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

The thermoacidophile and obligate elemental sulfur (S-8(0))-reducing anaerobe Acidilobus sulfurireducens 18D70 does not associate with bulk solid-phase sulfur during S-8(0)-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 degrees C. The production of polysulfide is likely due to the abiotic reaction between S-8(0) and 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 S-8(0) was detected in the cultivation medium, a result of the hydrolysis of polysulfides in acidic medium. To examine whether soluble polysulfides and/or nanoparticulate S-8(0) can serve as terminal electron acceptors (TEA) supporting the growth of A. sulfurireducens, total sulfide concentration and cell density were monitored in batch cultures with S-8(0) provided as a solid phase in the medium or with S-8(0) sequestered in dialysis tubing. The rates of sulfide production in 7-day-old cultures with S-8(0) sequestered 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 S-8(0) provided 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 S-8(0) particles 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, S-8(0) particles formed through abiological hydrolysis of polysulfide under acidic conditions appeared to serve as a growth-promoting TEA for A. sulfurireducens.