Humans have made use of this algae to make alginates. Red Algae is also a multicellular seaweed. It gets its color from phycoerythrin. This algae is found on the algal crest in coral reefs. Agar, carageenan, and nori are all derived from red algae.
Diatoms are unicellular organisms found in both marine and fresh water environments.
They are closely related to brown algae. Their shell, or frustule, is made of silica. They reproduce by continually splitting and getting smaller and smaller. We are also able to tell from diatoms the past pHs of the water, making them good bioindicators. Dinoflagellates are mostly marine.