Is $\ce{CO_2}$ in photosynthesis the only way for carbon to become organic?

Question

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.

Answer 1

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.

Source: Teara

Answer 2

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.

Answer 3

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