We have already noted that numbers of cyanobacteria are at the bottom of aquatic food chains, but members of all the pro-tistan algal phyla play similar roles. Diatoms, for example, are consumed by fish that feed on plankton. Up to 40% of a diatom's mass consists of oils that are converted into cod and other liver oils that are rich sources of vitamins for humans. The oils also may in the past have contributed to petroleum oil deposits. The National Renewable Energy Laboratory in Golden, Colorado, is working on a project to convert oil from cultured diatoms into a "clean" diesel fuel substitute.

Diatoms also have other extensive and more direct industrial uses. In the past, they apparently have been at least as numerous as they are now. As billions upon billions of them have reproduced and died, their microscopic, glassy shells have accumulated on the ocean floor, forming deposits of diatomaceous earth. These deposits have accumulated to depths of hundreds of meters in some parts of the world and are quarried in several areas where past geological activity has raised them above sea level. At Lompoc, California, beds of diatomaceous earth are more than 200 meters (650 feet) deep, while in the Santa Maria oil fields of California, deposits reach a depth of 1,000 meters (3,280 feet).

Diatomaceous earth is light, porous, and powdery material that contains about 6 billion diatom shells per liter (1.057 quarts), yet a liter weighs only eight-tenths of a kilogram (1.76 pounds). It also has an exceptionally high melting point of 1,750°C (3,182°F) and is insoluble in most acids and other liquids. These properties make it ideal for a variety of industrial and domestic uses, including many types of filtration. The sugar industry uses diatomaceous earth in sugar refining, and its use for swimming-pool filters is widespread. It is also used in silver and other metal polishes, in toothpaste, and in the manufacture of paint that reflects light. Reflectorized paint is used on highway markers and signs and on the automobile license plates of some states. Diatomaceous earth is also packed as insulation around blast furnaces and boilers and is manufactured into the construction panels used in prefabricated housing.

Pacific Coast fish farmers have experienced losses of salmon and cod when dense concentrations of the diatom Chaetoceros have developed in the aquaculture pens. The diatoms have long hollow spines that break off and penetrate

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