They have clorophyll as well as red pigments and carotenoids. Most live in the ocean. Some of them live symbiotically with jellyfishes, mollusks and corals. Many species of dinoflagellates produce toxins. The most famous one is Gonyaulax cantanella which cause red tide in oceans during the summer.
Red tide is causes by a red colored toxin that can make humans sick.
Red Algae Rhodophyta Red Algae is multicellular and is seaweed. It is autotrophic and contains clorophyll and phycobilin. Red Algae lives deep in the ocean. Brown Algae Phaeophyta Brown Algae is autotrophic and contains chlorophyll and fucoxanthin. They are multicellular and known as kelp.