The nitrogen cycle

Nitrogen is essential to all living things as a component of proteins and nucleic acids. Although elemental nitrogen makes up three quarters of the Earth's atmosphere, only a handful of life forms are able to utilise it for metabolic purposes. These are termed nitrogen-fixing bacteria, and incorporate the nitrogen into ammonia (Figure 16.3, reaction 1):

Figure 16.2 The carbon cycle - a closer look. Carbon is circulated as one of three forms, carbon dioxide, methane and organic compounds. Different organisms are able to utilise each form for their own metabolic requirements, converting it in the process to one of the others. Numbered arrows refer to reactions described in the text

Figure 16.2 The carbon cycle - a closer look. Carbon is circulated as one of three forms, carbon dioxide, methane and organic compounds. Different organisms are able to utilise each form for their own metabolic requirements, converting it in the process to one of the others. Numbered arrows refer to reactions described in the text

The nitrogenase enzyme complex responsible for the reaction is very sensitive to oxygen, and is thought to have evolved early in the Earth's history, when the atmosphere was still largely oxygen-free. Many nitrogen-fixing bacteria are anaerobes; those that are not have devised ways of keeping the cell interior anoxic. Azotobacter species, for example, utilise oxygen at a high rate, so that it never accumulates in the cell, inactivating the nitrogenase. Many cyanophytes (blue-greens) carry out nitrogen fixation in thick-walled heterocysts which help maintain anoxic conditions.

Some nitrogen-fixing bacteria such as Rhizobium infect the roots of leguminous plants such as peas, beans and clover, where they form nodules and form a mutually beneficial association (see Chapter 15).

Ammonia produced by nitrogen fixation is assimilated as amino acids, which can then form proteins and feed into pathways of nucleotide synthesis (2). Organic nitrogen in the form of dead plant and animal material plus excrement re-enters the environment, where it undergoes mineralisation (3) at the hands of a range of microorganisms, involving the deamination of amino acids to their corresponding organic acid. This process of mineralisation may occur aerobically or anaerobically, in a wide range of microorganisms, e.g.:

The process by which microorganisms convert organic matter to an inorganic form is termed mineralisation.

Figure 16.3 The nitrogen cycle. See the text for further details of reactions. Numbered arrows refer to reactions described in the text

CH3 CH3

I Deamination

COO-Alanine

COO-Pyruvate

The process of nitrification, by which ammonia is oxidised stepwise firstly to nitrite and then to nitrate, involves two different groups of bacteria (4, 5).

The nitrate thus formed may suffer a number of fates. It may act as an electron acceptor in anaerobic respiration, becoming reduced to nitrogen via a series of intermediates including nitrite (6). This process of denitrification occurs in anaerobic conditions such as waterlogged soils. Alternatively, it can be reduced once again to ammonia and thence converted to organic nitrogen (7).

Denitrification is the reduction, under anaerobic conditions, of nitrite and nitrate to nitrogen gas.

A final pathway of nitrogen cycling has only been discovered in recent years. It is known as anammox (anaerobic ammonia oxidation), and is carried out by members of a group of Gram-negative bacteria called the Planc-tomycetes (see Chapter 7). The reaction, which can be represented thus:

NH4+ + NO2- = N2 + 2H2O (8) has considerable potential in the removal of nitrogen from wastewater.

Anammox is the formation of nitrogen gas by the anaerobic oxidation of ammonia and nitrite.

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