Diazotrophs are microorganisms that 'fix' atmospheric nitrogen gas in to a more usable form such as ammonia. Examples of organisms that do this are rhizobia and Frankia (in symbiosis) and Azospirillum. A diazotroph is an organism that is able to grow without external sources of fixed nitrogen. All diazotrophs contain iron-molybdenum nitrogenase systems. Two of the most studied systems are those of Klebsiella pneumoniae and Azotobacter vinlandii. These systems are used because of their genetic tractability and their fast growth ¹. A microorganism or microbe is an organism that is so small that it is microscopic (invisible to the naked eye). ... Nitrogen fixation is the process by which nitrogen is taken from its relatively inert molecular form (N2) in the atmosphere and converted into nitrogen compounds useful for other chemical processes (such as, notably, ammonia, nitrate and nitrogen dioxide). ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless Atomic mass 14. ... Ammonia is a compound of nitrogen and hydrogen with the formula NH3. ... Rhizobia (from the Greek words Riza = Root and Bios = Life) are soil bacteria that fix nitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). ... Species Frankia alni Frankia is a genus of nitrogen fixing filamentous bacteria that live in symbiosis with actinorhizal plants, similar to Rhizobia. ... Nitrogenase (EC 1. ... Klebsiella pneumoniae is a gram-negative rod-shaped bacteria, and clinically the most important member of the Klebsiella genus of Enterobacteriaceae. ... Species A. vinladii Azotobacter are usually motile, oval, or spherical bacteria, form thick-walled cysts, and may produce large quantities of capsular slime. ...

Note 1: Dixon R and Kahn D. 2004. Genetic regulation of biological nitrogen fixation. Nat Rev Microbiol. 2(8):621-31.