By Cathryn Chaney | Updated Mar 24, 2022
Nitrogen, a key component of every protein, underpins life for plants and animals alike. Atmospheric nitrogen must first be converted into usable forms—such as ammonia and nitrates—through processes like lightning or microbial nitrogen fixation in soil. Plants absorb these compounds, and animals obtain nitrogen by consuming plant matter. When organic material decomposes or waste is excreted, soil bacteria and fungi decompose it back into ammonia, completing the cycle.
Soil-dwelling Nitrobacter species transform nitrites into nitrates. Four species—Nitrobacter winogradskyi, N. hamburgensis, N. agilis, and N. alkalicus—have been identified. A 2007 phylogenetic study published in Systematic and Applied Microbiology reported 30 distinct strains within these species. Nitrobacter thrives in soils and freshwater environments with moderate pH, but it does not grow in highly acidic habitats.
Nitrosomonas bacteria typically coexist with Nitrobacter in mixed consortia. They oxidize ammonia to nitrite, which Nitrobacter then converts to nitrate. This interdependence prevents nitrite accumulation that could inhibit Nitrosomonas activity.
In marine systems, two additional nitrite‑oxidizing bacteria—Nitrococcus mobilis and Nitrospina gracilis—perform a similar function. Nitrococcus mobilis, isolated from South Pacific waters, is a motile coccus with distinctive tubular cell membranes. Nitrospina gracilis is a slender, rod-shaped organism lacking an extensive membrane system. Nitrospina is now commercially available to aquarium hobbyists to help keep nitrate levels low in saltwater tanks by oxidizing toxic nitrites produced by fish.
