Azotobacter
Introduction
Azotobacter is a genus of usually motile, oval or spherical bacteria that form thick-walled cysts and may produce large quantities of capsular slime. They are aerobic, free-living soil microbes which play an important role in the nitrogen cycle in nature, binding atmospheric nitrogen, which is inaccessible to plants, and releasing it in the form of ammonium ions into the soil (nitrogen fixation).
Classification and Description
The genus Azotobacter is classified within the Pseudomonadaceae family. The species are chemoorganotrophs and are able to use a variety of simple organic compounds as sources of nutrition. They are also able to fix nitrogen while grown aerobically. The cells are typically oval and occur singly. They are often surrounded by a slimy layer of polysaccharide, which is particularly well developed when the cells are grown in a medium containing excess sugar.
Nitrogen Fixation
Azotobacter species are some of the most prominent and well-studied free-living nitrogen fixers. They are capable of converting atmospheric nitrogen into a form that plants can use, a process known as nitrogen fixation. This process is accomplished by the enzyme nitrogenase, which is only active in the absence of oxygen. Azotobacter species have developed a number of strategies to maintain a low-oxygen environment in their cells, including high rates of respiration and the production of extracellular polysaccharides to limit the diffusion of oxygen into the cell.
Role in Agriculture
Due to their nitrogen-fixing capabilities, Azotobacter species are used in agriculture to increase the productivity of soil. They are often used in conjunction with other nitrogen-fixing bacteria to enhance their effect. Azotobacter species are also known to produce a variety of substances that can inhibit the growth of pathogenic fungi and bacteria, making them useful in the biological control of plant diseases.
Species
There are several species of Azotobacter that have been identified, including A. vinelandii, A. chroococcum, A. beijerinckii, and A. nigricans. Each species has its own unique characteristics and capabilities in terms of nitrogen fixation and interactions with plants.