Diatom alga. Light micrograph of a diatom alga, Caloneis permagna. Diatoms are microscopic, unicellular organisms that occur abundantly in marine and freshwater plankton. They form amajor 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 and as a mild abrasive. Magnification: x133 at 35mm size.