how is atmospheric carbon in the form of CO2 converted into a molecule that is usable to all organisms?

The carbon-dioxide exists in the atmosphere, and it is dissolved in the water.

Explanation:

With the process of photosynthesis, it becomes organic, and by breathing, the organic gas goes back to carbon-dioxide.

The assimilation of carbon is the process when the carbon-dioxide converts to organic carbon with the help of living organisms. This organic carbon is then used for energy.

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