Nitrogen gas makes up roughly 78% of Earth's atmosphere — by far the most abundant gas surrounding every living thing. And yet nitrogen is also, paradoxically, one of the most common limiting nutrients for growth across ecosystems, because the two nitrogen atoms in atmospheric N₂ are bound together by an exceptionally strong triple bond that almost no organism can break directly. Every nitrogen atom built into your proteins and DNA had to be converted out of that unusable atmospheric form first, and only a narrow group of specialised organisms can actually do that conversion.
A bond too strong for most biology to break
The triple bond holding N₂ together is one of the strongest bonds in chemistry, and breaking it requires an enormous amount of energy — far more than ordinary metabolic processes in plants, animals, or most microorganisms can supply through normal biochemistry. Nitrogen fixation, the process of converting inert atmospheric N₂ into a chemically usable form like ammonia, is carried out naturally by only a limited set of organisms: certain bacteria and archaea, some free-living in soil or water, others living in a close symbiotic relationship with specific plants (most famously legumes like beans and clover, which house nitrogen-fixing bacteria in specialised root nodules). These organisms use a specialised enzyme, nitrogenase, capable of breaking the triple bond under conditions far milder than the industrial processes humans eventually developed to do the same thing artificially.
From fixed nitrogen to your own proteins
Once nitrogen has been fixed into ammonia by these specialised organisms, it can be further converted by other soil microbes into forms like nitrate that plants can directly absorb through their roots, incorporating the nitrogen into amino acids and other biomolecules. Animals then obtain their nitrogen by eating plants (or other animals that ate plants), meaning virtually every nitrogen atom in your own proteins and DNA passed through this narrow biological bottleneck at some point in its history — fixed from the atmosphere by a bacterium or archaeon, almost certainly at some point long before it ever became part of a plant, then an animal, then, eventually, you.
What we're still unsure about
The core biochemistry of nitrogen fixation and its role in the broader nitrogen cycle is well established and thoroughly studied — this isn't a matter of scientific dispute. What remains an active area of research is understanding, in fine detail, exactly how human activity — particularly industrial synthetic fertiliser production, which now fixes roughly as much nitrogen as all natural processes combined — has reshaped the global nitrogen cycle, and what the full long-term ecological consequences of that massive human-driven increase in fixed nitrogen actually are, including effects on water quality, biodiversity, and greenhouse gas emissions that ecologists are still working to fully quantify and predict.
This sits inside Biogeochemical Cycles (Carbon, Nitrogen, Water), one of seven topics in Ecology, one of six domains in Biology, one of seventeen subjects the app can quiz you on.