What forms the basis of all food chains in open ocean?

Phytoplankton forms the basis of all food chains in the open ocean. Phytoplankton are microscopic plant-like organisms that live in the upper layers of the ocean and use sunlight to photosynthesize. They are the primary producers of food in the open ocean, and they are eaten by zooplankton, small fish, and other organisms. These organisms are then eaten by larger fish, seabirds, and marine mammals. Phytoplankton are essential to the survival of all marine life.