with each other, is uncommon for other systems. Importance Nitrogen fixation plays an important role in the nitrogen cycle. Azotobacter also synthesizes some biologically active substances, including some phytohormones such as auxins, thereby stimulating plant growth. They also facilitate the mobility of heavy metals in the soil, thus enhancing bioremediation of soil from heavy metals, such as cadmium, mercury and lead. Some kinds of Azotobacter can also biodegrade chlorine-containing aromatic compounds, such as 2,4,6-trichlorophenol, which was previously used as an insecticide, fungicide, and herbicide, but later was found to have mutagenic and carcinogenic effects. Applications Owing to their ability to fix molecular nitrogen and therefore increase the soil fertility and stimulate plant growth, Azotobacter species are widely used in agriculture, particularly in nitrogen biofertilizers such as azotobacterin. They are also used in production of alginic acid, which is applied in medicine as an antacid, in the food industry as an additive to ice cream, puddings, and creams. Taxonomy The genus Azotobacter was discovered in 1901 by Dutch microbiologist and botanist Martinus Beijerinck, who was one of the founders of environmental microbiology. He selected and described the species Azotobacter chroococcum – the first aerobic, free-living nitrogen fixer. In 1909, Lipman described Azotobacter vinelandii, and a year later , which he named in honor of Beijerinck. In 1949, Russian microbiologist Nikolai Krasilnikov identified the species of which was divided in 1981 by Thompson Skerman into two subspecies – Azotobacter nigricans subsp. nigricans and Azotobacter nigricans subsp. achromogenes; in the same year, Thompson and Skerman described . In 1991, Page and Shivprasad reported a microaerophilic and air-tolerant type which was dependent on sodium ions. Earlier, representatives of the genus were assigned to the family Azotobacteraceae Pribram, 1933, but then were transferred to the family Pseudomonadaceae based on the studies of nucleotide sequences 16S rRNA. In 2004, a phylogenetic study revealed that A. vinelandii belongs to the same clade as the bacterium Pseudomonas aeruginosa, and in 2007 it was suggested that the genera Azotobacter, Azomonas and Pseudomonas are related and might be synonyms. References External links Pseudomonadales Bacteria genera Martinus Beijerinck Biology in the Netherlands