Final answer:
Soil microorganism respiration is the process that contributes to atmospheric CO2, contrasting with plant and phytoplankton photosynthesis, which reduce CO2.
Step-by-step explanation:
Among the processes listed, soil microorganism respiration acts as a source for atmospheric CO2. During this process, the microorganisms break down organic material and release carbon dioxide back into the atmosphere. This is part of the carbon cycle, where carbon moves between the atmosphere, oceans, and living organisms. In contrast, both plant photosynthesis and marine phytoplankton photosynthesis act to reduce CO2 levels by converting it into organic compounds. Atmospheric carbon dioxide can dissolve into the ocean, forming carbonic acid, and leading to the formation of carbonate and bicarbonate ions, which eventually contribute to oceanic sediments and the geological carbon reservoir.