diatom

Unicellular algae, some of which are colonial, green or brownish in colour (but all contain chlorophyll) and with siliceous and often highly sculptured cell walls. Diatoms make up much of the producer level in marine and freshwater food chains, and they have contributed to the formation of oil reserves. Deposits of diatomaceous earths were formed by the accumulation of diatom cell walls. (Source: ALL)