Involvement of Intermediate Sulfur Species in Biological Reduction of Elemental Sulfur under Acidic, Hydrothermal Conditions

Abstract

ABSTRACTThe thermoacidophile and obligate elemental sulfur (S80)-reducing anaerobeAcidilobus sulfurireducens18D70 does not associate with bulk solid-phase sulfur during S80-dependent batch culture growth. Cyclic voltammetry indicated the production of hydrogen sulfide (H2S) as well as polysulfides after 1 day of batch growth of the organism at pH 3.0 and 81°C. The production of polysulfide is likely due to the abiotic reaction between S80and the biologically produced H2S, as evinced by a rapid cessation of polysulfide formation when the growth temperature was decreased, inhibiting the biological production of sulfide. After an additional 5 days of growth, nanoparticulate S80was detected in the cultivation medium, a result of the hydrolysis of polysulfides in acidic medium. To examine whether soluble polysulfides and/or nanoparticulate S80can serve as terminal electron acceptors (TEA) supporting the growth ofA. sulfurireducens, total sulfide concentration and cell density were monitored in batch cultures with S80provided as a solid phase in the medium or with S80sequestered in dialysis tubing. The rates of sulfide production in 7-day-old cultures with S80sequestered in dialysis tubing with pore sizes of 12 to 14 kDa and 6 to 8 kDa were 55% and 22%, respectively, of that of cultures with S80provided as a solid phase in the medium. These results indicate that the TEA existed in a range of particle sizes that affected its ability to diffuse through dialysis tubing of different pore sizes. Dynamic light scattering revealed that S80particles generated through polysulfide rapidly grew in size, a rate which was influenced by the pH of the medium and the presence of organic carbon. Thus, S80particles formed through abiological hydrolysis of polysulfide under acidic conditions appeared to serve as a growth-promoting TEA forA. sulfurireducens.