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Chapter 11 : Microbes Recycle Nitrogen
The carbon cycle is intertwined with another major element cycle, namely, that of nitrogen. When the organic matter of dead organisms is mineralized by the actions of heterotrophic microbes, the nitrogen of proteins and nucleic acids is released in the form of ammonia. This is known as "ammonification" and is indicated as one phase of the nitrogen cycle depicted in this chapter. It was noted earlier that nitrate is an excellent nitrogen source for plant growth, and consequently, microbial production of this form of nitrogen in soils (nitrification) is of agricultural significance. Ammonia is as good as nitrate in providing nitrogen for plant growth, and the natural process of bacterial nitrogen (N2) fixation represents the possibility of a virtually inexhaustible supply of ammonia and useful organic nitrogen compounds. Nitrogen fixation is defined as the conversion of gaseous (atmospheric) N2 to ammonia and organic nitrogen and is the “final” phase of the Earth’s nitrogen cycle we will consider. Obviously, N2 fixation has been playing an important role in the recirculation of nitrogen atoms on Earth for millions of years. Nitrogenase consists of two enzyme proteins that act in concert; both of them contain metals that are essential for their enzymatic activities. The only unique aspect of N2 fixers is the transformation of N2 to NH3. Many genera of bacteria include species that are free-living N2 fixers, organisms that live and grow as unicellular forms in soil, natural waters, and other habitats. Nitrogen fixation is also widespread among genera of the cyanobacteria.