Phytoplankton are microscopic marine algae that are critical to the food chain. Phytoplankton living in the ocean today are divided into 2 main classes: dinoflagellates and diatoms. Both types provide food for a wide range of sea creatures. However, under certain conditions, phytoplankton can grow out of control and form harmful algal blooms, which produce toxic compounds that can harm marine life, birds, and even humans.