Diatoms occur in both fresh and saltwater.
The two halves of the shell fit together like the top and bottom of a petri dish. Diatoms are classified as centric or pinnate depending on their shape. Centric diatoms have radial symmetry and many plastids, while pinnate forms are generally bilaterally symmetrical with two plastids. Diatoms contain chlorophylls a and c, carotene, and xanthophyll pigments in their plastids and store the food they manufacture through photosynthesis as chrysolaminarin, leucosin, or oil droplets.