The Nitrogen Cycle: Nitrification and Denitrification Quiz

Nitrification is a two-step aerobic microbial oxidation process. In the first step, ammonia-oxidizing microorganisms such as Nitrosomonas and ammonia-oxidizing archaea convert ammonia to nitrite. In the second step, nitrite-oxidizing bacteria such as Nitrobacter and Nitrospira oxidize nitrite to nitrate. These chemolithotrophic organisms obtain energy for growth from these inorganic nitrogen oxidation reactions rather than from organic carbon compounds. Nitrification is a critical step in the soil nitrogen cycle and directly links nitrogen fixation and organic nitrogen mineralization to nitrate availability for plant uptake.

Ammonia monooxygenase is a membrane-bound copper-containing enzyme that catalyzes the first committed step of aerobic ammonia oxidation. It oxidizes ammonia to hydroxylamine using molecular oxygen and two electrons supplied by the quinone pool. The reaction requires a reductant because one oxygen atom is reduced to water while the other is incorporated into hydroxylamine. Hydroxylamine is subsequently oxidized to nitrite by hydroxylamine oxidoreductase, completing the first stage of nitrification. Ammonia monooxygenase is closely related to methane monooxygenase and can also fortuitously oxidize methane and various organic compounds.

Complete ammonia oxidation, abbreviated as comammox, was a landmark discovery announced in 2015 when certain Nitrospira species were found to possess genes for both ammonia oxidation and nitrite oxidation, enabling them to convert ammonia directly to nitrate without requiring cooperation between two separate organisms. This discovery fundamentally revised the century-old two-step model of nitrification and revealed that complete nitrification in soils and engineered systems is not exclusively dependent on a consortium of two different microbial guilds. Comammox Nitrospira appear to be particularly competitive under low ammonia concentrations.

Denitrification is a microbial respiratory process in which nitrate and nitrite serve as terminal electron acceptors in the absence of sufficient oxygen. The process proceeds stepwise through nitrite, nitric oxide, and nitrous oxide, with the final product being inert dinitrogen gas that returns to the atmosphere. It occurs predominantly under anaerobic or micro-aerobic conditions such as waterlogged soils, anaerobic sediment zones, and oxygen-depleted soil aggregates. Heterotrophic denitrifiers use organic carbon as the electron donor, and the process represents the primary pathway for returning fixed nitrogen to the atmosphere.

Dissimilatory nitrate reductase catalyzes the reduction of nitrate to nitrite as the first step of denitrification. Two main types exist: the membrane-bound Nar type that faces the cytoplasm and couples nitrate reduction to proton translocation across the membrane, and the periplasmic Nap type that lacks energy-conserving capacity. Both types contain a molybdenum cofactor at the active site where electron transfer to nitrate occurs. The electrons for nitrate reduction are supplied from the respiratory electron transport chain using the quinol pool, linking denitrification to cellular energy conservation during anaerobic respiration.

Nitrous oxide is a significant greenhouse gas and ozone-depleting substance that is released into the atmosphere primarily from agricultural soils where incomplete denitrification or nitrifier denitrification produces it as a by-product. Its global warming potential over a 100-year horizon is approximately 265 to 298 times greater than that of carbon dioxide, and it is the dominant ozone-depleting substance currently emitted by human activities. Agricultural nitrogen fertilizer application that drives both nitrification and denitrification in soils is the single largest anthropogenic source of nitrous oxide emissions globally.

Nitrous oxide reductase, encoded by the nosZ gene, is the only known biological enzyme capable of reducing nitrous oxide to dinitrogen gas, completing the denitrification pathway. It is a copper-containing enzyme that contains two specialized copper centers, CuA and CuZ. Nitrous oxide reductase is the most oxygen-sensitive enzyme in the denitrification pathway and is also inhibited by acetylene, making acetylene blockage a standard technique for measuring nitrous oxide production. When this enzyme is inactivated by oxygen exposure or low pH, denitrification is incomplete and nitrous oxide accumulates, contributing to greenhouse gas emissions from agricultural and aquatic environments.

Excess reactive nitrogen entering aquatic ecosystems from agricultural runoff and wastewater drives eutrophication, stimulating algal and cyanobacterial blooms. Decomposition of the excessive organic matter produced consumes oxygen through aerobic respiration, creating hypoxic or anoxic zones. Anoxic conditions in sediments accelerate denitrification and anaerobic ammonium oxidation, contributing to nitrogen loss. Incomplete denitrification releases nitrous oxide. The resulting dead zones, fish kills, loss of biodiversity, and harmful cyanobacterial blooms represent severe ecological consequences of nitrogen pollution in coastal and freshwater environments globally.

