Ammonia Oxidizing Bacteria and Their Impact on Nitrogen Cycling

Ammonia-oxidizing bacteria are among the most consequential microbes on Earth, driving the first and rate-limiting step of nitrification: converting ammonia into nitrite. That single reaction reshapes how nitrogen moves through soils, oceans, and engineered systems like wastewater treatment plants. Without these organisms, the nitrogen that fuels plant growth and aquatic food webs would largely stay locked in its reduced ammonium form, and the downstream chemistry of nitrate production, greenhouse gas emissions, and nutrient leaching would look dramatically different. Understanding what these bacteria do, where they thrive, and who competes with them is central to managing everything from farm productivity to ocean chemistry.

How Ammonia Oxidation Works

Ammonia-oxidizing bacteria, commonly abbreviated AOB, are chemolithoautotrophs. In plain terms, they get their energy from an inorganic chemical reaction rather than from sunlight or organic food. The reaction they rely on is the oxidation of ammonia (NH₃) to nitrite (NO₂⁻), which releases just enough energy for the bacteria to grow and fix carbon dioxide into organic matter. This process involves two main enzymatic steps. First, an enzyme called ammonia monooxygenase converts ammonia to hydroxylamine. Then a second enzyme, hydroxylamine oxidoreductase, converts hydroxylamine to nitrite. These same basic reactions also occur in certain methylotrophic and heterotrophic bacteria, but the autotrophic AOB are the classic performers and were the first to be studied in detail.¹

The nitrite that AOB produce does not sit idle. A second group of microbes, nitrite-oxidizing bacteria, quickly convert that nitrite into nitrate. Together, these two steps make up nitrification, the process that turns reduced nitrogen (ammonium) into oxidized nitrogen (nitrate). Nitrate is far more mobile in soil and water than ammonium, which clings to clay particles and organic matter. That mobility is both a gift and a curse: nitrate is the form of nitrogen most plants absorb easily, but it also leaches readily into groundwater and stimulates algal blooms in waterways.

The Discovery That Started It All

The scientific story of ammonia-oxidizing bacteria goes back to the late nineteenth century. Sergei Winogradsky, a Russian microbiologist, made two landmark discoveries: lithotrophy in sulfur-oxidizing bacteria and chemoautotrophy in nitrifying bacteria.² His work established that certain microbes could live without organic carbon, drawing energy purely from inorganic chemical reactions. This was a radical idea at the time, and nitrifying bacteria were the organisms that proved it. Isolating nitrifiers in pure culture turned out to be extremely difficult because they grow slowly and are easily outcompeted by faster heterotrophs. That difficulty persisted for decades and partly explains why some aspects of nitrifier biology remained mysterious well into the twenty-first century.

Diversity Among the Ammonia Oxidizers

Not all ammonia-oxidizing bacteria are closely related to one another. Molecular studies of their evolutionary trees showed that nearly all known AOB genera belong to a single monophyletic group within the beta subdivision of the proteobacteria, with one important exception: Nitrosococcus oceanus, a marine species, sits in the gamma subdivision instead.³ Within the beta group, the genera Nitrosomonas and Nitrosospira are the most commonly encountered in soils, freshwater, and wastewater systems. Different species within these genera have adapted to strikingly different conditions: some prefer high-ammonia, nutrient-rich environments while others specialize in low-ammonia oligotrophic waters.

Bacteria are no longer the only game in town when it comes to ammonia oxidation. Since the mid-2000s, ammonia-oxidizing archaea (AOA) have been recognized as major players, sometimes outnumbering AOB by orders of magnitude in certain environments. And more recently, a third group called comammox bacteria (short for “complete ammonia oxidizers”) was discovered. These are members of the genus Nitrospira that can perform both steps of nitrification, converting ammonia all the way to nitrate within a single cell, rather than handing off nitrite to a separate organism.⁴

