Biofertilizers like Rhizobium, Azospirillum, Azotobacter & Phosphobacteria are produced using lignite as a carrier material. They are also the main constituents of the outer shell of the cyst of "Azotobacter". For example, nitrogen-fixers such as "Azotobacter" (Proteobacteria) and "Bacillus" (Firmicutes) greatly improve plant performance. The nucleotide sequence of chromosomes of "Azotobacter vinelandii", strain AvOP, is partially determined. "A. chroococcum" is the most common species from "Azotobacter" to be isolated from soil samples. Unlike other "Azotobacter" species, iron was absolutely required for growth. Two of the most studied systems are those of "Klebsiella pneumoniae" and "Azotobacter vinelandii". An example of free-living bacteria is "Azotobacter". "Azotobacter vinelandii" can contain up to 80 chromosome copies per cell. Azotobacter chroococcum is a bacterium that has the ability to fix atmospheric nitrogen. "Azotobacter salinestris" is a Gram-negative, rod-shaped organism. Azotobacter vinelandii is Gram-negative diazotroph that can fix nitrogen while grown aerobically. Homocitrate ions play a certain role in the processes of nitrogen fixation by "Azotobacter". "Azotobacter" species are ubiquitous in neutral and weakly basic soils, but not acidic soils. For example, "Azotobacter chroococcum" forms a dark-brown water-soluble pigment melanin.