The Biochemistry Of Autotrophic Nitrification

Autotrophic nitrification is a two-step process, carried out by separate groups of bacteria: the ammonia and nitrite oxidizers, respectively. Autotrophic nitrifiers derive their C from CO2 or carbonates, rather than from organic matter, and are obligate aerobes. NH3 oxidation is characterized as

The first step in this oxidation is mediated by the membrane-bound enzyme ammonia mono-oxygenase, which can also oxidize a wide variety of organic, nonpolar low-molecular-weight compounds, including phenol, methanol, methane, and halogenated aliphatic compounds such as trichloroethylene:

The reaction is irreversibly inhibited by small quantities of acetylene, which inhibits ammonia mono-oxygenase and thereby provides a means for experimentally differentiating autotrophic from heterotrophic nitrification in soil. Hydroxylamine is further oxidized to nitrite by the reaction

NH2OH + H20 NH 2OHo xido reductase : NO2 + 4e— + 5H+.

Two of the four electrons released in this reaction are used in the prior NH3 oxidation step; the remaining two are used in electron transport, generating energy for cell growth and metabolism:

Intermediary compounds formed during the oxidation of hydroxylamine to nitrite can result in the formation of NO (Fig. 13.3), which can escape to the atmosphere and influence the photochemical production of ozone (O3) and the abundance of hydroxyl (OH) radicals in air, primary oxidants for a number of tro-pospheric trace gases including methane. Ammonia oxidizers also appear able to produce NO via NO-2 reduction, which results in the production of N2O, an important greenhouse gas that can also escape to the atmosphere. Nitrite reduction occurs when ammonia oxidizers use NO-2 as an electron acceptor when O2 is lim-iting—effectively becoming denitrifying nitrifiers! Denitrification is described later in this chapter.

In most soils the nitrite produced by ammonia oxidizers does not accumulate but is quickly oxidized to nitrate by the nitrite-oxidizing bacteria when they perform nitrite oxidation:

xt/^v- i tt s*. nitrite oxidoreductase . /iir+ ^ —

These reactions are membrane-associated and because nitrite oxidoreductase is a reversible enzyme, the reaction can be reversed to result in nitrate reduction to nitrite. Up to 80% of the energy produced during nitrification is respired via the Calvin cycle; growth efficiencies of the nitrifiers are correspondingly low. This

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