Anammox, short for anaerobic ammonium oxidation, is one of the most significant microbial processes removing biologically available nitrogen from ecosystems worldwide. Carried out by a specialized group of bacteria first found in wastewater sludge in the early 1990s, anammox converts ammonium and nitrite directly into harmless nitrogen gas under oxygen-free conditions.1PubMed. Anammox bacteria: from discovery to application The process accounts for a substantial share of nitrogen removal in ocean oxygen minimum zones, freshwater sediments, and even agricultural soils, and it has become increasingly important in engineering circles as a low-energy alternative for treating nitrogen-rich wastewater.
A Late Discovery That Rewrote the Nitrogen Cycle
For most of the twentieth century, textbooks described nitrogen removal from the environment as a two-step affair: nitrifying bacteria oxidize ammonium to nitrate, and denitrifying bacteria reduce that nitrate back to nitrogen gas. The idea that any organism could oxidize ammonium without oxygen seemed thermodynamically plausible but biologically implausible, until microbiologists working with wastewater reactors in the Netherlands noticed ammonium disappearing under strictly anaerobic conditions. The organisms responsible turned out to belong to the bacterial phylum Planctomycetota, a deeply branching lineage with unusual cell biology. None of them have been grown in pure culture to this day; researchers can only enrich them in mixed communities, though purification techniques using density-gradient centrifugation can push cell purity above 99.5%.2PubMed. Anammox organisms: enrichment, cultivation, and environmental analysis That stubborn resistance to laboratory cultivation is one reason anammox went unnoticed for so long and why some basic questions about these bacteria remain open.
Phylogenetic studies indicate that all known anammox bacteria share a single evolutionary origin. Analyses of both ribosomal RNA genes and the gene encoding their signature enzyme, hydrazine dehydrogenase, consistently point to a monophyletic group, meaning the metabolic trick of anaerobic ammonium oxidation appears to have evolved just once.3PubMed Central. Microbial divergence and evolution. The case of anammox bacteria Within that single lineage, several candidate genera have been identified, with “Candidatus Brocadia” and “Candidatus Kuenenia” among the most commonly detected in both engineered systems and natural environments.4PubMed Central. Potential contribution of anammox to nitrogen loss from paddy soils in Southern China
How the Reaction Works, and Why It Involves Rocket Fuel
The anammox reaction combines ammonium with nitrite to produce nitrogen gas, but the intermediate step is what makes the biochemistry remarkable. The enzyme hydrazine synthase fuses nitrogen from nitric oxide (derived from nitrite) with ammonium to produce hydrazine, the same compound once used as a propellant in spacecraft thrusters.5PubMed Central. Spectroscopic insights into the mechanism of anammox hydrazine synthase Hydrazine is extremely toxic and reactive, which raises an obvious question: how does the cell survive making it?
The answer lies in a structure called the anammoxosome, an intracellular compartment bounded by a membrane unlike anything else in biology. That membrane is built from ladderane lipids, molecules whose carbon chains contain fused cyclobutane rings stacked like the rungs of a ladder. These lipids are found nowhere else in nature. Ladderane membranes pack unusually tightly while remaining fluid, creating a barrier with special properties.6PubMed. Biophysical properties of membrane lipids of anammox bacteria: I. Ladderane phospholipids form highly organized fluid membranes Surprisingly, experiments with synthetic ladderane bilayers showed that these membranes are no better than ordinary lipid membranes at blocking hydrazine from diffusing through. What they are dramatically better at is slowing the movement of protons and hydroxide ions, with pH equilibration occurring five to ten times more slowly across ladderane membranes than across conventional ones.7PubMed Central. Ladderane phospholipids form a densely packed membrane with normal hydrazine and anomalously low proton/hydroxide permeability This suggests the anammoxosome’s primary job is to protect the proton gradient the cell uses to generate energy, rather than to cage hydrazine itself. The hydrazine is apparently consumed quickly enough by downstream enzymes that it never accumulates to dangerous concentrations.
Anammox in the Ocean
Once researchers knew what to look for, anammox turned up across the world’s oceans, concentrated in oxygen minimum zones where dissolved oxygen drops to near zero. These OMZs occupy a small fraction of total ocean volume but punch far above their weight in the global nitrogen budget. In the Eastern Tropical North Pacific, for instance, molecular tracers for anammox bacteria, specifically ladderane fatty acids, have been found throughout the OMZ water column, with their distribution tracking the availability of nitrite and the boundary of oxygen depletion.8Organic Geochemistry. Biomarker evidence for anammox in the oxygen minimum zone of the Eastern Tropical North Pacific
In some marine settings, anammox dominates nitrogen removal. In others it plays second fiddle to conventional denitrification. Measurements in the Arabian Sea OMZ, for example, found anammox activity to be patchy, with detectable rates at only a few of the depths sampled, while denitrification dominated overall nitrogen gas production.9Deep Sea Research Part I: Oceanographic Research Papers. Denitrification exceeds anammox as a nitrogen loss pathway in the Arabian Sea oxygen minimum zone Arctic marine sediments tell a similar mixed story: anammox accounted for roughly 1 to 35% of total nitrogen gas production depending on the site, with denitrification picking up the rest.10Limnology and Oceanography. Denitrification and anammox activity in Arctic marine sediments The relative balance between these two pathways shifts with local chemistry, organic carbon supply, and oxygen levels, which means that generalizing from one OMZ to another can be misleading.
