In my apartment, in San Francisco, I've just poured into the palm of my hand some of what may be the world's best defense against global warming. It arrived in a one-pint plastic container from EaglePicher Filtration and Minerals, Inc., a Reno, Nevada, firm that mined the stuff from an ancient lake bed nearby. As I rub it between my fingers, the white powder feels as silky as cornstarch. Even after I wash with soap and water, my fingertips are chalky, as though I've buffed them with extremely fine sandpaper.

The container holds the fossilized shells of countless diatoms, single-celled algae that inhabit oceans, lakes and other bodies of water. Like other phytoplankton, diatoms convert carbon dioxide, water and sunlight into food and release oxygen; unlike other phytoplankton, diatoms are encased in porous, intricately structured, often beautiful silica shells. Diatoms were all the rage in Victorian times, when genteel folk gathered around the parlor microscope to marvel at the jewel-like creatures. And ever since 1866, when Swedish chemist Alfred Nobel first mixed fossilized diatoms with nitroglycerin and rolled the stuff into a stick he called dynamite, the uses for diatom shells have been, well, exploding.

Today, processed diatomaceous earth (like the sample sent to me) is used to filter wine and swimming pool water, polish teeth and silver, and serve as filler in concrete, plastics and paper. It is mixed into house paint to flatten unwanted sheen and churned into golf courses and football fields to enrich and aerate the soil. Organic gardeners sprinkle it around roses and tomatoes; instead of poisoning hungry insect pests, pesticide-shy horticulturists make use of crushed diatom shells' sharp edges to cut the invaders to ribbons.

Yet the lowly diatom does its most important job on a far grander scale, as scientists are only now beginning to understand. By taking in carbon dioxide and sinking it to the ocean depths, diatoms dispose of huge amounts of the most abundant of the atmosphere's greenhouse gases. Recent satellite measurements suggest that phytoplankton process nearly half of all the carbon dioxide removed from the atmosphere by photosynthesis. These miniscule creatures, long regarded primarily as the bottom link of the marine food chain, capture nearly as much of the gas as all the trees, grasses and other land plants combined, majestic redwoods included. And of the many types of phytoplankton, diatoms are at once the most efficient carbon dioxide processors and the most prolific. A diatom, doubling about once a day, will beget 100 million descendants in a month.

How the organism manages to be so successful is becoming clearer. And the secret, researchers say, is its ornate shell. "Anybody who looks at a diatom is first struck by its beauty," says François Morel, a Princeton University oceanographer. "But with all that detailing, it's also wonderfully shaped to optimize the chemistry it needs to do."

Over time, a lot of atmospheric carbon dioxide dissolves in oceans and lakes, where it is converted into bicarbonate—a form that most plants and algae can't use. But diatoms and other phytoplankton possess an enzyme that converts bicarbonate back into usable carbon dioxide. Morel and collaborator Allen Milligan, an oceanographer at Rutgers' Institute of Marine and Coastal Sciences, discovered that the silica in the diatom's shell chemically speeds that conversion. Also, its latticework surface area, larger than that of a smooth shell, maximizes photosynthesis, giving the diatom more energy to grow and reproduce. "There have been other ideas over the years about how a diatom might benefit from a glass shell as protection, maybe, or as a heavy weight to help it gather nutrients" at lower depths, Milligan says. "But as far as we know, this is the first evidence that its chemistry is important."

The finding adds another dimension to understanding climate change, says Philip Froelich, an oceanographer at Florida State University at Tallahassee. "Potentially, diatoms are changing the chemistry of what's going on in the deep sea, and if so, we'll have to take that into account," he says. "It's too soon to say what sort of ramifications such changes might have. But it's safe to say that Morel and Milligan's discovery complicates an already complicated picture."

If a diatom isn't eaten and dies of old age (its life span is about six days), it sinks to the seafloor or lake bed, taking any carbon it processed with it. Buried diatom remains may then, over geological epochs, play another role in what scientists call the "carbon cycle" by contributing to the earth's store of fossil fuels, which, of course, yield carbon dioxide when burned.

Now that many authorities believe the atmosphere is loading up on carbon dioxide, with cars and factories and what have you producing the gas faster than diatoms and plant life can sequester it, maybe it's only natural that diatoms would figure in a proposed solution to the problem. There's at least 30 percent more carbon dioxide in the air today than before the Industrial Revolution, according to a 2001 report from the Intergovernmental Panel on Climate Change. To reduce those levels, some researchers and entrepreneurs have proposed boosting phytoplankton growth by "fertilizing" oceans with iron, a nutrient essential to the organism's growth. Preliminary tests in the equatorial Pacific Ocean and Southern Ocean since 1995 have shown that seeding seawater with iron does increase phytoplankton populations—in the short run.

But the prospect horrifies some ecologists. They say the approach could backfire, increasing atmospheric carbon dioxide levels by spurring the growth of the marine bacteria that feed on dead diatoms. "I could see significant warming happening even faster than anyone's imagined," says Kay Bidle, a marine biologist at the Rutgers marine institute and an expert on diatom ecology. Beyond that, Bidle and others say no one can predict how dumping iron into the ocean would affect marine life in the long run.

As I sit in my apartment, it's odd to think that the powdery diatomaceous earth in my hand helped cleanse the atmosphere, and humbling to think how much remains to be learned about an organism that has long beguiled nature lovers as well as scientists. More important, this new view of diatom chemistry suggests that answers to some of earth's biggest problems may be found in some of its tiniest inhabitants.