Sulfur Cycle

Microorganisms contribute greatly to the sulfur cycle; a simplified version is The sulfur cycle is similar to the nitrogen cycle in that, depending on the oxidation state of the sulfur species, it can serve as an electron acceptor, an electron donor, or both.

Sulfate, the fully oxidized species, is reduced by plants and microbes for use in amino acid and protein biosynthesis; this is described as assimilatory sulfate reduction.

By contrast, when sulfate diffuses into anoxic habitats, it provides an opportunity for microbial dissimilatory sulfate reduction. Here sulfate serves as a terminal electron acceptor during anaerobic respiration by a variety of microbes, including d-proteobacteria such Desulfovibrio and Desulfonema spp. and archaea belonging to the genus Archaeoglobus.

This results in sulfide accumulation. Sulfide, which is fully reduced, can then serve as an electron source for anoxygenic photosynthetic microorganisms and chemolithoautotrophs, including members of the phylum Chlorobi and the genus Thiobacillus, respectively. These microbes convert sulfide to elemental sulfur and sulfate.

A Simplified Sulfur Cycle
A Simplified Sulfur Cycle

Sulfate is fully oxidized, thus it serves as the terminal electron acceptor in anaerobic respiration, termed dissimilatory sulfate reduction. Alternatively, plants and microbes can donate the S atom to organic compounds during assimilatory sulfate reduction. H2S is fully reduced, so it can serve as an electron donor during chemo- and photolithotrophy. Because elemental sulfur and thiosulfate are neither fully oxidized nor reduced, they can serve as either electron donors or acceptors.

Proteobacteria Other microorganisms have been found to carry out dissimilatory elemental sulfur (S0 ) reduction. These include members of the genus Desulfuromonas, thermophilic archaea, and cyanobacteria in hypersaline sediments.

Sulfite (SO3 2-) is another critical intermediate that can be reduced to sulfide by a wide variety of microorganisms, including members of the genera Alteromonas, Clostridium, Desulfovibrio, and Desulfotomaculum.

Dimethylsulfoniopropionate (DMSP) is an important organic sulfur compound. It is produced by marine phytoplankton as a compatible solute. When these cells die and release DMSP, bacterioplankton (floating bacteria) use it as a sulfur and carbon source.

In the process, it is metabolized to dimethylsulfide (DMS) and released into the atmosphere. There DMS is rapidly converted into a variety of sulfur compounds that serve as nuclei for water droplet formation, contributing to the formation of clouds.

Because clouds help keep the Earth’s surface cool, it is hypothesized that increased DMS production could help mitigate the effects of global climate change.