Diatom alga. Light micrograph of the triangular- shaped diatom Triceratium ladus. Diatoms, members of a division of the algae family, are microscopic, unicellular organisms that occur abundantly in marine and freshwater plankton. They form an important part of the food chain. The body is composed of two halves which fit together like a tiny pill box. The cell walls contain silica, which is preserved in deposits known as siliceous or diatomaceous earths (ooze). Once hardened into rock it is called diatomite. The high porosity of this rock enables it to be used for filters, fillers & as a mild abrasive. Magnification: x120 at 35mm size.