Freshwater and Agricultural Soils
Anammox is not just a marine phenomenon. It operates in freshwater river and lake sediments too, though typically at a more modest scale. In river sediments from the Taihu Lake region of China, anammox contributed between about 1% and 11% of total nitrogen gas production, with rates peaking in summer and dropping in winter as water temperatures fell.11PubMed. Seasonal variation and controlling factors of anaerobic ammonium oxidation in freshwater river sediments in the Taihu Lake region of China In the sediments of eutrophic urban lakes, the contribution was somewhat higher, averaging around 10% of sediment nitrogen gas production over a full year, again with summer rates far exceeding winter rates.12Polish Journal of Environmental Studies. Seasonal Variation of Anammox and Denitrification in Sediments of Two Eutrophic Urban Lakes Temperature and the availability of nitrate in sediments appear to be the main controls in these systems.
Agricultural soils, particularly waterlogged paddy fields, are another environment where anammox bacteria thrive. Research on paddy soils in southern China found anammox contributing 0.6 to 15% of total soil nitrogen gas production, and scaling those rates up across the region’s rice-growing area led to an estimate that about 2.5 million metric tons of nitrogen per year may be lost through anammox in Chinese paddies alone, roughly 10% of applied ammonia fertilizer.4PubMed Central. Potential contribution of anammox to nitrogen loss from paddy soils in Southern China That is a non-trivial leak from the agricultural nitrogen budget. Studies across different soil types showed that anammox activity is strongly tied to soil pH and salinity, flourishing in alkaline soils and deeper soil layers while being largely absent in acidic ones.13Soil Biology and Biochemistry. Activity, abundance and community structure of anammox bacteria along depth profiles in three different paddy soils If you are a rice farmer losing nitrogen and wondering where it goes, anammox in the deeper, waterlogged soil horizons is part of the answer.
Microbial Partnerships That Keep Anammox Running
Anammox bacteria do not operate in isolation. They depend on neighboring microbes for the nitrite they consume and, in turn, influence what those neighbors do. One of the more important partnerships involves bacteria that carry out dissimilatory nitrate reduction to ammonium, or DNRA. Where anammox bacteria consume ammonium and nitrite, DNRA bacteria convert nitrate back into ammonium, effectively recycling nitrogen into a form anammox can use. In bioreactor experiments, when the ratio of incoming ammonium to nitrite shifted in ways that should have starved the anammox organisms, DNRA bacteria became more abundant and sustained reactor performance, a mutualistic handoff that kept nitrogen removal steady.14PubMed Central. Synergistic interactions between anammox and dissimilatory nitrate reducing bacteria sustains reactor performance across variable nitrogen loading ratios
Another form of cooperation involves comammox bacteria, organisms that carry out complete nitrification, converting ammonium all the way to nitrate in a single cell. Recent work has shown that the small molecule hydroxylamine can stabilize the coupling between comammox and anammox bacteria under low-ammonium conditions, boosting microbial network resilience and stimulating the expression of DNRA genes as a backup nitrogen retention route.15PubMed. Hydroxylamine stabilizes low-ammonium comammox-anammox cooperation through selective metabolic regulation of complete nitrification These interspecies metabolic links mean that the health of anammox populations is never just about anammox. It reflects the stability of an entire microbial community.
Wastewater Treatment and the Push Toward the Mainstream
Conventional biological nitrogen removal at sewage treatment plants is energy-intensive. The standard approach requires pumping large volumes of air to support nitrifying bacteria and feeding organic carbon to support denitrifiers. Anammox offers an appealing shortcut: because the bacteria get their energy from combining ammonium with nitrite rather than from organic carbon, and because the reaction occurs without oxygen, anammox-based treatment can dramatically reduce both aeration costs and the need for added carbon.
