Steve Zender, age 15, of Newport News, Virginia, for his question:

What exactly are diatoms?

Diatoms helped to make the cleansing powder you use to scour the bathtub and maybe they put the polisher in your toothpaste. These and other useful chores are performed by diatoms that lived ages ago. Countless living diatoms, too small to be seen, have far more important roles in nature's ecology. They form the base of the food chain that feeds all life in the seas. And aside from their usefulness, diatoms are among the most beautiful of all living things.

A few large diatoms are barely big enough to be seen with the naked eye. Most diatoms measure about 1,000 to the inch. It's worthwhile saving your money to buy a microscope, just to observe them. More than 10,000 species have been identified and many more types teem in all the fresh and salt waters of the world. Each one is a miniature work of art. The basic design is a tiny jewel box made from two matching shells of glassy silica. The shells are tinted with delicate shades of ambers, browns or greens and embossed with exquisite designs. The shapes come in squares and oblongs, circles and crescents. The delicate engraving comes in grooves and ridges, tiny pits and bumpy buttons.

The single celled diatoms live sealed inside their glassy shells. Since most shell builders are animals, we would expect them to be miniature members of the animal kingdom. However, the diatoms are single celled members of the plant world, microscopic cousins of the algae and other floating seaweeds. Like most plants, they contain chlorophyl and use it to manufacture their food from sunlight, water and carbon dioxide. Many species also contain other pigments that mask the basic green of the chlorophyl with tinges of brown and golden amber.

Diatoms teem in all the waters of the world but they are most abundant in cool and polar seas. There they form the basic ingredient in rich meadows of floating plankton. In the sea, half a ton of this plant food feeds 100 pounds of tiny animals. In turn, they add 10 pounds of meaty protein to larger fishes. And 10 pounds of fish protein can add a pound of weight to your growing body. Multitudes of diatoms form the base of the food pyramid that supplies all life in the water    from tiny shrimps to giant whales. But the system is not one sided    all give and no take. The diatoms need carbon dioxide to perform photosynthesis. And this carbon dioxide is provided by fishes and other animals that depend on the diatoms. This is one of the countless give and take systems in ecology.

The lovely little diatom shells are made from silica extracted from the water. Silica, hardest of the common minerals, is the durable material in tough quartz and gritty sand. The one celled diatoms live and die in countless numbers    but their hard shells endure for millions of years after they sink to the bottom and form thick deposits on the floors of the oceans.

In past ages, diatoms swarmed in shallow seas that later became dry land. Deposits of their shells are called diatomaceous earth. This material is mined to make cleansing powders, toothpastes and other products. The tiny shells are as hard as gritty sand, tough enough to scour a porcelain sink    but too small to scratch its smooth surface. The diatomaceous earth in a thimbleful of cleansing powder may contain 100 million tiny diatom shells that were created millions of years ago.