Nitrifying bacteria convert ammonia into nitrate through a chain of chemical reactions that keeps nitrogen cycling through soil, water, and the atmosphere. This process, called nitrification, underpins the fertility of agricultural land, the safety of drinking water, the function of wastewater treatment plants, and even the health of a home aquarium. Without these microorganisms steadily transforming one nitrogen compound into another, toxic ammonia would accumulate in ecosystems, and the global nitrogen cycle would grind to a halt.
The Two-Step Process and the Microbes Behind It
Nitrification has traditionally been understood as a relay race between two groups of microorganisms. In the first leg, ammonia-oxidizing bacteria and archaea convert ammonia into an intermediate compound called nitrite. In the second leg, nitrite-oxidizing bacteria pick up that nitrite and convert it into nitrate.1PubMed Central. Complete nitrification by a single microorganism The whole sequence starts with a copper-containing enzyme called ammonia monooxygenase, which catalyzes the first and rate-limiting step: turning ammonia into hydroxylamine.2PubMed Central. Simultaneous occupancy of Cu(C) and Cu(D) in the ammonia monooxygenase active site Hydroxylamine is then further oxidized to nitrite, completing the first half of the job. A separate set of organisms handles the nitrite-to-nitrate conversion.
The most well-known ammonia oxidizers are bacteria in the genus Nitrosomonas, and the classic nitrite oxidizers belong to Nitrospira and Nitrobacter. But bacteria are not alone in this work. Ammonia-oxidizing archaea, a completely different domain of life, also carry ammonia monooxygenase and kick-start the process in many environments.3PubMed Central. Inhibition of Ammonia Monooxygenase from Ammonia-Oxidizing Archaea by Linear and Aromatic Alkynes In some settings, archaea actually outnumber bacteria as ammonia oxidizers, which came as a surprise when researchers first realized it in the mid-2000s.
Comammox Organisms That Do It All Alone
For over a century, textbooks stated that no single organism could perform both steps of nitrification. That changed in 2015, when scientists discovered members of the Nitrospira genus capable of complete ammonia oxidation, meaning they convert ammonia all the way to nitrate without handing off the job to a partner.1PubMed Central. Complete nitrification by a single microorganism These organisms have been dubbed “comammox” bacteria (short for complete ammonia oxidizers), and since their discovery they have turned up in a surprising range of habitats, from wastewater treatment plants to drinking water systems.4Environmental Science & Technology. Comammox Nitrospira Bacteria Are Dominant Ammonia Oxidizers in Mainstream Nitrification Bioreactors Emended with Sponge Carriers Metagenomic evidence has confirmed their genetic toolkit, including ammonia monooxygenase and hydroxylamine dehydrogenase genes sitting on a single stretch of DNA, in drinking water distribution networks.5PubMed Central. Metagenomic Evidence for the Presence of Comammox Nitrospira-Like Bacteria in a Drinking Water System
The practical significance of comammox organisms is still being worked out. In wastewater bioreactors fitted with sponge carriers, they have been found to be the dominant ammonia oxidizers, which suggests they could be harnessed for more compact and efficient treatment systems. Their discovery also reshuffled what we thought we knew about how nitrification is distributed in nature: the old two-guild model is not wrong, but it is incomplete.
Archaea Versus Bacteria and the Role of Ammonia Concentration
Whether archaea or bacteria dominate ammonia oxidation in a given environment depends on how much ammonia is available. Competition experiments using freshwater isolates showed that ammonia-oxidizing archaea outcompeted bacteria under low-ammonia conditions, while bacteria dominated when ammonia was more abundant.6PubMed Central. Competition between Ammonia-Oxidizing Archaea and Bacteria from Freshwater Environments At an initial ammonium concentration of 50 micromolar, the archaeal strain won decisively; at 500 micromolar, bacteria took over. This pattern lines up with field surveys: archaea tend to rule in oligotrophic (nutrient-poor) waters and deep ocean sediments, while bacteria thrive in nutrient-rich soils and wastewater.
The split matters because it determines which organisms are actually doing the heavy lifting in a particular habitat, and therefore which environmental pressures could disrupt nitrification. Managing ammonia levels in a treatment reactor, for instance, can inadvertently shift the community from archaea to bacteria or vice versa, with consequences for how robustly the system performs.
What Controls How Fast Nitrification Happens
Temperature and pH are the two biggest physical knobs controlling nitrification rates. Research on nitrifying bacteria has identified an optimum pH near 7.8, and growth rates that climb steadily as temperature rises from about 15 to 25 °C.7Water Research. Effect of temperature and ph on the effective maximum specific growth rate of nitrifying bacteria Below that temperature range, nitrification slows dramatically, which is why constructed wetlands and outdoor wastewater lagoons often struggle with ammonia removal in winter.
