Why Do Photosynthetic Protists Play a Critical Role in Aquatic

Why do photosynthetic protists play a critical role in aquatic ecosystems?

A) Photosynthetic protists and prokaryotes are the major producers that form the critical base of food webs.
B) Photosynthetic dinoflagellates are in a mutualistic symbiotic association with coral, and coral reefs are one of the most diverse and productive biomes on earth.
C) They outcompete the non-photosynthetic parasitic protists.
D) They form colonies that cover the water's surface, which allows anaerobic organisms to thrive.
E) A and B