When Bacteria and Archaea Compete

Whether AOB or AOA dominate ammonia oxidation in a given environment depends largely on ammonia concentration. Laboratory competition experiments using freshwater isolates found that AOA outcompeted AOB under ammonia-limiting conditions, thriving at initial ammonium concentrations around 50 micromolar. When initial ammonium was raised to 500 micromolar, the situation reversed and AOB dominated. Under continuous ammonium limitation in chemostat cultures, the archaea consistently won out, suggesting they can scavenge ammonia at levels too low for bacteria to use efficiently.⁵

This pattern maps neatly onto what ecologists observe in the field. In nutrient-rich agricultural soils that receive heavy fertilizer applications, AOB tend to be the dominant ammonia oxidizers and are strongly linked to nitrification rates. In oligotrophic ocean waters or acidic forest soils where ammonia is scarce, archaea often take the lead. The practical consequence is that managing nitrogen cycling in different environments may require paying attention to different microbial communities.

Nitrogen Loss and Soil Acidification on Farms

For agriculture, ammonia-oxidizing bacteria are a double-edged sword. Farmers apply nitrogen fertilizer primarily as ammonium or urea, both of which AOB quickly convert to nitrite and then nitrate through the nitrification chain. Because nitrate is so mobile, it leaches out of the root zone during rainfall and irrigation, carrying nitrogen away from the crops that need it. This leaching, combined with gaseous losses as nitrous oxide, can cause up to half of applied nitrogen to be lost before plants can use it.⁶ That is an enormous economic and environmental cost: farmers pay for nitrogen that ends up polluting groundwater and contributing to climate change rather than growing food.

In grazed grasslands, the connection between AOB and nitrate leaching is particularly clear. A lysimeter study found that the amount of nitrate leached was significantly correlated with AOB population size (measured by their amoA gene copy numbers) and with soil nitrification rate, but not with the corresponding archaeal populations.⁷ In the nitrogen-rich urine patches left by grazing cattle, AOB rather than AOA appear to drive the bulk of nitrification and the resulting nitrogen losses.

Heavy nitrogen fertilization also reshapes the balance between AOB and AOA in soil. Research on intensively fertilized vegetable soils showed that the overall potential for ammonia oxidation declined significantly at the highest fertilization rates, with antibiotic inhibition experiments suggesting that bacteria rather than archaea accounted for the majority of the ammonia-oxidizing activity across all fertilization levels.⁸ Another downstream effect of nitrification in fertilized soils is acidification: when ammonium is oxidized and the resulting nitrate leaches away, hydrogen ions accumulate in the soil, gradually lowering pH.⁹ Over years of intensive fertilization, this can shift soils from conditions favoring AOB toward conditions where AOA become more active, further complicating nitrogen management.

Slowing Nitrification Down

Because AOB are the rate-limiting step for nitrification in many agricultural soils, slowing them down is one of the most promising strategies for keeping nitrogen in the root zone. Synthetic nitrification inhibitors like dicyandiamide (DCD) have been used for years and have proven highly effective at reducing nitrate leaching under grazed pastures.⁷ But researchers are increasingly interested in biological nitrification inhibitors (BNIs), compounds naturally released by certain plant roots, that could reduce the need for synthetic chemicals.

Recent work has revealed that different BNIs shut down AOB through surprisingly different mechanisms. One compound, gallic acid, acts as an irreversible inhibitor that halts ammonia oxidation within an hour at moderate concentrations, with no recovery after the compound is removed. Others, like MHPP and MBOA, are reversible: the bacteria bounce back to full activity once the inhibitor is gone. MHPP inhibits ammonia oxidation slowly and independently of how much ammonium is around, while MBOA acts quickly and becomes more potent at higher ammonium concentrations.¹⁰ These differences matter practically: a farmer choosing a BNI-producing cover crop or a BNI additive would need to match the inhibitor’s behavior to local soil conditions and fertilizer timing. New high-throughput screening methods are also enabling researchers to discover novel nitrification inhibitors more rapidly, including distinguishing whether a compound targets the ammonia monooxygenase or the hydroxylamine oxidoreductase step.¹¹