Oxygen is equally critical. Both steps of nitrification are aerobic, meaning the organisms need dissolved oxygen to do their work. In waterlogged soils or the deeper layers of biofilms, oxygen can become scarce, and nitrification stalls. This is actually exploited in engineered systems where operators want to suppress nitrification and instead route nitrogen toward other removal pathways like denitrification or anammox.
Why Agriculture Has a Complicated Relationship with Nitrification
Farmers apply nitrogen to crops primarily as ammonium-based fertilizers, which plants can absorb. But nitrifying bacteria rapidly convert that ammonium into nitrate, and nitrate behaves very differently in soil. It does not stick to soil particles the way ammonium does, so rainfall washes it down past the root zone and into groundwater. The process also generates nitrous oxide, a greenhouse gas roughly 300 times more potent than carbon dioxide over a century. Taken together, nitrification can cause up to half the applied nitrogen to be lost before plants ever use it.8PubMed. Nitrification in agricultural soils: impact, actors and mitigation
To slow the process down, farmers increasingly use nitrification inhibitors, chemical compounds that block ammonia monooxygenase and keep nitrogen in its ammonium form longer.9Environmental Science & Technology. Widespread Use of the Nitrification Inhibitor Nitrapyrin: Assessing Benefits and Costs to Agriculture, Ecosystems, and Environmental Health Products like nitrapyrin and dicyandiamide are applied alongside fertilizer, and their benefits range from reduced nitrate leaching and lower nitrous oxide emissions to improved crop yields, though the extent of the benefit depends heavily on local soil, climate, and management practices.10Sustainability. Effectiveness of N Fertilizers with Nitrification Inhibitors on Winter Barley Nutrition and Yield Chemical inhibitors are not without their own problems, though. Concerns about their toxicity and persistence in the environment have pushed researchers to explore biological and plant-derived alternatives.11PubMed Central. Nitrification a Boon or Curse to the Ecosystem: Nitrification Inhibitors and their Potential for Greener Agriculture
Nitrification in Wastewater Treatment
Municipal and industrial wastewater arrives at treatment plants loaded with ammonia from human waste, food processing, and industrial effluent. Nitrifying bacteria are the workhorses that convert this ammonia into nitrate, which can then be removed through denitrification (conversion to harmless nitrogen gas). Surveys of full-scale biological nutrient removal plants have found that nitrifying bacteria make up only about 1 to 10 percent of the total microbial population in activated sludge, yet they perform an outsized function.12PubMed Central. Nitrite oxidizing bacteria (NOB) dominating in nitrifying community in full-scale biological nutrient removal wastewater treatment plants In those plants, Nitrosomonas typically dominated the ammonia-oxidizing guild, while Nitrospira was the main nitrite oxidizer.
Because nitrifiers grow slowly compared with the heterotrophic bacteria that consume organic carbon, they are the first group to suffer when conditions go wrong. A three-year study of a full-scale plant treating high-strength industrial wastewater showed that nitrification performance gradually declined as influent concentrations of organic pollutants like phenol climbed, even though the ammonia-oxidizing communities themselves remained relatively stable in composition.13PubMed. Long term assessment of factors affecting nitrifying bacteria communities and N-removal in a full-scale biological process treating high strength hazardous wastewater Operators constantly balance aeration, sludge retention time, and influent loading to keep these small but essential populations healthy.
The Drinking Water Problem
Many water utilities use chloramines rather than free chlorine for residual disinfection in distribution pipes, because chloramines produce fewer harmful disinfection byproducts. But chloramines slowly release low levels of ammonia as they decay, and that ammonia feeds nitrifying bacteria living on pipe walls and in sediments. Nitrification inside drinking water pipes is a widespread operational headache that consumes disinfectant residual, leaving the water vulnerable to regrowth of other, potentially harmful, bacteria.14PubMed Central. Review of Nitrification Monitoring and Control Strategies in Drinking Water System The problem can cause taste, odor, and health complaints for customers downstream.
Surveys have confirmed that ammonia-oxidizing bacteria and their oxidation activity are highest in chloraminated water delivery networks.15PubMed. Occurrence of nitrifying bacteria and nitrification in Finnish drinking water distribution systems Utilities fight this with periodic free-chlorine “burns,” flushing programs, and careful pH management. In a pilot-scale chloraminated distribution system, researchers documented diverse communities of both ammonia- and nitrite-oxidizing bacteria colonizing the pipes, confirming that once nitrifiers establish themselves, they are difficult to eradicate.16PubMed Central. Ammonia- and nitrite-oxidizing bacterial communities in a pilot-scale chloraminated drinking water distribution system
Nitrification in Home Aquariums
If you have kept fish, you have dealt with nitrifying microorganisms whether you realized it or not. The “nitrogen cycle” that aquarium hobbyists talk about when setting up a new tank is exactly the two-step nitrification process: fish produce ammonia, biofilter media grows organisms that convert it to nitrite, and then other organisms convert nitrite to nitrate, which is far less toxic to fish. In established freshwater aquariums, the dominant ammonia oxidizers are actually archaea rather than bacteria, a finding that surprised aquarium microbiologists.17PLOS ONE. Temporal and Spatial Stability of Ammonia-Oxidizing Archaea and Bacteria in Aquarium Biofilters Average ammonia concentrations in mature tanks stayed low and constant, and all released ammonia was efficiently converted to nitrate.
