Sulfur metabolism

Sulfur is metabolized by all organisms, from bacteria and archaea to plants and animals. Sulfur can have an oxidation state from -2 to +6 and is reduced or oxidized by a diverse range of organisms. The element is present in proteins, sulfate esters of polysaccharides, steroids, phenols, and sulfur-containing coenzymes. Reduced sulfur compounds are oxidized by most organisms, including higher animals and higher plants. Some organisms can conserve energy (i.e., produce ATP) from the oxidation of sulfur and it can serve as the sole energy source for some lithotrophic bacteria and archaea. Sulfur oxidizers use enzymes such as Sulfide:quinone reductase, sulfur dioxygenase and sulfite oxidase to oxidize sulfur compounds to sulfate. Reduced sulfur compounds, such as hydrogen sulfide, elemental sulfur, sulfite, thiosulfate, and various polythionates (e.g., tetrathionate), are oxidized by chemotrophic, phototrophic, and mixotrophic bacteria for energy. Some chemosynthetic archaea use hydrogen sulfide as an energy source for carbon fixation, producing sugars. In order to have sufficient redox potential, microorganisms that use sulfur as an electron donor often use oxygen or nitrate as terminal electron acceptors. Members of the chemotrophic Acidithiobacillus genus are able to oxidize a vast range of reduced sulfur compounds, but are restricted to acidic environments. Chemotrophs that can produce sugars through chemosynthesis make up the base of some food chains. Food chains have formed in the absence of sunlight around hydrothermal vents, which emit hydrogen sulfide and carbon dioxide. Some bacteria use light energy to couple sulfur oxidation to carbon dioxide (CO2) fixation for growth. These fall into two general groups: green sulfur bacteria (GSB) and purple sulfur bacteria (PSB). However, some Cyanobacteria are also able to use hydrogen sulfide as an electron donor during anoxygenic photosynthesis. All PSB are part of the class Gammaproteobacteria and are found in two families: Chromatiaceae and Ectothiorhodospiraceae.