Heterotrophic Nitrification

A wide variety of heterotrophic bacteria and fungi have the capacity to oxidize NH+. So-called heterotrophic nitrification is not linked to cellular growth, as it is for autotrophic nitrification. There is evidence for two pathways for heterotrophic ammonia oxidation. The first is similar to that of autotrophic oxidation, in that the nitrifying bacteria have similar ammonia- and hydroxylamine-oxidizing enzymes. In fact these enzymes can oxidize a number of different substrates, and it may be that ammonia oxidation is only secondary to these enzymes' main purpose of oxidizing propene, benzene, cyclohexane, phenol, methanol, or any of a number of other nonpolar organic compounds.

The second heterotrophic pathway is organic and appears limited to fungi. It involves the oxidation of amines or amides to a substituted hydroxylamine followed by oxidation to a nitroso and then a nitro compound with the following oxidation states:

These reactions are not coupled to ATP and thus produce no energy. Alternatively, N compounds may react with hydroxyl radicals produced in the presence of hydrogen peroxide and superoxide, which may happen when fungi release oxidases and peroxidases during cell lysis and lignin degradation.

Heterotrophic bacteria such as Arthrobacter globiformis, Aerobacter aerogenes, Thiosphaera pantotropha, Streptomyces grisens, and various Pseudomonas spp. have been found to nitrify. The fungi Aspergillus flavus was first isolated as a nitrifier in 1954 and is the most widely studied of the nitrifying heterotrophs.

Interest in heterotrophic nitrification increased substantially in the late 1980s when it became clear that accelerated inputs of atmospheric ammonium to acid forest soils were being nitrified to nitrate with alarming effects on soil acidity, forest health, and downstream drinking water quality. It was assumed until recently that most of this nitrification was heterotrophic; we know now that most nitrification in acid soils is autotrophic (De Boer and Kowalchuk, 2001), although the exact mechanisms by which nitrification occurs at low pH are not well understood. Heterotrophic nitrification thus appears important in some soils and microenvironments, perhaps particularly where autotrophic nitrifiers are chemically inhibited (see below), but are thought now to rarely dominate the soil nitrifier community.

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