Nitrite-oxidizing bacteria (NOB) are microorganisms that perform an important step in the global nitrogen cycle. As chemolithoautotrophs, they derive energy from inorganic chemical reactions to build their own organic compounds from carbon dioxide. Their primary function is converting nitrite into nitrate, an action that connects the work of ammonia-oxidizing bacteria with organisms that use nitrate.
The growing awareness of environmental pollutants has brought renewed attention to NOB. In agricultural settings, they influence the effectiveness of nitrogen-based fertilizers by converting them into nitrate. This process contributes to the movement of nitrogen through the soil and can impact groundwater quality. Understanding these organisms is important for managing nutrient cycles and mitigating environmental contamination.
The Biochemical Process of Nitrite Oxidation
The core function of nitrite-oxidizing bacteria is the biochemical oxidation of a nitrite ion (NO₂⁻) into a nitrate ion (NO₃⁻). This reaction requires oxygen, making it an aerobic process summarized as NO₂⁻ + ½O₂ → NO₃⁻. This is the second of two main stages of nitrification, following the initial oxidation of ammonia to nitrite by other microorganisms.
This conversion is catalyzed by the enzyme nitrite oxidoreductase (NXR), a complex protein associated with the cell’s internal membranes. The enzyme removes electrons from nitrite and passes them along an electron transport chain. This flow of electrons is coupled to the pumping of protons across a membrane, creating a gradient that drives the synthesis of adenosine triphosphate (ATP), the cell’s main energy currency.
The energy generated from oxidizing nitrite fuels the bacteria’s life processes, including the fixation of carbon dioxide to create organic matter. The efficiency of this energy capture determines the organism’s ability to grow and reproduce. This process allows these bacteria to thrive on an inorganic chemical energy source that is toxic to many other forms of life.
Prominent Genera of Nitrite Oxidizing Bacteria
Several distinct genera of bacteria perform nitrite oxidation, each adapted to different environmental conditions. For a long time, Nitrobacter was considered the archetypal nitrite oxidizer. Members of this genus are found in soils and freshwater and are considered r-strategists, meaning they thrive where nitrite concentrations are high, such as in tilled agricultural soils.
In many ecosystems, bacteria of the genus Nitrospira are now understood to be more widespread and abundant. They are found in a vast range of habitats, from soils and oceans to wastewater treatment plants. Nitrospira are considered K-strategists, meaning they are highly efficient at scavenging for nitrite even when concentrations are very low, giving them a competitive advantage in many natural settings.
Other genera are adapted to more specific environments, particularly marine systems. Nitrococcus is a genus of spherical, motile bacteria that inhabit saltwater environments. Nitrospina is another marine genus often found in low-nutrient regions of the ocean. The discovery of these varied genera has expanded the understanding of NOB diversity.
Ecological Significance in the Nitrogen Cycle
Nitrite-oxidizing bacteria are integral to the nitrogen cycle because they prevent the buildup of nitrite, a compound toxic to aquatic life. In ecosystems like soils, rivers, and oceans, they ensure the rapid conversion of nitrite to nitrate. High levels of nitrite can interfere with oxygen transport in the blood of animals like fish, leading to cellular suffocation.
The nitrate produced by these bacteria is a primary source of nitrogen for plants and phytoplankton. Nitrate is highly soluble and readily absorbed by plant roots and aquatic microorganisms, supporting their growth and forming the base of many food webs. In the oceans, the nitrate generated by NOB accounts for a substantial portion of the fixed nitrogen available to marine life, fueling primary productivity.
By linking nitrite production to the formation of usable nitrate, these bacteria act as a regulatory control point in the nitrogen cycle. Their activity directly influences the availability of nitrogen for primary producers. This in turn affects the overall health and productivity of an ecosystem.
Practical Applications and Environmental Management
The function of nitrite-oxidizing bacteria is managed in engineered systems to maintain environmental quality. In wastewater treatment plants, these microbes are used for biological nutrient removal. They carry out the final step of nitrification, converting nitrite into nitrate, which is part of a larger strategy to reduce nitrogen discharged into waterways and prevent eutrophication.
These bacteria are also important for maintaining healthy conditions in aquaculture and home aquariums. In these closed aquatic systems, fish waste releases ammonia, which is converted to nitrite by other bacteria. NOB then oxidize the toxic nitrite into the much less harmful nitrate, a process known as “cycling” a new tank, which prevents nitrite from reaching lethal concentrations.
The activity of NOB in managed environments is sensitive to conditions like pH, temperature, and dissolved oxygen levels. Operators of wastewater facilities and aquaculture farms monitor and control these parameters to ensure the microbial communities function optimally. For instance, maintaining sufficient aeration is necessary because nitrite oxidation is an aerobic process, ensuring the bacteria can effectively detoxify the water.