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In any discussion of colonization of space one of the most important aspects is the carbon cycle. Within atmosphered planets and small moons, excluding the gas giants carbon dioxide is one of the most common gas. It is relatively common on Venus, Mars, comets. The conjugate salt of carbon dioxide is also common on Earth. In space in general we talk about how to get rid of it, as on Earth. But carbon dioxide is a resource for self-sufficient colonization. Aspects of the cycle CH4 + 2O2 = CO2 + 2H2O. By mass CO2 (D=44; H20, D=18) represents the major mass product of the reaction. This is why rocket engines run fuel rich, its more efficient to heat some methane up and accelerate it than to afford a higher mass fraction of O2 to accelerate CO2. The inorganic conversion of CO2 into other substances is generally not energetically favored. n CO2 + n H2O → (CH2O)n + n O2 is the process used by plants however the energy efficiency of the process is low when compared to the area which plants use. CO + H2O + A ⇌{\displaystyle \rightleftharpoons } CO2 + AH2 can be used with a hydride donor and carbon monoxide dehydrogenase enzyme something that simple microbes can do and could be made more efficient than plants but is still a rather rate limited reaction. Its difficult to increase the rate because even though hydride donor proteins are rather unstable, carbon dioxide is one of the most stable forms of carbon when oxygen is present. CO can also be made from Zn and CaCO3 (limestone) and There are several reasons to have carbon dioxide in space, one is that hydrogen (liq) has a relative density of 0.07, much less than other fuels and as a consequence is difficult to store in a pressurized form in the vacuum of space. Converting carbon dioxide to methane (liquid) improves its storability (0.42262 grams per liter). While the mass fraction of a rocket that runs on methane is higher, the energy can be compacted to about 66% of the space as liquid hydrogen, it has a lower boiling point (−161.49 °C versus −252.879 °C) and lower rates of leakage loss. One of the problems with a liquid hydrogen system is that there are few gases the boiling points below hydrogen that can be used as refrigerants to cool hydrogen under pressure and liquefy it. There are several gases that can be used as refrigerants for methane and thus keeping it liquefied is also easier. Despite the presence of methane on smaller bodies and on mars (30 parts per billion) it is not sufficiently present to justify extracting it from the atmosphere. And the problem with Mars is water, but there is water in space it is just difficult to get to. Water can be locked into Carbon dioxide CaO+ H20 + 2CO2 -slight acidity-> Ca(HCO3)2 Under basic conditions it generates free carbonate and become Calcium Carbonate. The Carbonate can be removed easily and water extracted and CO2 released. On comets there is a relative abundance of CO2 ammonia and water. THese can be used to form Ammonium Bicarbonate, which is relatively stable excepting in water (in space frozen hydrates ammonium bicarbonate would simply lyophilize at a very slow rate). NH3 + CO2 + H20 ---> NH4HCO3 This compound when dissolved in water and subject ed to a vacuum will sublimate water on the first condensate plate, ammonia on the second plate and pull CO2 out of the system (or captured). The captured CO2 and NH3 can be used to make water and Oxidants for space craft. The problem with CO2 largely revolves around recycling it. If the desire is to elongate compounds the CH3-Li + CO2 ----> CH3CO2 Li which can be further reduced with reducing agents. Alkylmagnesium bromides and Alkyl lithides are not the easiest compounds to work with (sensitivity to oxygen donors) or store, thus and as a fuel, methane is probably a better option than ethane, propane or butane, since these don't greatly increase density. If one does manage to get to carbon mono-oxide cheaply the tyranny of carbon dioxide can be broken. Carbon monoxide with chlorine gas can be used to make phosgene which can then be used to make acid chlorides which is useful for making ethers (which might be useful as fuels). Also used to make polycarbonate plastics. CO can extend the length of a carbon chain by hydroformylation reaction creating an aldehyde of an alkene one carbon shorter in length. The aldehyde can be reduced with hydrogen and dehydrated to form an alkene with an additional methyl group. CO can also be used to create methanol. CO + 2 H2 → CH3OH, while methanol corrodes aluminum and is not suitable for storage, the CH30H can be converted about to create Methyl Ether which could be used as a fuel. CO2 can also be used with methanol to create acetic acid, which is a very stable 2 carbon compound that can be stored and later hydrogenated to form ethane, ethanol and ethyl ether. Methanol or Ethanol can be reduced to produce methane or ethane. So, despite its bad reputation, carbon-monoxide production in space would eventually be a thing> Getting carbon-monoxide cheaply from CO2 has its challenge. In space the industrial process is very heat intensive, the biological process is very slow and requires the addition of biologically useful organo-hydrides https://www.sciencedaily.com/releases/2017/12/171205155418.htm This paper describes the use of graphene to use electricity to efficiently catalyze the conversion of CO2 into CO. The reaction is two part, Part I a two water molecule gives up their hydrogen and creates O2, these then attack the oxygen of CO2 creating water and CO. The rate of reaction is about 12.7% efficient which using solar panels at 30% efficiency is 3.81% ensolance efficient. But this is about many times more efficient than plants.