In the Carbon cycle, decomposers breakdown dead phytoplankton and return the carbon in their bodies to the atmosphere as carbon dioxide. A decomposer in any organism, especially a soil bacterium, fungus, or invertebrate, that decomposes organic material. Some decomposers in the ocean include bacteria, marine worms, and mollusks.
The process in the carbon cycle is cellular respiration. Cellular respiration is a series of metabolic processes that take place within a cell. Organisms from all kingdoms of life, including bacteria, archaea, plants, protists, animals, and fungi can all perform cellular respiration. In the Carbon cycle cellular respiration is used by autotrophs and heterotrophs to break down carbon containing molecules, such as sugars, to release the energy stored in them.
Fossil fuels play a big part in the carbon cycle. Fossil fuels are a natural fuel such as coal or gas, formed in the geological past from the remains of living organisms. When plankton fossilize, geological forces bury and compress them, and transform them into oil. Which is the dense liquid of carbon that we use to fuel our cars, trucks and buses.