Ruminants are animals that have a special digestive organ, the rumen, that is a unique ecosystem, in which anaerobic microorganisms digest insoluble feed materials, such as, cellulose and starch. Bacteria, protozoa, and fungi of the rumen produce volatile fatty acids that are used by the ruminant. In addition to their role in the digestive process, rumen microorganisms synthesize vitamins and amino acids used by the ruminant. Ammonia produced by nitrogen fixation or by ammonification from organic nitrogen compounds can be assimilated into organic matter, or it can be oxidized to nitrate by the nitrifying bacteria. Losses of nitrogen from the biosphere occur as a result of denitrification, in which nitrate is converted back to N 2 .
Bacteria play major roles in both the oxidative and reductive sides of the sulfur cycle.
Because sulfide is toxic, and also reacts with various metals, sulfate reduction is an important biogeochemical process. Dimethyl sulfide is the major organic sulfur compound of ecological significance in nature. Bacterial and chemical transformation of these metals is of great geological and ecological importance. Bacterial ferric iron reduction occurs in anoxic environments, and results in the mobilization of iron from swamps, bogs, and other iron-rich aquatic habitats. Bacterial oxidation of ferrous iron occurs on a large scale at low pH, and is very common in coal-mining regions, where it results in a type of pollution called acid mine drainage.