What is artificial nitrogen fixation

Nitrogen fixation

Nitrogen fixation, a process by which atmospheric nitrogen is converted into ammonia. The activation of molecular nitrogen and its reduction to ammonia depends on the catalytic activity of the enzyme nitrogenase. The ammonia is then incorporated into various nitrogen compounds in the cell through the process of ammonia assimilation. Nitrogenase is - especially in anaerobic organisms - a very unstable enzyme. The S. is a fundamentally important process for the nitrogen economy of the soil and water and it represents an essential stage of the nitrogen cycle of the biosphere. Certain free-living terrestrial microorganisms, especially the genera Clostridium and Azotobacter, have the ability of S. Other microorganisms fix nitrogen in symbiosis with higher plants, such as legumes. There are also many examples of the symbiotic association between microorganisms and non-leguminous plants, e.g. an actinomycete (Frankia spp.) from the nitrogen-fixing root nodules of the alder (Alnus) isolated. In the water, especially in the ocean, the most important nitrogen fixers are the cyanobacteria. The S. by cyanobacteria is of practical importance for rice cultivation in the tropics. The lichens (a symbiotic association between a cyanobacterium and a fungus) are of great ecological importance because they are able to colonize locations in which extreme climatic conditions prevail or which are poor in food. The carbon and nitrogen requirements of the lichens are covered by photosynthesis and S. Such symbiotic systems are therefore largely supplied by the atmosphere and their nutritional requirements for the rest of the environment are relatively low. Lichen are the first to colonize an infertile environment and pave the way for later settlement by plants that have more demanding nutritional needs. This can be done in poor soils Nostoc-Gunnera-System approximately 70 g of atmospheric nitrogen per m2 and fix year.

Diazotrophy, or the ability to perform S., is a special characteristic of relatively less prokaryotic organisms (diazotrophs). It has not yet been detected in any eukaryote. At Clostridium pasteurianum, which contributes to the nitrogen enrichment of arable soils, both the reducing power and the ATP, which are required for the S., are provided by phosphoroclastic pyruvate cleavage. In cell-free enzyme preparations, the pyruvate can be replaced by ATP or an ATP-generating system and a reducing agent (hydrogen or an electron donor). Suitable reducing agents include sodium dithionite and potassium borohydride. The nitrogenase can also - in the presence of a ferredoxin-dependent hydrogenase - transfer electrons from molecular hydrogen to nitrogen. In most nitrogen-fixing systems, the natural electron donor is a ferredoxin. In certain cases, it is replaced by other electron-transferring proteins, e.g. flavodoxin or rubredoxin. Four molecules of ATP are required for the transfer of an electron pair. The step-by-step nitrogen reduction on the surface of the nitrogenase may take place via enzyme-bound intermediates. Free intermediates between ammonia (the product) and N2 (the substrate of the S.) were not observed.

The most extensively studied nitrogen fixation system is the symbiotic socialization between members of the Leguminosae and Rhizobium. Most often the legume is used for these examinations Glycine max (Soybean) used. An infection of the plant roots by virulent rhizobia leads to the formation of root nodules, which have the ability to fix nitrogen. Pure ones can also fix under laboratory conditions Rhizobium-Cultures nitrogen, provided that a pentose (e.g. arabinose) and a dicarboxylic acid (e.g. fumarate or succinate) are present in the culture medium. In the course of the infection process, the Rhizobium-Cells their rod-like shape and change into spherical bacterioids. The reduction of nitrogen to ammonia and the assimilation of ammonia take place in these bacterioids. The carbon compounds for the respiration of the bacterioids and for the ammonia assimilation are provided by the plants. The amino acids are exported to the host plant tissues. For the S. by legumes in root nodules Leghemoglobin and the leghemoglobin concentration is an indicator of the ability to fix nitrogen. Leghemoglobin is not found in nitrogen fixation systems in non-legumes. Inside the cells of the root nodules, the bacterioids are immersed in a solution of leghemoglobin, which is enclosed by a membrane covering. The transport speed of oxygen through an unstirred leghemoglobin solution is eight times faster than its diffusion speed through water. This facilitated diffusion of oxygen to the bacterioids enables a high breathing rate, which is necessary in order to produce the relatively large amounts of ATP that are required by the nitrogenase. In contrast, oxygen interferes with the laboratory preparation of active bacterioids because phenols and polyphenol oxidases are present in the host plant tissue. These can be inactivated by adsorption on polyvinylpyrrolidone in the presence of ascorbic acid. The nitrogen-fixing bacterial suspension can be isolated from homogenized root nodules under strictly anaerobic conditions, e.g. by centrifuging the homogenate under argon or by destroying the polyphenol oxidase activity. The bacterioids can then be treated for nitrogenase like any other source of bacteria. The subsequent cell disruption and enzyme purification by selective precipitation and column chromatography must be carried out under strictly anaerobic conditions because the nitrogenase is irreversibly inactivated by oxygen. This is particularly critical in later cleaning stages, because the nitrogenase's sensitivity to oxygen increases in the course of the cleaning process. The separate protein components of nitrogenase are both inactivated by oxygen, with the Fe protein being the most sensitive.