According to https://en.wikipedia.org/wiki/Carbon_cycle, organic carbon has a cycle. My question is how does carbon enter the cycle in first place? As far as I can say, inorganic carbon enters the cycle only as CO2 that is transformed by photosynthesis.

If atmospheric CO2 can turn into organic carbon, and, as far as I know, only volcanoes add CO2 to the air. So current organic carbon comes from volcanoes or was always part of atmosphere?

If that is true, then photosynthetic life has always been constrained to the carbon released by volcanic activity.

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    $\begingroup$ Much of the Earth's atmosphere was originally CO2. This was converted to O2 and carbon when life evolved photosynthesis. (It's more complicated than that, of course. See e.g. en.wikipedia.org/wiki/Great_Oxidation_Event for a basic introduction.) $\endgroup$
    – jamesqf
    Oct 1, 2019 at 5:32
  • $\begingroup$ Much of the CO2 was dissolved into the ocean, then the acid eroded rocks, and now it's carbonate rock maybe? $\endgroup$ Feb 7 at 2:52

3 Answers 3


The answer is no. One important mechanism that also fixes carbon is chemosynthesis. One good example of this production is around hydrothermal vents in the bottom of the ocean.

Chemosynthesis is the biological production of organic matter from single-carbon compounds like $\ce{CO2}$ and nutrients using the energy generated by the oxidation of inorganic or organic molecules (instead of solar energy). This diagram shows one of the example pathways that produce organic carbon (sugars) from $\ce{CO2}$, water, and a source of chemical energy coming from hydrogen sulfide produced by volcanic activity at the hydrothermal vent.

The source of the $\ce{CO2}$ in this case is dissolved carbon dioxide in the saltwater. The existing $\ce{CO2}$ concentration in these environments is enhanced in some cases (but not necessarily) by the volcanic activity in the area.

Hydrothermal vent chemistry Source: Teara


In addition to chemosynthesis by living organisms, carbon dioxide can be reduced to methane and more complex hydrocarbons by serpentization.

Serpentization is a high-temperature, high-pressure reaction between mafic minerals (principally olivines and pyroxenes) and water to form a less mafic mineral, from the serpentine group (including serpentine itself and talc, among others). The reaction displaces iron and magnesium oxides. The magnesium oxide is purely basic and emerges combined with water to form magnesium hydroxide (brucite) or perhaps with carbon dioxide to give magnesium carbonate (magnesite) or other carbonate minerals. The iron oxide is a reducing agent, tending to convert from the $\ce{FeO}$ component of the mafic minerals to $\ce{Fe3O4}$ (magnetite) with displacement of hydrogen from water. As described in the Wikipedia article, carbon dioxide may also be reduced, especially in the presence of the water, to produce hydrocarbons:

Laboratory experiments have confirmed that at a temperature of 300 °C (572 °F) and pressure of 500 bars, olivine serpentinizes with release of hydrogen gas. In addition, methane and complex hydrocarbons are formed through reduction of carbon dioxide. The process may be catalyzed by magnetite formed during serpentinization.[1] One reaction pathway is:[2]

$\ce{18\overset{forsterite}{Mg2SiO4} + 6\overset{fayalite}{Fe2SiO4} + 26 H2O + CO2 -> 12\overset{serpentine}{Mg3Si2O5(OH)4} + 4\overset{magnetite}{Fe3O4} + \overset{methane}{CH4}}$

Serpentization is widely studied by planetary scientists because when they search for signatures of life, they reckon with serpentization as an alternate source of the organic matter they observe -- and as a source for the raw materials organisms might use.

Cited References

  1. Berndt, Michael E.; Allen, Douglas E.; Seyfried, William E. (1 April 1996). "Reduction of CO2 during serpentinization of olivine at 300 °C and 500 bar". _Geology 24 (4): 351–354. Bibcode:1996Geo....24..351B. DOI: 10.1130/0091-7613(1996)024<0351:ROCDSO>2.3.CO;2.

  2. Russell, M. J.; Hall, A. J.; Martin, W. (2010). "Serpentinization as a source of energy at the origin of life". _Geobiology 8 (5): 355–371. https://doi.org/10.1111/j.1472-4669.2010.00249.x. PMID 20572872. S2CID 41118603.


Life began near alkaline warm water vents about 3.8 billion years ago. It was carbon-based right from the beginning. Composed entirely of little carbon (lipid) sacks of water we call cells. That carbon all came from CO2. CO2 atmospheric concentrations then were about twenty times those of today (about 8,000-10,000ppm.) And there is fifty times more CO2 dissolved in water than there is in the atmosphere! A short (in geological timeframes) 200 million years later photosynthesis evolved. Photosynthesis is nearly as old as life itself. And arguably the most important metabolism in life on earth. Photosynthesis freed living organisms' dependence on warm water vent energy and allowed life to spread anywhere there is sunlight! Its formula is sunlight plus CO2 plus H2O with the green enzyme chlorophyll converts sunlight energy into living high energy sugar bonds, brings the carbon (2nd most abundant element in living species) into our carbon-based life on earth, gives the "environment" its green colour. All of life's carbon, all of life's energy, and as a bonus every molecule of the oxygen we breathe! There is nothing more important to life on earth. And life-essential CO2 has been dangerously and inexorably declining since life's birth to within 30ppm of the beginning of the death of all things. Recycling more CO2 by using fossil fuels has been the greatest contribution to the sustainability of life on earth that human beings have ever done. It delays the death of all things from a lack of CO2. Here's Dr. Patrick Moore, co-founder of Greenpeace. Thank you PragerU... https://www.youtube.com/watch?v=WDWEjSDYfxc

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    $\begingroup$ The statement "Recycling more CO2 by using fossil fuels has been the greatest contribution to the sustainability of life on earth that human beings have ever done. It delays the death of all things from a lack of CO2." is just false. The carbon cycle without humans is able to keep CO2-levels relatively stable by itself. Using PragerU as a source is laughable. $\endgroup$
    – smichel
    Jan 18 at 13:07

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