The technology has been successfully deployed at scale for treating high-strength nitrogen streams, such as the liquid that drains off digested sewage sludge. Extending it to mainstream municipal wastewater, which is much more dilute and cooler, has proven harder. The slow growth rate of anammox bacteria is the central bottleneck. Doubling times are measured in weeks rather than hours, which means that if cells wash out of a reactor faster than they reproduce, the population crashes. Strategies to retain biomass, such as growing bacteria in dense granules or on biofilm carriers, are essential. Other hurdles include suppressing nitrite-oxidizing bacteria that compete for the same substrate and maintaining the right ammonium-to-nitrite ratio in a waste stream whose composition fluctuates constantly. Low winter temperatures further slow growth and complicate the picture.16PubMed. Challenges, solutions and prospects of mainstream anammox-based process for municipal wastewater treatment
Despite those challenges, the economic incentive is large enough that pilot and full-scale installations continue to proliferate, especially in Europe and East Asia. Engineers are also exploring how sidestream anammox reactors, which treat the concentrated reject water, can seed mainstream treatment by gradually acclimating biomass to lower temperatures and diluted conditions.
Oxygen, Climate Change, and Expanding Dead Zones
Anammox bacteria are anaerobes, but their sensitivity to oxygen turns out to be more nuanced than once thought. Marine species tolerate significantly more dissolved oxygen than freshwater ones. A marine species in the “Candidatus Scalindua” group showed a half-inhibitory oxygen concentration roughly four to seven times higher than that of freshwater anammox species, and its activity recovered even after exposure to ambient air for up to 24 hours.17PubMed Central. Oxygen tolerance and detoxification mechanisms of highly enriched planktonic anaerobic ammonium-oxidizing (anammox) bacteria That resilience matters for understanding where anammox can function in the real ocean, because oxygen concentrations at the edges of OMZs are not zero; they are low but variable.
This oxygen tolerance also has implications for climate projections. Marine nitrogen loss via anammox takes place in less than a tenth of a percent of the ocean’s volume, so even modest changes in the size of low-oxygen zones can significantly shift the global nitrogen budget.18PLoS One. Oxygen sensitivity of anammox and coupled N-cycle processes in oxygen minimum zones Warming oceans hold less dissolved oxygen and stratify more strongly, both of which expand OMZs. If anammox bacteria in the marine environment are active at oxygen levels roughly twenty times higher than what inhibits lab cultures, as field measurements suggest, then the effective volume of ocean acting as a nitrogen sink may already be about ten times larger than calculations based on lab thresholds would predict.18PLoS One. Oxygen sensitivity of anammox and coupled N-cycle processes in oxygen minimum zones Future expansion of OMZs could enlarge that volume further, accelerating the loss of biologically available nitrogen from the sea. For marine productivity, which is often nitrogen-limited, this is not a trivial feedback loop.
Reading the Isotope Fingerprints
Figuring out how much nitrogen anammox removes in a given environment is difficult because the end product, nitrogen gas, is the same molecule produced by denitrification. One powerful diagnostic tool is the nitrogen isotope signature left behind. Anammox bacteria preferentially consume the lighter nitrogen isotope, leaving the remaining ammonium pool enriched in the heavier isotope by a characteristic margin. Batch culture experiments have pinned this enrichment effect at roughly +24 to +29 per mil for ammonium. The conversion of nitrite, meanwhile, imprints an unusual inverse isotope effect during its oxidation to nitrate, on the order of -31 per mil, which is geochemically distinctive.19PubMed Central. Nitrogen isotope effects induced by anammox bacteria These isotopic fingerprints may explain previously puzzling offsets between nitrate and nitrite isotope ratios observed in ocean OMZs.
The same isotopic approach can be applied to engineered systems. Tracking how the nitrogen isotope signatures of ammonium, nitrite, and nitrate shift through an anammox reactor provides a non-invasive way to monitor whether the process is running correctly and to estimate the fraction of nitrogen removal attributable to anammox versus competing reactions.20Scientific Reports. Nitrogen isotope effects can be used to diagnose N transformations in wastewater anammox systems For plant operators, this could eventually replace more labor-intensive tracer experiments.
Emerging Contaminants and Reactor Resilience
As anammox-based wastewater treatment scales up, the organisms increasingly encounter pollutants that were never part of their evolutionary history. Microplastics and antibiotics are two categories of concern. In experiments exposing anammox granular sludge to a combination of polystyrene nanoplastics and the antibiotic ciprofloxacin at low concentrations, nitrogen removal efficiency held steady at roughly 89 to 91% over the study period. The bacteria’s specific activity did dip markedly during the first few weeks of exposure before recovering over the following months, partly through upregulation of resistance genes.21Journal of Environmental Chemical Engineering. Nitrogen removal adaptation and stability loss of anammox granular sludge under prolonged low-concentration PS-NPs and ciprofloxacin exposure The catch is that prolonged exposure degraded the physical structure of the granular sludge itself. The bacteria secreted more water-attracting extracellular polymers, loosening the tight granule architecture that keeps biomass from washing out of the reactor. Over longer timescales, that structural weakening could undermine the very retention strategy that makes mainstream anammox viable. Operators designing real-world systems will need to account for the fact that a reactor can appear to be performing well on nitrogen removal numbers while quietly losing the granule integrity that keeps everything together.