The Sulfur Cycle

Sulfur is found in all living organisms as a constituent of some proteins, vitamins, and hormones. Like carbon and nitrogen, sulfur cycles between the atmosphere, lithosphere, and hydrosphere, but unlike carbon and nitrogen, sulfur has a major reservoir in the lithosphere. The bulk of sulfur is found in sediments and rocks in the form of sulfate minerals (primarily gypsum, CaSO4) and sulfide minerals (primarily pyrite, FeS2); although the ocean constitutes the most significant reservoir of sulfur in the form of inorganic sulfate. As shown in Table 1, sulfur can be found in a range

Table 2 Key processes in the sulfur cycle

Process [Corresponding reaction]

Sulfide/sulfur oxidation [H2S ! S°! SO2-] Aerobic

Anaerobic Sulfate reduction (anaerobic) [SO2 ! H2S]

Sulfur reduction (anaerobic) [S°! H2S]

Sulfur disproportionation [S2OI-! H2S + SO2-]

Organic sulfur compound oxidation or reduction [CH3SH ! CO2 + H2S] [DMSO ! DMS]

Desulfurylation [Organic -S ! H2S]

of oxidation states ranging from the highly reduced sulfide (-2) to the fully oxidized sulfate (SOij-) (+6). Several intermediate oxidation states of sulfur can act as either electron donors or electron acceptors depending on environmental conditions, the most notable two being elemental sulfur (S0) and thiosulfate (S2O2-).

Many diverse microbial processes contribute to the ecologically important steps in the biogeochemical cycling of sulfur (summarized in Table 2). In many ways the S cycle resembles the N cycle, but there are two important distinctions: (1) the sulfur cycle has no equivalent to nitrogen fixation that brings an element into the biosphere from the atmosphere and (2) photo-synthetic microorganisms use sulfide and elemental S as an electron source for reducing carbon dioxide during autotrophy. Similar to fully reduced forms of carbon (methane) and nitrogen (ammonia), the fully reduced form of sulfur (sulfide) is thermodynamically unstable in the presence of oxygen and other potential oxidants; thus, chemolithotrophic bacteria (e.g., Thiobacillus; Beggiatoa) use oxygen as a final electron acceptor - oxidizing the electron donor, sulfide, to sulfate.

If the sulfate is transported to a habitat that lacks molecular oxygen, the sulfate can serve as an electron acceptor. Many anaerobic heterotrophic bacteria (e.g., Desulfovibrio spp. and Desulfobacter spp.) derive energy by carrying out dissimilatory sulfate reduction that uses sulfate as an external final electron acceptor to form sulfide during anaerobic respiration. Anaerobic sulfate-reducing bacteria (SRB) are a widespread and metabolically diverse group represented by several genera. SRB are important in the anaerobic degradation of organic matter

Organisms

Sulfur chemolithotrophs

Thiobacillus, Beggiatoa, many others Purple and green phototrophic bacteria, some chemolithotrophs

Desulfovibrio, Desulfobacter

Desulfuromonas, many hyperthermophilic Archaea

Desulfovibrio, and others

Thiobacillus

Phototrophic purple bacteria, Parasporobacterium paucivorans Many microbes

Reproduced from Madigan MM, Martinko J, and Parker J, Brocks Biology of Microorganism, 9th edn., © 2000. Electronically reproduced by permission of pearson Education, Inc., Upper Saddle River, New Jersey.

in most aquatic habitats, where these microorganisms function (much like methanogens) as terminators of the anaerobic food chain. Sulfate reduction is a crucial respiratory process in marine habitats that contain an abundance of sulfate and lack other thermodynamically more favorable electron acceptors. Once formed, sulfide has the potential to support a variety of chemolitho-trophic organisms capable of oxidizing reduced forms of inorganic sulfur to more oxidized sulfur species.

The production of hydrogen sulfide from elemental sulfur has been shown to be catalyzed by a variety of bacteria, fungi, plants, and animal tissues as well as abiotic mechanisms. However, the reduction of elemental sulfur to sulfide in an energy-yielding process is performed exclusively by prokaryotic microorganisms found in both Bacteria and Archaea. This process, know as elemental sulfur reduction or sulfur respiration (Table 2), has been typically evidenced by sulfur-dependent growth on nonfermentable substrates such as hydrogen or formate. Most SRB cannot reduce elemental sulfur, and for many species, it has been found to be inhibitory. However, some members of the genera Desulfomicrobium and Desulfovibrio have been shown to grow with elemental sulfur as an electron acceptor while a larger group reduces elemental sulfur without growth. The reductase enzyme involved in this process has been identified as a tetraheme cytochrome c3 in several species; hydrogen serves as a source of electrons. Membrane preparations from active bacteria couple the sulfur-dependent hydrogen oxidation to ATP synthesis, and it appears that sulfur respiration in these organisms is due to the relaxed specificity of this c-type cytochrome resulting in an abbreviated electron transfer chain without the involvement of enzymes directly responsible for the reduction of sulfate to sulfide. Given the widespread distribution of sulfur-utilizing microorganisms and their diverse metabolic activities, it is not surprising that they play a key role in the recycling of sulfur and carbon.

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