoautotrophic, hydrogenotrophic, and contained SQR but not DSR or SOX. In addition, the predominance of the Chlorobium–Chlorobaculum–Prosthecochloris lineage among cultured GSB could be due to the horizontally acquired DSR and SOX systems. Finally, based upon structural, biochemical, and phylogenetic analyses, a uniform nomenclature is suggested for sqr genes in prokaryotes.
Highlights
Green sulfur bacteria (GSB; order Chlorobiales) are strictly anaerobic, photosynthetic bacteria that predominantly oxidize reduced sulfur compounds for photoautotrophic growth (Overmann, 2000, 2008; Garrity and Holt, 2001)
I. album is a non-phototrophic relative of GSB, whose genome has recently been sequenced to reveal more about the physiology and evolution of this organism
Phylogeny of ribosomal RNA and core metabolic functions Analyses of the SSU rRNA gene and several cellular core proteins in the GSB strains with sequenced genomes revealed an overall congruent phylogeny (Figures 2A,B), which was in agreement with previous analyses performed on a smaller number of strains (Bryant et al, 2011)
Summary
Green sulfur bacteria (GSB; order Chlorobiales) are strictly anaerobic, photosynthetic bacteria that predominantly oxidize reduced sulfur compounds for photoautotrophic growth (Overmann, 2000, 2008; Garrity and Holt, 2001). They occur in anoxic, aquatic environments where light and sulfide coincide. GSB constitute the only cultured members of the order Chlorobiales, and the phylum Chlorobi was proposed based on GSB (Garrity and Holt, 2001; Imhoff, 2008). Preliminary analyses suggest that these organisms are photoheterotrophs that are physiologically distinct from well-studied Chlorobiales (Bryant et al, 2011)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
