What is the primary function of phytoplankton in marine ecosystems?

Phytoplankton play a crucial role in marine ecosystems primarily through photosynthesis and oxygen production. As microscopic organisms that float in the upper part of the ocean, phytoplankton convert sunlight into energy, using carbon dioxide and water in a process similar to terrestrial plants. This photosynthetic activity not only forms the basis of the marine food web—providing energy for various marine life forms—but also generates a significant portion of the Earth's oxygen, contributing to the atmospheric balance essential for life.

The other options, while they represent important functions in marine environments, do not primarily relate to phytoplankton. For example, wave energy absorption pertains to physical dynamics of ocean waves rather than biological processes. Nutrient recycling is a broader ecological function that involves various organisms and processes in the ecosystem, not just phytoplankton alone. Coral reef formation involves the contributions of different organisms, like corals, and is distinct from the primary functions of phytoplankton. Thus, the key function of phytoplankton as the main producers in ocean ecosystems emphasizes their critical role in both the energy flow and oxygen supply in marine environments.