Ecology: the Biology of Interactions. 2.08. The Biogeochemical Cycle of Sulfur
Sulfur is a fairly abundant element in the Earth’s crust (fifteenth in chemical abundance), occurring in various oxidation states. It is an essential component of proteins. Sulfur is consumed (by autotrophs and most heterotrophic microorganisms) in oxidized form, as...
Ukrainian language (latest version) / Russian language (updates discontinued) 2.07. The Biogeochemical Cycle of Nitrogen
2.07. Biogeochemical cycle of nitrogen
D. Shabanov, M. Kravchenko. Ecology: the Biology of Interactions Chapter 2. Biospherology
2.09. The Biogeochemical Cycle of Phosphorus
2.08. The Biogeochemical Cycle of Sulfur Sulfur is a fairly abundant element in the Earth’s crust (fifteenth in chemical abundance), occurring in various oxidation states. It is an essential component of proteins (0.8–2.4%), where it is present in sulfhydryl (–SH) and disulfide (–S–S–) groups. Sulfur is consumed (by autotrophs and most heterotrophic microorganisms) in oxidized form, as sulfates. This process is called assimilatory sulfate reduction. Unlike the two elements considered earlier, sulfur is scarcely represented in the atmosphere. Hydrogen sulfide in the atmosphere is rapidly oxidized by oxygen, and sulfur oxides are deposited on the Earth’s surface with rain. We have already noted that a certain transfer of sulfur through the atmosphere occurs in the form of dimethyl sulfide (see section 2.03). During the decomposition of organic matter, sulfur is released in reduced form with the release of a gas having an unpleasant odor, hydrogen sulfide. In addition to hydrogen sulfide, other foul-smelling substances may be formed during this process, for example mercaptans, which are also eventually converted to hydrogen sulfide. The fact that the smell of these substances seems repulsive to us is a consequence of their being markers of organic matter decomposition under anoxic conditions. A number of organisms are also capable of dissimilatory sulfate reduction. In this process, in an oxygen-free environment, sulfates rather than oxygen are used as oxidants (and therefore the process is also called sulfate respiration). The result of the activity of such organisms is clearly visible in silt deposits. The layer of silt (detritus) into which oxygen penetrates is usually brown and lacks an unpleasant odor. Where oxygen does not penetrate, the silt is black (because of iron(II) sulfide) and has a disgusting smell. This is the result of activity by, for example, vibrios of the genus Desulfovibrio. In hydrothermal habitats, where reserves of volcanic sulfur are present in a reducing environment, thermophilic sulfur-reducing archaea and bacteria live at temperatures of 40–100°C. In an oxidizing environment, hydrogen sulfide is oxidized to sulfur and sulfates. This is associated with the activity of various groups of organisms. Anaerobic phototrophic bacteria carry out anaerobic photosynthesis, oxidizing hydrogen sulfide to sulfur and sulfates. These bacteria are pigmented and may be brown, green, or purple. Elemental sulfur may accumulate in the bodies of such bacteria as the purple sulfur bacteria Chromatium in the form of separate granules. For lithotrophic sulfur-oxidizing bacteria and archaebacteria, the energy source is not light but precisely the oxidation of hydrogen sulfide. Representatives of this group include thiobacilli and extremely thermoacidophilic sulfur-dependent archaea. A remarkable example of the activity of thiobacilli is provided by “black smokers,” hydrogen sulfide vents on the ocean floor where thiobacilli exist in endosymbiosis with vestimentiferans and mollusks. The same group includes the filamentous colorless sulfur bacteria Beggiatoa, which inhabit treatment facilities. Finally, many other heterotrophs, particularly many fungi, are capable of oxidizing hydrogen sulfide. Reserves of sulfur and sulfates are both the result of the activity of living organisms and of inorganic origin. The sulfur cycle is strongly influenced by humans. Burning sulfur-containing substances (for example, brown coal) leads to emissions of sulfur oxide SO2 into the atmosphere. This compound is toxic in itself and is one of the causes of acid rain (see section 6.11). Ukrainian / Russian 2.07. The Biogeochemical Cycle of Nitrogen
D. Shabanov, M. Kravchenko. Ecology: the Biology of Interactions Chapter 2. Biospherology
2.09. The Biogeochemical Cycle of Phosphorus
2.09. Biogeochemical cycle of phosphorus