Air is mostly nitrogen, yet almost no living thing can use it
Every protein and strand of DNA needs nitrogen, and the air around us is full of it. The catch is the triple bond holding each nitrogen pair together, which is too strong for most life to break. A handful of microbes crack it with a special enzyme, and nearly everything else ultimately depends on them.
Fixation means turning inert nitrogen gas into usable compounds such as ammonia and nitrates. Lightning does it naturally in the air, and industry does it at scale through the Haber process, which underpins fertilisers, medicines, dyes and explosives. In living things the job falls to enzymes called nitrogenases, built around iron and usually molybdenum, occasionally vanadium. The reaction is costly: each molecule of nitrogen converted consumes 16 units of ATP, the cell's energy currency, and releases a molecule of hydrogen as a by-product.
Oxygen is the enzyme's enemy, destroying it quickly. Microbes cope in different ways: some live only where oxygen is absent, some burn it off by respiring hard, and legume root nodules bind it up with a protein called leghemoglobin. Many cyanobacteria confine the enzyme to dedicated cells called heterocysts. Nitrogenase itself is ancient, thought to have appeared between 1.5 and 2.2 billion years ago, with some isotope evidence hinting at around 3.2 billion.
The ocean fixes at least as much nitrogen as the land, and a single colonial cyanobacterium, Trichodesmium, may account for almost half of all marine fixation. One alga, Braarudosphaera bigelowii, has gone further, absorbing a cyanobacterial partner that became a nitrogen-fixing organelle known as a nitroplast. On land, legumes such as clover, soybeans, peanuts and rooibos host rhizobia bacteria in root nodules; when the plant dies, the fixed nitrogen enriches the soil, which is why traditional crop rotations often include a clover year. Red clover alone can add 50 to 200 pounds per acre.
The discovery unfolded slowly. Jean-Baptiste Boussingault identified biological fixation in 1838, and experiments later showed plants could not take nitrogen straight from the air. The German agronomists Hermann Hellriegel and Hermann Wilfarth worked out the role of nodule bacteria in the late 1880s, and in 1901 Martinus Beijerinck showed that the soil bacterium Azotobacter chroococcum could fix atmospheric nitrogen, the first such free-living organism to be identified.
Source: Nitrogen fixation