AOB in Oceans and Estuaries

In the open ocean, ammonia-oxidizing archaea generally outnumber AOB, but bacteria still occupy important niches. In oxygen minimum zones, where dissolved oxygen plummets to near zero at intermediate depths, both AOB and AOA have been found. Off the coast of northern Chile, Nitrosospira-like bacteria dominated clone libraries constructed from both the oxygen-rich surface waters and the suboxic waters of the oxygen minimum zone, though the communities shifted in functional gene composition along the steep oxygen gradient.¹² In laboratory models mimicking oxygen minimum zone conditions, researchers demonstrated cooperation between AOA and anammox bacteria: the archaea provided nitrite to the anammox organisms, which then converted ammonium and nitrite directly to nitrogen gas, effectively removing reactive nitrogen from the system.¹³

Estuaries, where rivers meet the sea, present ammonia oxidizers with a particularly harsh gradient of salinity. Long-term monitoring of the Schelde estuary in Europe found that Nitrosomonas species were abundant at brackish sites while Nitrosospira appeared in early spring at the marine sites, with salinity and temperature identified as the main factors controlling both AOB and AOA diversity and distribution.¹⁴ Another estuary study found that the high-salinity end harbored a remarkably stable but low-diversity AOB community, dominated by Nitrosospira-like organisms not closely related to any cultured strains.¹⁵ Daily salinity fluctuations, which are normal in tidal estuaries, appear to reduce the diversity of both AOB and AOA communities even further.¹⁶ The upshot is that estuarine nitrogen cycling depends heavily on which ammonia-oxidizing species can tolerate local salinity conditions, and rising sea levels or altered freshwater flows could reorganize those communities.

Ocean Acidification and the Future of Marine Nitrification

Ocean acidification poses a direct threat to ammonia oxidation in seawater. When researchers experimentally lowered pH by small amounts (0.05 to 0.14 units) at sites across the Atlantic and Pacific, nitrification rates fell in every case. At long-term monitoring stations near Bermuda and Hawaii, ammonia oxidation rates dropped by roughly 36 to 38 percent. Across all experiments, rates declined by 8 to 38 percent, with the largest absolute decreases occurring where baseline rates were highest, off the California coast.¹⁷ The projections are sobering: ocean acidification could reduce marine nitrification rates by 3 to 44 percent within the coming decades, with cascading effects on nitrous oxide production, the supply of oxidized nitrogen to upper ocean layers, and nitrogen cycling as a whole.

The reason pH matters so much is chemical rather than purely biological. As seawater becomes more acidic, the equilibrium between ammonia and ammonium shifts toward ammonium, the charged form that AOB and AOA cannot use directly. Less free ammonia means less substrate available for ammonia monooxygenase, so the reaction slows even if the organisms themselves are not harmed. This makes ocean acidification a uniquely tricky problem: the microbes may remain present and healthy while losing access to the substrate they need.

Wastewater Treatment and Engineering Challenges

Municipal and industrial wastewater treatment plants depend on AOB to remove ammonia, which is toxic to aquatic life and consumes dissolved oxygen in receiving waters. In most activated sludge systems, AOB are the workhorses. Studies using DNA-based stable isotope probing, a technique that tracks which organisms are actively consuming a labeled substrate, have confirmed that AOB rather than AOA dominate active ammonia oxidation in full-scale wastewater plants, even in cases where AOA slightly outnumber AOB in total gene counts.¹⁸ Among the active species, Nitrosomonas europaea and related lineages consistently show up as the dominant contributors.¹⁹

Cold weather is the Achilles’ heel of nitrification in wastewater plants. When water temperatures drop below about 15°C, ammonia concentrations in treated effluent often spike. Research on full-scale sequencing batch reactors revealed that this failure is not because AOB die off in winter. Their population abundance remains relatively stable through cold months. Instead, they dramatically reduce expression of the amoA gene that encodes the critical ammonia monooxygenase enzyme. In other words, the bacteria are present but less metabolically active. Interestingly, a minority population of Nitrosospira briensis-like organisms maintained the highest ratio of gene expression to gene abundance across all four seasons, suggesting that some nitrifiers remain active even in cold water.²⁰ This finding has practical implications: engineering strategies that enrich for cold-tolerant nitrifiers could help prevent winter treatment failures.

