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Diatom alga. Coloured scanning electron micrograph (SEM) of an unidentified diatom alga. It has an intricately patterned cell wall, or frustule, made of silica and pectin. The wall is in two halves which fit neatly together. Diatoms can be circular (centric, as here) or elongated (pennate). They are found in both marine and freshwater, and are a major constituent of plankton. As such, they are of great importance to the food chains in their environment. Magnification: x1, 300 at 5x7cm size. x4500 at 7x9.5ins (KEYSTONE/SCIENCE PHOTO LIBRARY/MICROFIELD SCIENTIFIC LTD)