Sulfur Circulation on Earth and Bacteria

Abstract

Sulfur (S) on Earth changes its form e.g. from hydrogen sulfide (H₂S) to sulfate via elemental sulfur (S^o), and sulfate is changed again to hydrogen sulfide as shown in Fig. 4.1. Furthermore, hydrogen sulfide forms pyrite (FeS₂) and sulfuric acid is formed from this compound. Most of these processes are performed by the actions of bacteria. Hydrogen sulfide is also evolved from hot spring and volcanoes, and occurs when dead animals, the excreta of animals, and dead plants are decomposed by bacteria. The compound is oxidized to sulfuric acid by the sulfur-oxidizing bacteria and photosynthetic sulfur bacteria via elemental sulfur. The change of hydrogen sulfide to elemental sulfur occurs also abiotically in the presence of molecular oxygen. When the oxidation of hydrogen sulfide to sulfuric acid occurs in soil, the sulfuric acid formed usually reacts with calcium carbonate to produce calcium sulfate. Calcium sulfate serves as the sulfur source for plants, while it is also reduced to hydrogen sulfide by the sulfate-reducing bacteria. When the sulfuric acid formed is not neutralized and remains as an acid, it pollutes the environment acidically and sometimes corrodes concrete. When elementall sulfur formed during the oxidation of hydrogen sulfide is not further oxidized, it produces sulfur ores or mountains of elemental sulfur.