General Microbiology - Schlegel, H. 1987

Molecular nitrogen fixation
Nitrogen fixation by free-living bacteria

Until 1949, The ability to fix molecular nitrogen was considered a trait exclusive to a few Bacteria, primarily belonging to the genera Clostridium and Azotobacter. This view changed with the Introduction of the isotopic tracer method (¹⁵N₂) and the acetylene reduction assay, which detects Nitrogenase—the enzyme complex that binds N₂. It then became apparent that many other bacteria possess this capability, including the majority of anoxygenic phototrophic bacteria, numerous cyanobacteria, facultative anaerobes (Klebsiella pneumoniae, Bacillus polymyxa), chemolithoautotrophic bacteria (Xanthobacter autotrophicus, Alcaligenes latus), as well as methylotrophic, sulfate-reducing, and methanogenic bacteria.

Azotobacter species fix nitrogen with exceptional efficiency (about 20 mg of nitrogen per 1 g of sugar consumed). Several species of Azotobacter are known, inhabiting various environments (Table 13.1). All of them are Gram-negative, relatively large, and under certain conditions motile by means of flagella; all are strict aerobes capable of oxidizing a wide range of Organic compounds. In A. chroococcum, the Cells are joined in pairs. Abundant slime production and the presence of dark pigments (Melanins) give the colonies a characteristic appearance. Under nutrient-depleted conditions, cysts with thick Cell walls ("arthrospores", "microcysts") are formed.

Nitrogen-fixing bacteria have been found in the ROOT zone of many cultivated plants; Azotobacter paspali grows on the root surface of Paspalum notatum, while the bacterium Azospirillum lipoferum has been discovered in the rhizosphere of Digitaria decumbens.

Nitrogen Fixation BY free-living cyanobacteria is of considerable importance, particularly in rice paddies (where they fix 30–50 kg of nitrogen per hectare annually). The ability to fix nitrogen has been confirmed in pure cultures of approximately 40 cyanobacterial species. These organisms are among the primary colonizers of barren soils (such as volcanic deposits). They can be found in the most extreme habitats—both in Antarctica at temperatures near freezing and in hot springs. They live either solitarily or in Symbiosis with Fungi (Lichens). In inland waters and certain oceanic regions, massive proliferations of cyanobacteria, commonly known as "Water blooms", occur annually. The extent to which cyanobacteria contribute to nitrogen accumulation and biomass production in marine environments remains to be fully determined.

Class="center">Table 13.1. Representatives of the Azotobacter group and their distribution

Trace Elements required for nitrogen fixation. Molybdenum is essential for the fixation of molecular nitrogen (Table 13.2). This heavy metal is a structural component of nitrogenase.

Furthermore, the fixation of N₂ by legumes requires cobalt. It is a constituent of coenzyme B₁₂, which Functions as a cofactor for the Enzymes methylmalonyl-CoA mutase and nucleotide reductase. Like Lactobacillus leichmannii, rhizobia also depend on this cofactor. In some nitrogen-fixing organisms, molybdenum can be replaced by vanadium.

Table 13.2. Molybdenum requirement for MOLECULAR NITROGEN FIXATION by Azotobacter vinelandii cells