Microbes and the Cycling of Nitrogen
Nitrogen is an important part of proteins and nucleic acids. This vital nutrient is recycled from organic compounds to ammonia, ammonium ions, nitrite, nitrate, and nitrogen gas by a variety of processes, many of which depend on microbes. Different organisms prefer nitrogen in different forms. The accompanying figure illustrates nitrogen cycling. Note that nitrification (the conversion of ammonium to nitrite and nitrate) in soil is carried out by two genera of bacteria: Nitrosomonas and Nitrobacter. Denitrification - the loss of nitrate from soil to form gaseous nitrogen compounds (N2O, NO, and N2) - is dependent on other kinds of bacteria.
Some prokaryotes are essential to the nitrogen cycle because of their role in nitrogen fixation, the conversion of nitrogen gas to ammonium ions. These ions can then be used to build amino acids. In aquatic environments cyanobacteria are the most significant nitrogen fixers. In soil some nitrogen-fixing bacteria are free-living, such as members of the genus Clostridium; others live in symbiotic relationship with leguminous plants (such as peas and clover). Symbionts, such as Rhizobium, may contribute ten times more nitrogen to soils than free-living bacteria. As we shall see, these symbionts develop intimate relationships with their host plants that require complex communications.
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