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.
These organisms have cellulose plates with grooves which contain flagella 2 flagella at right angles. When there is a population build up, a red tide occurs. Zooaxanthellae, a dinoflagellate group, is a mutualist with coral animals. They have a flexible outer layer of proteins. They also have chloroplasts and an eyespot that can sense light.