Talmudic Question #15
Why are only prokaryotes known to carry out biological nitrogen fixation?
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It seems to me that there are two possibilities. (An aside, was the early earth rich or poor in available N? It seems likely that it was rich in it, but it's not my area)
1) Microbes evolved this function early in evolutionary time, and formed the basis for other organisms to use biologically fixed N — this, along with inorganic processes, provided sufficient N for other life. Other organisms that evolved found biological N to be a non-limiting resource. (perhaps Nitrogenase evolved in an anoxic world, so the restriction on O2 was not an issue?)
2) Microbes evolved this late, in a competition for niche space, utilizing their adaptability to create a space for themselves to live. Other organisms lack the capacity to develop this specialized function (energy intensive, requires anoxia, removal of H2 byproduct), or found it more economical to co-opt microbial N fixation.
Microscopically, organisms acquire nutrients "one at a time"; separate CNPS sources and a separate Energy source are plausible. Macroscopically, plants stand out as acquiring nutrients separately. They've co-opted microbial N‑fixation.
Along these lines, is the symbiosome a prelude to an legume N‑fixing endosymbiont (a nitroplast, or some more pleasant name).
A bit of a rambling comment, since I'm supposed to be writing an exam 🙂
The simple answer is that amateur karyotes just don't have the stuff.
The more complex answer (the Talmud never settles for one answer) might be, in part, that, in fact, eukaryotes DO have the ability to fix nitrogen — by forming symbiotic associations with prokaryotes that have the enzymes to do it with...which is the strategy eukaryotes have used to do most everything they do, from aerobic respiration to photosynthesis. Any more questions?
Reb Ben
It seems that nitrogen fixation was invented just once in 4 billion years. No new ways to do it have been found by prokaryotes or eukaryotes since then.
It may not be possible to re-invent it in the presence of oxygen.