The Haber process is a chemical reaction in which diatomic nitrogen gas reacts with hydrogen gas to form ammonia. The reaction equation is: N2 + 3H2 --> 2NH3. Although this reaction looks deceptively simple, the Haber process generates ammonia only under conditions of high pressure and temperature, on the order of 200 atmospheres and 400 degrees Celsius. Iron oxides or osmium can act as catalysts to speed up the reaction.
In the realm of microbiology, certain bacteria have developed the ability to convert atmospheric nitrogen into ammonia. These nitrogen-fixing bacteria live in the soil as well as in nodules at the roots of legumes and other plants. These bacteria have evolved an elaborate strategy that allows them to convert nitrogen gas into ammonia at much lower temperatures and pressures than the Haber process.
Nitrogen-fixing bacteria rely on an enzymatic complex called nitrogenase to split the triple bond holding N2 together and reduce each nitrogen atom with hydrogen to form ammonia. Nitrogenase contains clusters of molybdenum, iron and sulfur that somehow coordinate this multistep reaction.
Although the details are incompletely understood, two features are well established; first, nitrogenase functions exclusively in an anaerobic environment. A special iron sulfur protein called Shethna scavenges free oxygen to prevent it from inactivating nitrogenase. Second, each mole of N2 converted to NH3 requires an input of 16 molecules of ATP -- a huge (but ultimately worthwhile) investment of energy for a unicellular bacterium.