(also thiobacterium), any of the microorganisms that oxidize reduced compounds of sulfur. The study of sulfur bacteria served as the basis for establishing che-mosynthesis by S. N. Vinogradskii in 1887. Sulfur bacteria include the photosynthesizing purple and green bacteria that oxidize hydrogen sulfide anaerobically in the light, thiobacteria, and sulfur bacteria proper—colorless microorganisms having sulfur inclusions in their cells. Sulfur bacteria develop mostly on the surface of hydrogen sulfide silt, where the reduction of sulfates occurs. Most frequently encountered are the filamentous (Beggiatoa, Thiothrix) and the unicellular (Thiospira, Thio-vulum, Macromonas) forms.

Thiobacteria, which include the small Pseudomonadales of the Thiobacillus genus, are the most important in a geochemical sense. The energy from the oxidation of reduced compounds of sulfur is used by these microorganisms for the autotrophic assimilation of carbon dioxide. T. thioparus develops in media of pH > 5, while T. thiooxidans and T. ferrooxidans develop in strongly acidic media, down to a pH of 1. The development of thiobacteria leads to the formation of sulfuric acid and attendant chemical weathering, which causes the destruction (in open excavations) of sulfur deposits. Sulfur bacteria can also damage man-made structures (for example, Angkor Wat). T. thiooxidans oxidizes only sulfur, while T. ferrooxidans is also capable of oxidizing sulfides and Fe²⁺ and is responsible for the acidic nature of mine water. The capacity of these bacteria to decompose metal sulfides is used in the bacterial leaching of metals from ores; it also serves as the basis for bacterial hydro-metallurgy.