Final answer:
Many prokaryotes function as decomposers in the carbon cycle, breaking down organic material and releasing carbon dioxide, which is then used by primary producers.
Step-by-step explanation:
A primary role of many prokaryotes in the carbon cycle is that of decomposers. Decomposers help recycle nutrients in an ecosystem, including carbon, by breaking down dead organisms and organic waste. Prokaryotic decomposers, which include bacteria and archaea, carry out the essential process of decomposition, releasing carbon dioxide back into the atmosphere as a byproduct. This carbon dioxide returns to the carbon cycle, where it can be used by primary producers like plants and photosynthetic bacteria to create new organic compounds through photosynthesis, thereby supporting entire ecosystems.