Actinobacteria sp. are fermenting organisms, which do not require oxygen to survive. These organisms break down organic material and convert it to inorganic material. Which part of the carbon cycle does this describe?

The organisms break down organic material and return its carbon to inorganic forms, which is decomposition. Fermentation is a way some organisms obtain energy without oxygen, but it does not change the process being described: breaking down dead or waste organic matter. Photosynthesis instead takes in inorganic carbon to make organic material.

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