By far the greatest share of total world nitrogen fixation is performed biologically by nitrogen-fixing prokaryotes, which fix approximately 170 million Mg (megagrams or metric tons) of nitrogen per year. About 80 million Mg is fixed industrially by humans. A smaller amount of nitrogen is fixed in the atmosphere by nonbiological means such as lightning, volcanic eruptions, and forest fires. Rain brings these atmospherically formed products to the ground.
Several groups of bacteria fix nitrogen. In the oceans, various photosynthetic bacteria, including cyanobacteria, fix nitrogen. In fresh water, cyanobacteria are the principal nitrogen fixers. On land, free-living soil bacteria make some contribution to nitrogen fixation, but they fix only what they need for their own use and release the fixed nitrogen only when they die.
Other nitrogen-fixing bacteria live in close association with plant roots (Figure 37.5). They release up to 90 percent of the nitrogen they fix to the plant and excrete some amino acids into the soil, making nitrogen immediately available to other organisms. The plant obtains fixed nitrogen from the bacterium, and the bacterium obtains the products of photosynthesis from the plant. Such associations are excellent examples of mutualism, an interaction between two species in which both species benefit. They are also examples of symbiosis, in which two different species live in physical contact for a significant portion of their life cycles.
Bacteria of the genus Rhizobium fix nitrogen only in close, mutualistic association with the roots of plants in the legume family. The legumes include peas, soybeans, clover, alfalfa, and many tropical shrubs and trees. The bacteria infect the plant's roots, and the roots develop nodules in response to their presence. The various species of Rhizobium show a high specificity for the species of legume they infect. Farmers and gardeners coat legume seeds with Rhizobium to make sure the bacteria are present. Some farmers alternate their crops, planting clover or alfalfa occasionally to increase the available nitrogen content of the soil.
The legume-Rhizobium association is not the only bacterial association that fixes nitrogen. Some cyanobacteria fix nitrogen in association with fungi in lichens or with ferns, cycads, or nontracheophytes. Rice farmers can increase crop yields by growing the water fern Azolla, with its symbiotic nitrogen-fixing cyanobacterium, in the flooded fields where rice is grown. Another group of bacteria, the filamentous actino-mycetes, fix nitrogen in association with root nodules on woody species such as alder and mountain lilacs.
How does biological nitrogen fixation work? In the four sections that follow, we'll consider the role of the enzyme ni-trogenase, the mutualistic collaboration of plant and bacterial cells in root nodules, the need to supplement biological nitrogen fixation in agriculture, and the contributions of plants and bacteria to the global nitrogen cycle.
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