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What environments do you think intelligent life forms could live in besides a nitrogen/oxy mix? Would a nitrogen/methane (like Titan's, but warmer and with a higher g) ecology be possible? How about argon substituting nitrogen? A CO2/methane planet, although I have doubts about energy yield? I don't really have an education in this, does anyone with knowledge have an opinion?
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One thing i don't really understand is whatever the animal body in general processes the nitrogen from air (which is needed for DNA, aminoacids, proteins etc...) or it gets it entirely from ingested food? I know for example that N is a must for all plants, as a fertilizer in soil, but is it required for human breathing ?
Amimals get their nitrogen (element) from food obtained from other animals or plants. The higher plants get their nitrogen from nitrates but also some plants can get nitrogen (element) from the air, via a symbiosis with Nitrogen (N2) fixating bacteria. Some algae can directly fix the nitrogen, I believe.
In the case of Mars, even with a global deficit in nitrogen in the planet ( not quiet sure of that but we can imagine the worst) , I believe these N2 fixating bacteria could provide the basis for the nitrogen concentration of the alimentary chain, in a local ecosystem. Even so, compressing the martian air could be necessary, because with 0.2 mb pN2 in the air, these nitrogen fixating bacteria wouldn't have much to feed and fix.
Regarding the basis of what is needed for a metabolism, provided the elements N, C, O and H are present, ultimatly what is needed is a source of energy assimilable and controlable by this metabolism to build complex molecules. For plants it's the sun light via photosynthesis, for anaeobic bacteria it could be chemical energy based on oxydo-reduction potentials present in the ambiant media, in short, a reductor and an oxydant molecule (but not necessarily oxygen), have to be available. If you look at the metabolic chain of "respiration" in the mitochondria of aerobic organisms, the complete oxydation of (one mole of ) glucose by oxygen O2 lead ultimately to C02 an water and could provide 686 kcal of heat, but it doesn't happen like that, otherwise the mitchondria would just burn, so the process is splitted in smaler steps that produce smaller amount of energy stored in the form of chemical bonds (the famous phosphate bonds in ATP). These small amount of stored energy are then used for the construction of big molecules (DNA, proteins etc).
Are other sources of energy possible, for example, can the heat, I mean a temperature gradient present in the media, like in an hydrotermal vent or a radioactive source, be directly used as a source of energy for a metabolism to build molecules ? I would say that if there was a way for organisms to store the energy contained in the temperature gradient in smaller, manageable quantities, then maybe, a thermic-based metabolism could be possible, but as far as I know, it's not the case. MAybe I am wrong, but I don't know any organism that use the thermic energy directly. Why's that, that plants can use energy contained in photons but not the molecular agitation in thermic gradient ?
It's probably something related to the entropy of the system and the nature of the chemical bond from which is based our "complexity": as "heat" is cinetic energy but also pure entropy or disorder, while a photon is energy but then, what is its entropy ? the issue is not so obvious.
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