Jacob S. Joffe

The oxidation of sulfur by microörganisms

Two groups of phenomena are to be considered in the study of the sulfur cycle: (1) the reduction phenomena and (2) the oxidation phenomena. The first result in the production of hydrogen sulfide and the second in the oxidation of H2S to S and of the latter to H2SO4. Both groups of phenomena result from activities of microörganisms. Winogradsky was the first investigator to have definitely demonstrated the rôle of microbes in the oxidation of the H2S to S, and of the latter to H2SO4, but it is characteristic that the organisms studied by Winogradsky (Beggiatoa and Thiothrix), never turned the medium acid: this was explained by the presence of sufficient CaCO3 in the culture to neutralize any acid formed. Keil isolated the two organisms in pure culture only about thirty years after the work of Winogradsky was carried out. The sulfur oxidizing bacteria were usually divided into four groups, namely (1) Thread-forming colorless bacteria, accumulating sulfur within their cells. The Beggiatoa and Thiothrix are representative of this group. (2) Non-thread forming, colorless bacteria, accumulating sulfur within their cells. Here are referred forms (Thioploca, Thiovulum etc.) of various sizes and shapes, the distinguishing differences being the facts that they oxidize H2S, accumulate sulfur within their cells, are colorless and non-thread forming. Some of these have been isolated in pure culture. (3) Purple bacteria. Some of these seem to play a part in the sulfur cycle, although none of the sulfur forms have yet been isolated in pure culture. (4) Colorless, non-thread forming sulfur oxidizing bacteria which do not accumulate sulfur within their cells, but which produce an abundance of sulfur (from H2S and thiosulfates) outside of their cells. The two characteristic and most important forms belonging to this group are the Thiobacillus denitrificans, anaerobic, deriving its oxygen from the decomposition of nitrates; and Thiobacillus thioparus, which oxidizes thiosulfates, H2S, and S, allows an extensive accumulation of sulfur from the first two and allows the medium to become distinctly acid.