Infinity may not be found in a grain of sand, but MIT professor Penny Chisholm has glimpsed it in a drop of water. Chisholm is a professor of environmental studies, engineering, and biology (she calls herself a "microbial ecobiologist"), but her research is built around a very small topic -- the tiniest, most abundant photosynthetic microbe in the ocean, a speck called Prochlorococcus. By studying the minuscule phytoplankton, she hopes to learn very big things.

"They're greatly underappreciated because they're so small," says Chisholm, 59. "They form the basis of the food webs in the oceans." Not to mention that they're the most common form of phytoplankton, which carry out half of the planet's photosynthesis. A better understanding of their ability to pull carbon dioxide out of the air might help scientists figure out a way to counteract global warming.

Even to the most dedicated biologists, Prochlorococcus may appear to be an unpromising subject. "They have no features," acknowledges Chisholm. "They just look like little black specks." But she believes that these microbes are, in fact, a window onto a larger vista. A milliliter of ocean water can contain 100,000 Prochlorococcus cells.

"They're the minimum form of life," she says. "We've decoded their genome sequence and found that it's made of 1,700 genes. That's the smallest number of genes that can convert sunlight into life."

By examining every aspect of Prochlorococcus -- from its genome to its effect on the biosphere as a whole -- Chisholm is practicing what she calls "cross-scale systems biology." In her lab at MIT, scientists from fields as disparate as engineering, oceanography, and biochemistry are trying to understand every aspect of this form of life and its role in the planet. By doing so, they want to contribute to a "reframing" of the study of life sciences in which a living system is understood from the cellular to the global level.

"Biology has become fragmented in the past 30 years," says Chisholm. " But to really understand a biological system, you have to study all of its scales."

Chisholm came to the oceans relatively late. Born in the Upper Peninsula of Michigan, she was a senior at Skidmore College when she took an interest in lake plankton.

"If you look under your microscope at plankton you see an invisible forest that's teeming with life," she says. This enthusiasm led to a PhD at SUNY Albany in limnology (the study of lakes). As a post-doc, she switched to saltwater at the Scripps Institution of Oceanography in San Diego. She's been at MIT for the past 30 years.

"She's made huge strides in ocean ecology," says Mick Follows, a principal research scientist at MIT's department of earth, atmosphere, and planetary sciences. "But beyond that, her work is helping our understanding of genetics and ecology in general."

To illustrate the importance of ecological studies, Chisholm indulges in a metaphor, describing a person sick from an electrolyte imbalance. "Now, the earth itself has an electrolyte imbalance, with CO{-2} and methane levels far exceeding what they should be." Microbes like Prochlorococcus are essential regulators of the earth's atmosphere. Also, studying how the microbe produces hydrogen could have applications for renewable energy.

"We're investing tremendous amounts of money into the biomedical sciences, but if we don't find a way to do the same for the health of the earth, we'll have healthy people living on a planet that won't sustain their grandchildren," says Chisholm.

The better we understand the planet, from its bacteria to its atmosphere, the better we can decide how to preserve it, says Chisholm, picking up a small, glass globe -- a toy "Ecosphere" -- that holds a splash of seawater, a few red shrimp, algae, and microbes. It's a self-contained system of air, water, and dirt.

"We don't understand how this works," says Chisholm, looking at the globe. "We're just starting out."

Hometown: Watertown.

Family: Husband, Don Sisson. Two cats, Jack and Gus.

Hobbies: Skiing. "Every year we go to Steamboat Springs, Colo., and meet up with the same group of oceanographers." She has a cabin back in Michigan, on Lake Superior. Also, serves on the board of the World Wildlife Fund.

Likes: The idea of having an institute like the National Institutes of Health, but for earth sciences.

Dislikes: The idea of "geoengineering," manipulating planetary functions to solve problems like global warming (for instance, building huge mirrors to reflect the sun's rays away from earth). "It would likely cause more problems than it would solve. Nature provides the best model for sustainable human systems."

To hear her: Chisholm will be speaking Wednesday at 6 p.m., at an MIT Museum salon-style event. In the library at the Hotel @ MIT, 20 Sidney St., in Cambridge. Call 617-253-4444 or go to web.mit.edu/museum.