This carbon is then absorbed by phytoplankton, microscopic organisms in the ocean that photosynthesize to produce their energy (note this role can also be filled by macroscopic species of seaweed). The more carbon dioxide is put into the atmosphere, the faster phytoplankton can multiply thus draining the ocean of carbon dioxide faster. To achieve equilibrium the oceans suck carbon dioxide out of the atmosphere even faster, thus combating the rising levels of carbon dioxide in both our atmosphere and in the ocean itself. If we are unable to achieve this equilibrium, the ocean will become saturated with carbon, which can lead to the acidification of the water. Acidification in turn can make it harder for both microscopic and macroscopic organisms to use calcium in the water to create their shells. Photosynthesizing creatures, like phytoplankton, are one of the only forces which can counteract this progress, and by increasing their biomass, slow or reverse the course of ocean acidification.
At the next step of the carbon cycle, krill, zooplankton, and jellyfish then eat these phytoplankton. When these animals breathe they can release the carbon back into the ocean, which can exchange this carbon with the atmosphere, this completes the carbon cycle. Most of the carbon in an organism is the material that makes up its body, when creatures die, their bodies begin to fall to the bottom of the ocean as marine snow.
Most of this marine snow is eaten by fish and other animals living in the Twilight Zone of the ocean. At night, these animals migrate up to the surface of the ocean and release the carbon as they defecate and respire, this keeps more carbon up near the surface of the ocean allowing the phytoplankton to continue absorbing it.