Biological and abiological sulfur reduction at high temperatures.

Reduction of elemental sulfur was studied in the presence and absencè of thermophilic sulfur-reducing bacteria, at temperatures ranging from 65 to 110 degrees C, in anoxic artificial seawater media. Above 80 degrees C, significant amounts of sulfide were produced abiologically at linear rates, presumably by the disproportionation of sulfur. These rates increased with increasing temperature and pH and were enhanced by yeast extract. In the same medium, the sulfur respiration of two recent thermophilic isolates, a eubacterium and an archaebacterium, resulted in sulfide production at exponential rates. Although not essential for growth, sulfur increased the cell yield in both strains up to fourfold. It is suggested that sulfur respiration is favored at high temperatures and that this process is not limited to archaebacteria, but is shared by other extreme thermophiles.