Nitrogen and phosphorous biochemistry

This is a section from Hypothetical types of biochemistry which was all original research. However it is interesting, so I'll keep it here

Nitrogen and phosphorus also offer possibilities as the basis for biochemical molecules. Like carbon, phosphorus can form long chain molecules on its own, which would potentially allow it to form complex macromolecules were it not so reactive. However, in combination with nitrogen, it can form much more stable covalent bonds and create a wide range of molecules, including rings (a class of compounds called phosphazenes).

Earth's atmosphere is approximately 78% nitrogen, but this would probably not be of much use to a phosphorus-nitrogen (P-N) life-form since molecular nitrogen (N2) is nearly inert and energetically expensive to "fix" due to its triple bond. On the other hand, one could say that some Earth plants such as legumes can fix nitrogen using symbiotic bacteria contained in their root nodules, but those bacteria have to exist before the nitrogen fixation process they perform can actually take place. On Earth, the intense temperatures created by lightning split atmospheric nitrogen in order to make it available for the first nitrogen containing organisms to use. A nitrogen dioxide (NO2) or ammonia (NH3) atmosphere would be more useful. Nitrogen also forms several oxides, such as nitric oxide, nitrous oxide, and dinitrogen tetroxide, and all would be present in a nitrogen-dioxide-rich atmosphere.

  • In a nitrogen dioxide atmosphere, P-N plant analogues could absorb nitrogen dioxide from the air and phosphorus from the ground. The nitrogen dioxide would be reduced, with analogues to sugar being produced in the process, and waste oxygen would be released into the atmosphere. Animals based on phosphorus and nitrogen would consume the plants, use atmospheric oxygen to metabolize the sugar analogues, exhaling nitrogen dioxide and depositing phosphorus, or phosphorus-rich material, as solid waste.
  • In an ammonia atmosphere, P-N plants would absorb ammonia from the air and phosphorus from the ground, then oxidize the ammonia to produce P-N sugars and release hydrogen waste. P-N animals are now the reducers, breathing in hydrogen and converting the P-N sugars to ammonia and phosphorus. This is the opposite pattern of oxidation and reduction from a nitrogen dioxide world, and from the known biochemistry of Earth. It would be analogous to Earth's atmospheric carbon supply being in the form of methane instead of carbon dioxide.

Debate continues, as several aspects of a phosphorus-nitrogen cycle biology would be energy deficient. Also, nitrogen and phosphorus are unlikely to occur in the ratios and quantity required in the universe. Carbon, being preferentially formed during nuclear fusion, is more abundant and is more likely to end up in a preferred location.

An ammoniated atmosphere would be possible and stable at first view (in a reductive environment), and this type of environment would be preferentially present on massive planets which are more likely to retain hydrogen slowing down its escape to space, and have thick atmosphere that better protect ammonia from radiations; like super-Earth with mass in range between the Earth and the little giant planets like Uranus and Neptune. But it is doubtful that an atmosphere rich in nitrogen dioxide could even exist. Since the nitrogen oxides are all endoenergetic compared with molecular nitrogen and oxygen; and they are oxidizing, they would decompose by stellar radiation and by catalysis on the surface of rocks when they are produced. Unlike nitrogen dioxide, the chemically similar gas nitrogen trifluoride is not endoenergetic and is more stable, but the relative rarity of fluorine means that NF3 is unlikely to be present in large enough concentrations in any planetary atmosphere.