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?

Decomposition.

Explanation:

Decomposition is simply a breakdown process of raw organic materials to a finished compost. This is a gradual complex process, in which both chemical and biological processes has to be in order for organic matter to form compost.

The decomposition that occurs organic matter by biological action takes place in nature since life first appeared on our planet. Lately, man has attempted to control and directly utilize the process for sanitary recycling and reclamation of organic waste material. 

The two forms of decomposition are:

ANAEROBIC (without oxygen) decomposition.

AEROBIC (with oxygen) decomposition and stabilization.

In both of these processes, bacteria, fungi, molds, protozoa, actinbacteria, and other saprophytic organisms feed upon decaying organic materials initially, while in the later stages of decomposition mites, millipedes, centipedes, springtails, beetles and earthworms further breakdown and enrich the composting materials. The organisms will vary in the pile due to temperature conditions, but the goal in composting is to create the most favorable environment possible for the desired organisms.