The primary function of phytoplankton in ocean ecosystems is to produce oxygen and serve as the base of the marine food web. As autotrophic organisms, phytoplankton conduct photosynthesis, converting sunlight and carbon dioxide into organic matter and oxygen, which supports marine life. They play a crucial role in carbon cycling, helping to regulate atmospheric CO2 levels. Furthermore, phytoplankton are essential in supporting higher trophic levels, including fish and marine mammals, making them vital for maintaining the health of marine ecosystems.