Biochemistry - The Chemical Reactions of Living Cells Volume 3 - D. Metzler 1980

Metabolism of Nitrogenous Compounds
N2 Fixation and Other Transformations of Inorganic Nitrogen Compounds
Reduction of Elemental Nitrogen

One of the most remarkable reactions in Nitrogen METABOLISM is The conversion of diatomic nitrogen (N₂) into ammonia. It has been estimated that in 1974, Biological Nitrogen Fixation contributed 17.5∙10¹⁰ kg of nitrogen to the earth (compare this with the fixation of 4∙10¹⁰ kg of nitrogen via Chemical Reactions) [2]. To better illustrate the sheer scale of this amount, consider that a single square meter of a field planted with nodulating legumes, such as soybeans, fixes 10—30 g of nitrogen per year.

The fixation of N₂ by Clostridium pasteurianum and several other bacterial species was discovered in 1893 by Winogradsky [2a]. Subsequent research demonstrated that iron and molybdenum must be present in the culture medium for nitrogen fixation to occur. Both CO and N₂O exert an inhibitory effect. It was originally hypothesized that ammonia is the end product of this process. Alternatively, it was suggested that more oxidized compounds, such as hydroxylamine, are initially incorporated into organic molecules. Since 1960, following the isolation of Cell-free preparations capable of fixing nitrogen, major advances have been made in this field [3—5]. A significant breakthrough was the discovery that all nitrogen-fixing Bacteria share The ability to reduce acetylene to Ethylene. Apparently, the capacities for Nitrogen Fixation and acetylene reduction are closely interrelated. This simple and sensitive acetylene reduction assay allows for the easy measurement of the nitrogen-fixing potential of Cells.

Using this assay, it was soon demonstrated that the ability to fix nitrogen is not restricted to isolated bacterial species, but is widely distributed among many prokaryotes. Among these organisms, the most thoroughly studied are Azotobacter, Clostridium pasteurianum (studied by Winogradsky), Klebsiella (a bacterium related to E. coli), and Rhizobium, the symbiotic ROOT-nodule bacteria of legumes. Rhizobium deserves special attention because N₂ reduction takes place within the nodules of plants infected by the bacteria1). Inside the nodules, the bacteria transform into "bacteroids," accompanied by the synthesis of a specialized plant-Gene-encoded Hemoglobin known as leghemoglobin [7, 8]. Legumes are not the only plants that form symbioses with nitrogen-fixing organisms. Certain angiosperms host nitrogen-fixing actinomycetes, while some gymnosperms harbor nitrogen-fixing blue-green Algae. Nitrogen fixation also occurs in the leaf nodules of certain plants infected by Klebsiella bacteria. Although its nutritional significance remains unclear, nitrogen-fixing strains of Klebsiella have been isolated from the intestines of certain inhabitants of New Guinea.

Quantitatively, cyanobacteria (blue-green algae) are The most significant among free-living nitrogen-fixing organisms. For instance, in rice paddies, blue-green algae can fix anywhere from 2.4 to 10 g of nitrogen per square meter annually.

¹⁾ However, it has recently been demonstrated that certain free-living Rhizobium bacteria can also fix nitrogen [6].