Some advanced treatment systems pair nitrification with the anammox process to save energy and reduce costs. In these systems, AOB partially oxidize ammonia to nitrite, which anammox bacteria then combine with remaining ammonium to produce nitrogen gas. The tricky part is maintaining enough nitrite for anammox without allowing nitrite-oxidizing bacteria to consume it all. Selectively suppressing nitrite oxidizers while keeping AOB active is an active area of engineering research.²¹

Comammox and the Greenhouse Gas Advantage

The discovery of comammox Nitrospira raised an intriguing question: does performing both nitrification steps in a single cell produce less nitrous oxide, a potent greenhouse gas? The evidence so far says yes. When comammox bacterium Nitrospira inopinata was compared with canonical AOB and AOA during aerobic ammonia oxidation, the comammox organism produced substantially lower yields of nitrous oxide and other reactive nitrogen gases. Canonical AOB, by contrast, emitted considerably more.²² If comammox bacteria turn out to be widespread in environments where nitrification is active, their lower greenhouse gas footprint could mean that current estimates of biological nitrous oxide emissions need adjusting. From an engineering perspective, selectively promoting comammox organisms in wastewater systems or soils could shrink the climate impact of nitrogen processing.

Heavy Metal Stress and Bioremediation Potential

AOB are sensitive to heavy metal contamination, which matters because many agricultural and industrial soils carry elevated levels of metals like cadmium, copper, nickel, and lead. Screening studies on AOB isolated from contaminated sites have shown that all tested strains are highly sensitive to high concentrations of these metals, with copper proving more toxic than cadmium, nickel, or lead.²³ Cadmium contamination in soil has been shown to reduce the potential nitrification rate in a dose-dependent manner, with a 10 mg per kilogram dose cutting nitrification activity by about 40 percent within a week. At very low cadmium doses, a brief stimulatory effect was observed before inhibition set in, following a hormetic pattern.²⁴ This sensitivity makes AOB useful as bioindicators for soil health: a sudden drop in nitrification activity can flag metal contamination before it becomes visible in crop performance.

AOB on Your Skin

One of the more unexpected applications of ammonia-oxidizing bacteria has nothing to do with soil or water. A topical live biotherapeutic product called B244, based on ammonia-oxidizing bacteria, has been tested for the treatment of atopic dermatitis (eczema). In a phase 2b randomized controlled trial, patients who applied B244 to their skin showed significantly greater reductions in itch severity compared to those using a vehicle control. Both dose groups achieved a mean reduction in itch scores of about 34 percent from baseline by week four, compared with a smaller reduction in the placebo group.²⁵ The rationale is that AOB on the skin surface convert the ammonia in sweat into nitrite and nitric oxide, molecules that have anti-inflammatory and antimicrobial properties. While this is still early-stage clinical research, it illustrates just how versatile ammonia-oxidizing bacteria turn out to be once you start looking beyond their traditional ecological roles.

Ammonia Oxidizers in Extreme Environments

AOB have been found in some surprisingly hostile habitats. Mono Lake in California, an alkaline, hypersaline lake with a pH above 9, harbors ammonia-oxidizing bacteria related to the Nitrosomonas europaea group. Researchers concluded that the physiological diversity of nitrifiers likely exceeds what their genetic sequences alone would predict, and that these organisms represent members of a well-known lineage that have acclimated to extreme alkaline and high-salinity conditions.²⁶ The ability of AOB to persist in such environments is a reminder that nitrogen cycling is not limited to moderate, temperate settings. It operates wherever ammonia and a suitable electron acceptor are available, and the bacteria have evolved to follow.

Newer molecular tools are also refining our understanding of which organisms are truly active in any given environment, as opposed to merely present. Stable isotope probing techniques, which feed microbes a labeled nitrogen or carbon source and then track which organisms incorporate the label into their DNA, have proven especially valuable in paddy soils, where the waterlogged conditions create a patchwork of aerobic and anaerobic zones.²⁷ These methods have repeatedly shown that gene counts alone can be misleading: a numerically abundant population may be metabolically dormant while a rarer one does most of the work. For nitrogen cycling research, knowing who is active matters far more than knowing who is there.