Ecology: the biology of interaction. II-08. Biogeochemical cycle of Sulfur

II-8. Biogeochemical Cycle of Sulfur
Sulfur is a fairly widespread element in Earth's crust (fifteenth in chemical abundance), occurring in various oxidation states. It is a necessary component of proteins (0.8-2.4%), entering into their composition as sulfhydryl groups (-SH) and disulfide groups (-S-S-).
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 previously considered, sulfur is almost absent from the atmosphere. Hydrogen sulfide in the atmosphere is rapidly oxidized by oxygen, and sulfur oxide precipitates to the Earth's surface along with rain. We have already indicated that a certain amount of sulfur is transported through the atmosphere as dimethyl sulfide (see Section II-3).
Upon the decomposition of organic matter, sulfur is released in reduced form with the emission of a gas having an unpleasant odor—hydrogen sulfide. In addition to hydrogen sulfide, other foul-smelling substances can form during this process, for example, mercaptans, which over time also convert to hydrogen sulfide. The fact that the odor of these substances seems repulsive to us is a consequence of their being markers of organic matter decomposition under anaerobic conditions.
A number of organisms are capable of conducting dissimilatory sulfate reduction. In this case, in an anaerobic environment, sulfates are used as oxidizers rather than oxygen (in connection with this, such a process is also called sulfate respiration). The result of the activity of such organisms is clearly visible in mud deposits. The layer of mud (detritus) into which oxygen penetrates is usually brown in color and has no unpleasant odor. Where oxygen does not penetrate, the mud is black in color (due to iron(II) sulfide) and has a repulsive odor. This is the result of activity, for example, of Desulfovibrio vibrios. In hydrothermal habitats, where volcanic sulfur is available in the reducing environment, thermophilic sulfur-reducing archaea and bacteria live (at temperatures of 40-100°C).
In oxidizing environments, hydrogen sulfide is oxidized to sulfur and sulfates. This is associated with the activity of various groups of organisms. Anaerobic phototrophic bacteria conduct anaerobic photosynthesis, oxidizing hydrogen sulfide to sulfur and sulfates. These bacteria are colored and have brown, green, or purple color. Neutral sulfur can accumulate in the bodies of such bacteria as separate granules, as in purple sulfur bacteria Chromatium. For lithotrophic sulfur-oxidizing bacteria and archaea, the source of energy is not light but rather the oxidation of hydrogen sulfide itself. Representatives of this group are thiobacilli and extremely thermoacidophilic sulfur-dependent archaea. An excellent example of thiobacilli activity is given by "black smokers"—hydrogen sulfide vents on the ocean floor, where thiobacilli exist in endosymbiosis with vestimentiferans and mollusks (see Section III-5). Also belonging to this group are the filamentous colorless sulfur bacteria Beggiatoa, which inhabit wastewater treatment facilities. Finally, many other heterotrophs, including many fungi, are capable of oxidizing hydrogen sulfide.
The sulfur cycle is subject to serious human influence. The combustion of sulfur-containing substances (such as brown coal) leads to the release of sulfur oxide SO2 into the atmosphere. This compound is toxic in itself and, moreover, is one of the causes of acid rain (see Section VI-11).