Nitrogen Cycle

Nitrogen exists in redox states from –3 to +5, and this is reflected in the transformations that drive nitrogen cycling. depending on its level of oxidation, nitrogen species can serve as electron acceptors in anaerobic respiration (e.g., NO3 and NO2 ) or as electron donors (e.g., NO2 and NH4 +) in chemolithotrophy.

This can lead to some confusion, but the trick is to consider the oxidation state of the nitrogen species: is it fully oxidized (i.e., NO3 )? If so, it can only accept electrons. Conversely, if it is fully reduced (NH4 +), its role is limited to electron donor.

Nitrite (NO2 ) is neither fully oxidized nor reduced, so it can function as either electron donor or acceptor. We begin our discussion of this cycle with nitrogen fixation—the reduction of the inorganic gaseous molecule N2 to its organic form (e.g., amino acids, purines, and pyrimidines).

Nitrogen fixation is performed only by some bacteria and archaea. Although the nitrogenase enzyme is sensitive to oxygen, nitrogen fixation can be carried out under both oxic and anoxic conditions. Microbes such as Azotobacter spp. and cyanobacteria in the genus Trichodesmium fix nitrogen aerobically, while free-living anaerobes such as Clostridium spp. also fix nitrogen.

A Simplified Nitrogen Cycle
A Simplified Nitrogen Cycle

Perhaps the best-studied nitrogen-fixing microbes are the bacterial symbionts of leguminous plants, including rhizobia, their a-proteobacterial relatives, and a few b-proteobacteria. Other bacterial symbionts fix nitrogen as well.

For instance, actinomycete species in the genus Frankia fix nitrogen while colonizing many types of woody shrubs, and heterocyst-forming cyanobacteria Anabaena spp. fix nitrogen when in association with the water fern Azolla spp.

The product of N2 fixation is ammonia (NH3), which is immediately incorporated into organic matter as an amine. These amine N atoms are introduced into proteins, nucleic acids, and other biomolecules. Eventually these organic molecules are degraded (dissimilated) and mineralized, and the nitrogen is released as ammonium (NH4 +).

Many microbial genera are capable of dissimilation of organic nitrogen substrates, but complete mineralization requires an assemblage of microbes. Because ammonium is fully reduced, it can only donate electrons, and this is what happens in the two-step chemolithotrophic process of nitrification (figure).

In the first step, ammonium is oxidized to nitrite (NO2 ), and in the second step, NO2 is oxidized to nitrate (NO3 ). No single microbial genus can perform both steps of nitrification. For example, some mesophilic archaea and bacteria in the genera Nitrosomonas and Nitrosococcus play important roles in ammonia oxidation, while Nitrobacter spp. and related bacteria carry out nitrite oxidation.

Both steps of nitrification are usually aerobic, with O2 as the terminal electron acceptor. An exception is the b-proteobacterium Nitrosomonas eutropha, which oxidizes ammonium anaerobically to nitrite and nitric oxide (NO) using nitrogen dioxide (NO2) as an acceptor in a denitrification-related reaction (see figure).

The production of nitrate is important because it can be reduced and incorporated into microbial and plant cell biomass in a process known as assimilatory nitrate reduction.

Alternatively, some microorganisms use nitrate as a terminal electron acceptor during anaerobic respiration. When respired, the nitrogen is not incorporated into cellular material, so this is called dissimilatory nitrate reduction.

A variety of microbes, including Geobacter metallireducens and Desulfovibrio spp., are capable of dissimilatory nitrate reduction. When nitrate is fully reduced to dinitrogen gas (N2), nitrogen is removed from the ecosystem and returned to the atmosphere through a series of reactions collectively known as denitrification.

This dissimilatory process is performed by a variety of heterotrophic bacteria, such as Pseudomonas denitrificans. The major products of dissimilatory nitrate reduction include nitrogen gas (N2) and nitrous oxide (N2O), although nitrite (NO2 ) also can accumulate (see figure).

N2O, an important greenhouse gas, is also produced during ammonia oxidation to nitrite; that is, the first step of nitrification. This is particularly true of marine ammonia oxidizing archaea.

The anammox reaction (anoxic ammonium oxidation) is an anaerobic reaction performed by chemolithotrophs in the phylum Planctomycetes. Here ammonium ion (NH4 +) serves as the electron donor and nitrite (NO2 ) as the terminal electron acceptor; it is reduced to nitrogen gas (N2) (see figure).

In effect, the anammox reaction is a shortcut to N2, proceeding directly from ammonium and nitrite, without having to cycle first through nitrate (figure).

The discovery that planctomycete bacteria oxidize considerable amounts of NH4 + to N2, thereby removing ammonia from the environment, has sparked keen interest in the wastewater management field, where high levels of ammonia are undesirable.