Microbes and the Sulfur Cycle

Inorganic sulfur cycles through the environment as sulfate, elemental sulfur, and sulfide.

Volcanoes and hydrothermal vents emit sulfur gases like hydrogen sulfide, making vital contributions to the sulfur cycle.

For example, in anoxic, organic-rich environments near hydrothermal vents, sulfate-reducing bacteria convert sulfate mainly to hydrogen sulfide.

This gas, toxic to most life forms, is converted by various microorganisms into less harmful sulfate or elemental sulfur.

In oxygen-rich conditions, sulfur-oxidizing bacteria like Thiobacillus convert hydrogen sulfide to sulfur or sulfate, sometimes forming sulfuric acid and lowering local pH.

On the other hand, green and purple sulfur bacteria convert sulfide into sulfur granules, which Thiomargarita and similar microbes further oxidize to sulfate.

Elemental sulfur can be reduced back to sulfide by specialized anaerobic bacteria and Archaea.

In addition to inorganic transformations, marine algae form dimethyl sulfide, a volatile organic sulfur compound, which is converted in the atmosphere into aerosols that promote cloud formation and help regulate Earth’s climate.