Nitrogen is an element found in all proteins, and is essential for plant and animal life. Gaseous nitrogen in the air has to be fixed into compounds, either by lightning or by soil-dwelling bacteria, before it can be used by plants. These compounds include ammonia and nitrates. Animals can then take in nitrogen by eating plants. When living matter dies or nitrogen-containing wastes are excreted, bacteria and fungi convert the organic nitrogen back into ammonia.
The bacteria species in soil that convert nitrites to nitrates all belong to the genus Nitrobacter. There are four identified species: Nitrobacter winogradskyi, Nitrobacter hamburgensis, Nitrobacter agilis and Nitrobacter alkalicus. In 2007, a phylogenetic study of the genus Nitrobacter, published in "Systematic and Applied Microbiology," identified 30 different strains for some of the species. Nitrobacter exists in soils and fresh water where the pH is moderate. It does not grow in highly acidic habitats.
Nitrobacter bacterial species typically occur along with bacteria from the genus Nitrosomonas in mixed bacterial communities called consortia. The two kinds of bacteria are codependent, since Nitrosomonas produces the nitrites that Nitrobacter needs, and Nitrobacter cleans up the nitrites that can suppress the Nitrosomonas if allowed to build up.
In the ocean, there are two additional types of bacteria that oxidize nitrites into nitrates. These are Nitrococcus mobilis and Nitrospina gracilis. Nitrococcus mobilis, isolated from South Pacific waters, is a large motile coccus with unique tubular cell membranes. Nitrospina gracilis is long, slender and rod-shaped, without an extensive cell membrane system. Nitrospina has become commercially available to people who maintain saltwater aquariums, to aid in keeping nitrate levels down in tank water. The bacteria help oxidize toxic nitrites that fish produce.