Bacteria may still play a crucial role during the initial cycling period, when ammonia concentrations spike to levels high enough to stress or kill fish. Research suggests that bacteria, with their preference for higher ammonia concentrations, could be important for first establishing nitrification and for heavily stocked tanks that experience chronically elevated ammonia.18PLOS ONE. Aquarium Nitrification Revisited: Thaumarchaeota Are the Dominant Ammonia Oxidizers in Freshwater Aquarium Biofilters This helps explain why bottled “starter bacteria” products, which typically contain bacterial strains, can speed up the cycling process even though archaea eventually take over in the established tank.
Nitrification in the Open Ocean
In marine ecosystems, nitrification is tightly coupled to the supply of ammonia from decomposing organic matter and to competition with phytoplankton, which also consume ammonium. In sunlit surface waters where phytoplankton bloom, the algae can outcompete nitrifiers so effectively that nitrification rates plunge. Experimental work has shown that intensifying competition from phytoplankton can reduce nitrification rates from around 60 nanomoles per liter per day to less than 1, essentially shutting the process down in the upper water column during blooms.19PubMed Central. Ammonium Uptake by Phytoplankton Regulates Nitrification in the Sunlit Ocean Below the photic zone, where light limits phytoplankton growth, nitrifiers have the ammonia supply largely to themselves, and nitrification rates climb. This vertical partitioning helps explain why nitrate concentrations in the ocean increase with depth.
Connections to Denitrification and Anammox
Nitrification does not happen in isolation. The nitrate and nitrite it produces feed directly into other nitrogen-removing processes. Denitrifying bacteria take nitrate and reduce it stepwise back to nitrogen gas, which escapes to the atmosphere. Anammox (anaerobic ammonium oxidation) bacteria combine ammonia with nitrite to produce nitrogen gas through a different pathway. In engineered wetlands, simultaneous nitrification, anammox, and denitrification have been shown to work together, achieving over 90 percent nitrogen removal in planted systems.20PubMed. Denitrification- and anammox-dominant simultaneous nitrification, anammox and denitrification (SNAD) process in subsurface flow constructed wetlands
There is even cross-feeding between the guilds. Ammonia-oxidizing bacteria produce nitric oxide as a byproduct, and research has shown that this nitric oxide can significantly boost anammox activity, essentially giving the anammox organisms a substrate they need to function efficiently.21PubMed. Anammox activity improved significantly by the cross-fed NO from ammonia-oxidizing bacteria and denitrifying bacteria to anammox bacteria Denitrifying bacteria do the same. These metabolic handoffs mean that the overall efficiency of nitrogen removal from water depends on cooperation among microbes that are often studied as if they were independent actors.
Heavy Metals, Microplastics, and Other Stressors
Nitrifying bacteria are sensitive to a range of toxic compounds, and because they grow slowly, they recover from damage more sluggishly than many other microbial groups. Heavy metals are classic inhibitors. Controlled experiments with pure cultures of Nitrosomonas and Nitrobacter found that copper was roughly ten times more toxic than nickel: it took 50 mg/L of nickel to cause the same degree of ammonia-oxidation inhibition as just 5 mg/L of copper.22Water Science and Technology. Effects of heavy metals on nitrifying bacteria Nitrosomonas, the ammonia oxidizer, was at least as sensitive as Nitrobacter, meaning the first step in nitrification tends to fail before the second when heavy metals are present.
More recently, microplastics have emerged as a concern. In experiments simulating landfill leachate treatment, both cadmium and polyvinyl chloride microplastic particles inhibited ammonia oxidation rates, and the effects were roughly additive when both pollutants were present together.23PubMed. Evaluation of partial nitrification efficiency as a response to cadmium concentration and microplastic polyvinylchloride abundance during landfill leachate treatment Given that both heavy metals and microplastics are increasingly common in wastewater and stormwater runoff, their combined effects on nitrification performance are a growing area of concern for treatment plant operators.
