Nitrogen FixingBefore life can get started, there has to be abundant H, C, O, and N available, because these are the most basic of life's building blocks. Photosynthesis uses the complex molecule RuBisCO to split CO2 into C and O2, with the C used to make the sugar glucose (C6H12O6). But how do living cells get N? Every living cell requires millions of N.M.60
The earth's atmosphere is mostly nitrogen gas, N2, but the atoms are so tightly bound together that it is practically inert, and unusable by living cells. Ammonia (NH3) will do, but inorganic sources (such as a byproduct of lightning discharge) are not reliably available in sufficient amounts to supply living cells, or at least to allow them to reproduce in abundance.
The solution to this quandary is a fussy, very complex molecular factory, the nitrogenase molecule. This molecule is virtually the same throughout the hundred or so bacterial species that can fix nitrogen, and it is the only known molecule that can perform the task.M.61
The overall equation for nitrogen fixation is:
N2 + 8 H+ + 8 e− + 16 ATP → 2 NH3 + H2 + 16 ADP + 16 P
nitrogen + 8 protons + 8 electrons + 16 ATP -> 2 ammonia + hydrogen + leftovers
where ATP is the energy "battery" and leftovers are recycled in the light reactions of photosynthesis.
At its core is a Molybdenum (occasionally Vanadium) atom, a rare element, but sufficiently abundant for the purpose. At tremendous expenditure of energy (relative to the normal biological processes), and at a very slow pace (1.25 s to convert a single N2 molecule to two atoms of ammoniaM.62), the nitrogenaseM.63 molecule can break nitrogen gas into N atoms and form ammonia, which then makes N available for use.
By "fussy" I mean that oxygen, the normal waste product of photosynthesis, poisons nitrogenase, and elaborate precautions must be made to keep it away the nitrogenase. Most bacteria and all eukaryotes are unable to fix nitrogen for this reason. The only possible solution to this is that before advanced life could exist on earth, it was necessary to fill the earth with organic matter that would feed these advanced species.
Is it not clear now, why it took the very first living species well over 2 billion years to prepare the Earth for "greening" (Day 3)M.64 and for advanced animal life (Days 5 and 6)?
In contrast to the carbon-fixing RuBisCO, which is the most abundant protein (40-50% of all the protein on earth), Nitrogenase is extremely rareM.65: It has been said that all of the nitrogenase in the world would fit into a single bucket.M.66
Only a few species—about 100—of bacteria and archaea are known to fix nitrogen, and then only, as I said, with extraordinary cautionary preparations. One of these is cyanobacteria (the phylum of blue-green bacteriaM.67), which appear to be among the very first living species, able to conduct photosynthesis and also to fix nitrogen.
In the case of cyanobacteria, special cells called heterocysts are produced to fix nitrogen. These cells have thick walls to isolate the cell contents, and then produce nitrogen, receiving sustenance from neighboring cyanobacteria. Because oxygen poisons nitrogenase, these special cells cannot perform some of the normal tasks of the regular cyanobacteria cell; in particular, they are dependent on adjacent cells for a supply of food and energy (ATP), which is needed in abundance to feed the nitrogenase.
In a typical low-nitrogen medium, about one in 15 cells in a (modern) cyanobacteria chain is a heterocyst (see figure). Frequently the immediate neighbor to a heterocyst is another specialized cell called an akinete, which can survive under harsh conditions—freezing, starvation and dehydration—for long periods of time. Since the early earth was constantly changing with no permanent dry land or shorelines, the ability to survive and resume growth in another locality or time was important. In addition the ability to go into a kind of suspended existence also allowed the cyanobacteria to drift with the ocean currents and distribute life and nutrients worldwide.