Phytoplankton play a crucial role in the oceanic carbon cycle by acting as a primary producer. Through the process of photosynthesis, these microscopic organisms convert carbon dioxide into organic matter, which forms the base of the marine food web. They not only absorb carbon dioxide from the atmosphere but also release oxygen, thus contributing to the overall gas exchange in the ocean. When phytoplankton die, a portion of their carbon sinks to the ocean floor, effectively sequestering carbon for long periods. This process helps mitigate climate change by reducing atmospheric CO2 levels.