Surviving Extreme Environments
Although nitrifiers perform best near neutral pH, certain species have evolved to thrive in highly acidic or alkaline conditions. In acid soils, ammonia-oxidizing archaea have acquired specialized proton-pumping enzymes through horizontal gene transfer, and they fortify their cell membranes to resist the flood of protons that would otherwise kill them. One well-studied acid-tolerant archaeon appears to have a less permeable membrane and extra sugar units on its outer surface layer to block proton entry.24PubMed Central. Nitrification in acidic and alkaline environments Acid-tolerant ammonia-oxidizing bacteria deploy a parallel strategy, using carbonic anhydrases to scavenge protons and cation transporters to maintain an electrical charge across their membranes that resists proton influx.
These adaptations matter because many soils worldwide are naturally acidic, and acid rain has lowered the pH of many more. If nitrification only worked near neutral pH, huge swaths of terrestrial ecosystems would accumulate ammonia. Instead, specialized nitrifiers keep the nitrogen cycle running even in conditions once thought to be inhospitable to the process.
Nitrifiers as Symbionts in Marine Sponges
Marine sponges pump enormous volumes of water through their bodies, filtering bacteria, organic particles, and dissolved nutrients. Many species host permanent communities of ammonia-oxidizing archaea and nitrite-oxidizing bacteria inside their tissues. In the sponge Coscinoderma matthewsi, researchers identified two novel nitrifying lineages living together and confirmed that the sponge passes these symbionts directly to its larvae, a form of vertical inheritance that suggests a long co-evolutionary relationship.25PubMed Central. Co-occurring nitrifying symbiont lineages are vertically inherited and widespread in marine sponges For the sponge, the benefit is waste removal: the nitrifiers convert the ammonia generated by the sponge’s own metabolism into less toxic nitrate. For the nitrifiers, the sponge provides a steady supply of ammonia and a sheltered habitat. The relationship illustrates how nitrification is not just a soil or water phenomenon but is woven into the biology of animals.
Nitrifying Bacteria and the Decay of Historic Buildings
An unexpected place where nitrifiers cause trouble is on the surfaces and pores of stone buildings. Field surveys of historic buildings in Germany found ammonia-oxidizing and nitrite-oxidizing bacteria in the majority of stone samples, with ammonia oxidizers present in 55 percent of samples and nitrite oxidizers in 62 percent.26PubMed. Biodeterioration of natural stone with special reference to nitrifying bacteria Unlike surface-dwelling heterotrophic bacteria and fungi, which colonize stone within months, nitrifiers took several years to establish and were sometimes found in their greatest numbers beneath the surface. By producing nitric acid during the oxidation of ammonia, they dissolve calcareous stone from within. The bacteria showed a preference for calcareous materials with medium-sized pores, which provide the right combination of moisture retention and gas exchange.
This slow, hidden corrosion is a concern for the preservation of cathedrals, monuments, and other heritage structures. Traditional cleaning and sealing approaches often target the visible biological growth on a stone surface while leaving the nitrifying community underneath largely untouched.
Emerging Biotechnology Using Nitrifiers
Engineers are finding ways to couple nitrification with electricity generation. In one prototype system, a microbial fuel cell loaded with heterotrophic nitrifying and denitrifying bacteria treated ammonia-contaminated wastewater while simultaneously producing a small electrical current. Over roughly 200 days of operation, it achieved 99 percent ammonia removal, 95 percent total nitrogen removal, and continuous bioelectricity output.27Journal of Cleaner Production. Sustainable ammonia-contaminated wastewater treatment in heterotrophic nitrifying/denitrifying microbial fuel cell The power densities are still far too low for grid-scale use, but the concept points toward wastewater treatment plants that could partially offset their own energy costs.
Bioelectrochemical systems are also being explored in the other direction: applying a small electrical current to biocathodes to boost nitrification. Researchers have found that electrically stimulated biofilms supported higher abundances of key nitrifiers and increased the activity of ammonia oxidase and nitrite oxidase enzymes.28Environmental Technology & Innovation. Enhancement mechanism of biocathodes on nitrification in the bioelectrochemical system from a microbial perspective The approach could eventually help treatment plants handle surges in ammonia loading without expanding their physical footprint.
An Ancient Evolutionary Origin
Ammonia-oxidizing archaea are not newcomers. Molecular clock analyses estimate that the ancestral lineage capable of ammonia oxidation diverged from non-ammonia-oxidizing relatives roughly 1.16 billion years ago, around the same time the supercontinent Rodinia was assembling.29Oxford Academic. The Evolution Pathway of Ammonia-Oxidizing Archaea Shaped by Major Geological Events As global temperatures cooled and atmospheric oxygen rose, mesophilic (moderate-temperature) traits appeared in the lineage around 950 million years ago. The split between the major marine and acid-soil lineages dates to roughly 500 million years ago, during the early Paleozoic era. These timescales mean that nitrification has been shaping Earth’s nitrogen cycle for the better part of a billion years, long before the appearance of land plants, let alone agriculture. The process we now try to manage with chemical inhibitors and engineered reactors is one of the oldest continuous metabolic functions on the planet.