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Nitrogen fixation is a very expensive process in terms of energy, and it has to be as N2 is a really stable molecule and it simply takes a lot of energy to conv

Nitrogen fixation is a very expensive process in terms of energy, and it has to be as N2 is a really stable molecule and it simply takes a lot of energy to convert it to something else. No way to get around the thermodynamics of that reaction. I did a quick search and found an abstract of an old paper (https://www.nature.com/articles/267149a0 https://www.nature.com/articles/267149a0) that estimates around 15% of total energy used for nitrogen fixation. The value probably depends a lot on various factors, but it is certainly a significant amount.

dnautics 8y ago
It's not just that, but biology has evolved nitrogenase only once (it's not trivial) and the process is very inefficient. Something like five? Molecules of hydrogen get produced, that's not counting the six hydrogen atoms that get incorporated in to the ammonia. It's enough of a big deal that some microorganisms salvage the leftover hydrogen for energy. There are some microorganisms that salvage their neighbors leftover hydrogen. Also nitrogenase is very porous and oxygen sensitive so for photosynthetic organisms sequestering the nitrogen fixation away from co2 fixation is a major challenge. Nostoc deals with it by evolving terminal cell differentiation... Cyanothece has a night/day genetic switch, and higher plants fix nitrogen as far away from the leaves as possible.
- raverbashing 8y ago
  > but biology has evolved nitrogenase only once (it's not trivial) and the process is very inefficient. Something like five? Molecules of hydrogen get produced, that's not counting the six hydrogen atoms that get incorporated in to the ammonia. It's enough of a big deal that some microorganisms salvage the leftover hydrogen for energy. There are some microorganisms that salvage their neighbors leftover hydrogen. Wow that's crazy. And to think life absolutely depends on it, so I guess it used natural sources of ammonia. (And to think the same substance that's so dear to plants is excreted by animals, including us)
  - dnautics 8y ago
    life doesn't depend on it, there's plenty of ur-ammonia around.
- masklinn 8y ago
  > It's not just that, but biology has evolved nitrogenase only once (it's not trivial) I don't know, on the one hand it may have evolved only once[0], on the other hand it seems to have been hit fairly early on (1.5~2.2Ga, quickly following significant expansion of oxygen production so the needs for additional bioavailable nitrogen could have been a direct consequence of that and just not necessary before) and spread around by lateral genetic transfer, significantly lowering the chances that it'd evolve again. [0] it may also have evolved multiple times in a short time-span, and have had no need to re-evolve since
  - dnautics 8y ago
    fair point, we can never know for sure, but there are other mechanisms (like carbon fixation) that have clearly been multi-evolved... you would think that biology would at least have found an oxygen tolerant version for the modern era, as it has for carbon fixation (RuBisCo is not the only way)
- dredmorbius 8y ago
  There are numerous biological mechanisms which seem to have evolved only once. That's not to say there is not a better way, but the task of outcompeting an established incumbent is formidable. RNA, DNA, mitochondria, flowering plants, much of mammalian brain structure (including emotions), offhand. Various enzymes and hormones are remarkably consistent across vaast swathes of the genetic tree.

dnautics 8y ago

It's not just that, but biology has evolved nitrogenase only once (it's not trivial) and the process is very inefficient. Something like five? Molecules of hydrogen get produced, that's not counting the six hydrogen atoms that get incorporated in to the ammonia. It's enough of a big deal that some microorganisms salvage the leftover hydrogen for energy. There are some microorganisms that salvage their neighbors leftover hydrogen. Also nitrogenase is very porous and oxygen sensitive so for photosynthetic organisms sequestering the nitrogen fixation away from co2 fixation is a major challenge. Nostoc deals with it by evolving terminal cell differentiation... Cyanothece has a night/day genetic switch, and higher plants fix nitrogen as far away from the leaves as possible.

raverbashing 8y ago
> but biology has evolved nitrogenase only once (it's not trivial) and the process is very inefficient. Something like five? Molecules of hydrogen get produced, that's not counting the six hydrogen atoms that get incorporated in to the ammonia. It's enough of a big deal that some microorganisms salvage the leftover hydrogen for energy. There are some microorganisms that salvage their neighbors leftover hydrogen. Wow that's crazy. And to think life absolutely depends on it, so I guess it used natural sources of ammonia. (And to think the same substance that's so dear to plants is excreted by animals, including us)
- dnautics 8y ago
  life doesn't depend on it, there's plenty of ur-ammonia around.
masklinn 8y ago
> It's not just that, but biology has evolved nitrogenase only once (it's not trivial) I don't know, on the one hand it may have evolved only once[0], on the other hand it seems to have been hit fairly early on (1.5~2.2Ga, quickly following significant expansion of oxygen production so the needs for additional bioavailable nitrogen could have been a direct consequence of that and just not necessary before) and spread around by lateral genetic transfer, significantly lowering the chances that it'd evolve again. [0] it may also have evolved multiple times in a short time-span, and have had no need to re-evolve since
- dnautics 8y ago
  fair point, we can never know for sure, but there are other mechanisms (like carbon fixation) that have clearly been multi-evolved... you would think that biology would at least have found an oxygen tolerant version for the modern era, as it has for carbon fixation (RuBisCo is not the only way)
dredmorbius 8y ago
There are numerous biological mechanisms which seem to have evolved only once. That's not to say there is not a better way, but the task of outcompeting an established incumbent is formidable. RNA, DNA, mitochondria, flowering plants, much of mammalian brain structure (including emotions), offhand. Various enzymes and hormones are remarkably consistent across vaast swathes of the genetic tree.