Polysulfides as Intermediates in the Oxidation of Sulfide to Sulfate by Beggiatoa spp

Zero-valent sulfur is a key intermediate in the microbial oxidation of sulfide to sulfate. Many sulfide-oxidizing bacteria produce and store large amounts of sulfur intra- or extracellularly. It is still not understood how the stored sulfur is metabolized, as the most stable form of S0 under standard biological conditions, orthorhombic α-sulfur, is most likely inaccessible to bacterial enzymes. Here we analyzed the speciation of sulfur in single cells of living sulfide-oxidizing bacteria via Raman spectroscopy. Our results showed that under various ecological and physiological conditions, all three investigatedBeggiatoa strains stored sulfur as a combination of cyclooctasulfur (S8 ) and inorganic polysulfides (Sn 2− ). Linear sulfur chains were detected during both the oxidation and reduction of stored sulfur, suggesting that Sn 2− species represent a universal pool of bioavailable sulfur. Formation of polysulfides due to the cleavage of sulfur rings could occur biologically by thiol-containing enzymes or chemically by the strong nucleophile HS− asBeggiatoa migrates vertically between oxic and sulfidic zones in the environment. MostBeggiatoa spp. thus far studied can oxidize sulfur further to sulfate. Our results suggest that the ratio of produced sulfur and sulfate varies depending on the sulfide flux. Almost all of the sulfide was oxidized directly to sulfate under low-sulfide-flux conditions, whereas only 50% was oxidized to sulfate under high-sulfide-flux conditions leading to S0 deposition. With Raman spectroscopy we could show that sulfate accumulated inBeggiatoa filaments, reaching intracellular concentrations of 0.72 to 1.73 M.