Anammox, discovered in a wastewater treatment reactor in the 1990s, is the anaerobic ammonium oxidation process carried out by planctomycete bacteria such as Candidatus Kuenenia and Candidatus Scalindua. These bacteria couple the oxidation of ammonium with the reduction of nitrite, producing dinitrogen gas directly without the need for organic carbon as an electron donor. Anammox is now recognized as one of the most important pathways for nitrogen loss from the ocean, contributing an estimated 30 to 50 percent of total marine nitrogen removal and fundamentally revising the understanding of nitrogen budget in marine ecosystems.

Assimilatory nitrate reduction is an anabolic process in which nitrate is reduced to ammonium specifically to serve as a nitrogen source for biosynthesis of amino acids, nucleotides, and other nitrogen-containing compounds. It is regulated by nitrogen availability and repressed when ammonium is plentiful. Dissimilatory nitrate reduction is a catabolic respiratory process in which nitrate serves as a terminal electron acceptor for energy generation under anaerobic conditions, with the reduced products either released as nitrogen gases in denitrification or retained as ammonium in dissimilatory nitrate reduction to ammonium.

Most denitrifying bacteria are in fact facultative anaerobes that preferentially use oxygen as a terminal electron acceptor when it is available and switch to nitrate only when oxygen becomes limiting. This is because aerobic respiration is thermodynamically more favorable than nitrate-based anaerobic respiration and yields more ATP per electron transferred. Denitrification genes are typically expressed only when oxygen levels fall below a threshold and nitrate is available. Some organisms such as Paracoccus denitrificans can grow efficiently under both fully aerobic and anaerobic denitrifying conditions depending on oxygen availability.

In denitrifying bacteria, the reduction of nitrite to nitric oxide is catalyzed by one of two structurally and evolutionarily unrelated enzymes. The copper-containing nitrite reductase encoded by nirK is a trimeric enzyme using copper as the catalytic metal, while the cytochrome cd1 enzyme encoded by nirS is a homodimeric enzyme containing both heme c and heme d1 groups. Both catalyze the same reaction and function at similar efficiencies under their respective optimal conditions. Organisms are known to carry either nirK or nirS but not both simultaneously, and the two genes have different phylogenetic distributions across denitrifying microbial communities.

The denitrification pathway consists of four sequential enzymatic steps, each catalyzed by a different enzyme. Because these enzymes have different expression patterns, oxygen sensitivities, and cofactor requirements, conditions that differentially inhibit or delay expression of individual enzymes can cause toxic intermediates to accumulate. Nitric oxide is a highly reactive radical that is toxic even at nanomolar concentrations and must be rapidly detoxified. Nitrous oxide accumulates when nitrous oxide reductase is inhibited by oxygen or low pH. The balance between expression and activity of the four denitrification enzymes therefore determines both the efficiency of nitrogen removal and the amount of greenhouse gases emitted.

Nitrification and denitrification are indeed opposing processes that together regulate the balance of reactive nitrogen in terrestrial and aquatic ecosystems. Nitrification oxidizes ammonia to nitrate, making nitrogen more water-soluble and mobile but also vulnerable to leaching and denitrification losses. Denitrification reduces oxidized nitrogen back to inert dinitrogen gas, permanently removing reactive nitrogen from the ecosystem. Together with nitrogen fixation and assimilation, these two processes form the core of the global nitrogen cycle and determine how much reactive nitrogen is retained in soils and water versus returned to the atmosphere.

Complete denitrification converts nitrate entirely to dinitrogen gas, representing complete removal of reactive nitrogen from the ecosystem. Incomplete denitrification halts at an intermediate step, releasing nitric oxide or nitrous oxide due to absence of downstream enzymes, differential gene regulation, oxygen inhibition of nitrous oxide reductase, or unfavorable pH conditions. From an agricultural and environmental perspective, incomplete denitrification is highly significant because it contributes to atmospheric nitrous oxide accumulation, a potent greenhouse gas and ozone-depleting substance, and nitric oxide emissions that contribute to tropospheric ozone and aerosol formation.