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Abstract:
Sulfur storage in bacteria was first documented by Sergei Winogradsky over a century ago, and yet our understanding of the chemical nature of stored sulfur is still fairly limited. The most thermodynamically stable form of sulfur under standard, biological conditions is orthorhombic α-sulfur, which consists of puckered S8 rings (Roy and Trudinger 1970), but is not likely that bacterial enzymes can utilize such a large hydrophobic molecule. Moreover, organisms utilizing the reverse-acting, cytoplasmic Dsr complex transport sulfur from extracellular or periplasmic storage vesicles across one or more membranes via an unknown form (reviewed in Ghosh and Dam 2009). In this study, Raman spectroscopy was used to investigate the link between sulfur speciation and enzymatic pathways of sulfur utilization in ecologically and physiologically distinct Beggiatoa and Thiomargarita. Results showed that under microaerobic conditions and circumneutral pH stored sulfur exists in an equilibrium distribution of S8 rings and inorganic polysulfides. These linear Sn2- chains were formed as intermediates during both sulfur oxidation and reduction. Sulfur rings could be activated either biologically via membrane-bound thiol groups and glutathione or chemically by the strong nucleophile HS¯. Raman spectroscopy was also used to identify other cellular compounds such as polyhydroxyalkanoates, cytochromes, and sulfate. Unexpectedly high concentrations of internal sulfate (up to 2 M) were detected in Beggiatoa, though the function of the sulfate remains unknown.
