Assorted diatoms. Interphako interference contrast light micrograph of a circular arrangement of various diatoms. Diatoms are planktonic unicellular algae. They have a mineralised cell wall (frustule) divided into two halves. The frustule contains silica and provides protection and support. Diatoms form an important part of the plankton at the base of the marine and freshwater food chains. Magnification: x143 when printed at 10 centimetres across.