November 14, 1996
Harvard
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  Microbiologist-Aquanaut Colleen Cavanaugh Receives Tenure

By William J. Cromie

Gazette Staff

Not many people have visited with 6-foot-long, bright red worms on the boiling bottom of the ocean. It sounds like a Jules Verne fantasy, but newly tenured Professor of Biology Colleen Cavanaugh has seen these and other strange creatures on voyages to the bottom of the deep sea.

As a first-year graduate student, she discovered what makes life possible in a hell-on-earth where the sun never shines, temperatures can exceed 250 degrees F, and the ocean exerts pressures of thousands of pounds on every square inch of an animal's body. Giant worms, huge clams and mussels, and strange shrimp thrive in such conditions because of one-celled bacteria who live on and inside them. The bacteria turn sulfur, methane, and other inedibles into organic molecules that their hosts feed on.

"These bacteria are the primary producers of biological matter; they form the base of the food chain, at hydrothermal vents on the bottom of the Atlantic and Pacific," Cavanaugh says.

She went on to find the same kind of partnership among clams living in shallow eelgrass beds and mudflats along the coast of New England. For these discoveries and her reputation as a teacher and mentor, Cavanaugh was recently given tenure retroactive to Jan. 1.

The news was received with delight by other faculty of the Department of Organismic and Evolutionary Biology. "Colleen focuses on a fascinating and important problem in evolutionary biology, the symbiotic relationships between marine animals and chemosynthetic bacteria who live within their tissues," notes Professor of Biology Andrew Knoll. "Her research integrates approaches that range from molecular biology to deep sea exploration in the tradition of the great naturalists. I'm delighted that the excitement generated by her lab will animate Harvard biology for years to come."

The Inside Life

An important evolutionary angle of Cavanaugh's research involves the belief that important parts of our cells and those of other animals and plants were once symbiotic bacteria. She is tracing the history of how free-living bacteria take up life inside other creatures, then become integral parts of their hosts.

Examples include chloroplasts, now present in every plant cell and vital for turning sunlight into food. All plant and animal cells also boast mitochondria, former bacteria that provide energy for cell metabolism, much the same way that sulfur-eating bacteria provide fuel and food for giant tube worms.

Growing up in Detroit, Cavanaugh never imagined she would pursue such lineages and lead the life of a scientific Captain Nemo. She did, however, announce in the second grade that she wanted to be a scientist.

"In English class you had to write, in science class you didn't," she recalled. "So science became my first choice."

By the seventh grade, Cavanaugh leaned toward biology and ecology. "I thought about working on the Great Lakes, and decided to major in biology at the University of Michigan," she said.

In her sophomore year, Cavanaugh heard about a course in marine ecology at Woods Hole, Mass. The course turned out to be one of those events that change a person's direction in life.

Her research involved wading into cold water to study the mating habits of horseshoe crabs. But she "fell in love" with Woods Hole and the relaxed camaraderie and cross-fertilization among biologists, geologists, and other scientists of the sea. Leaving all that at the end of the course turned out not to be a problem because her car had broken down and she could not get back home.

Cavanaugh began searching for a waitress job, but luck intervened and she replaced a last minute dropout in a Boston University undergraduate research program. She went back to working with horseshoe crabs. She also met then later married Phillip Gschwend, an M.I.T. graduate student who was studying the development of lobster claws.

In 1977, Cavanaugh earned her undergraduate degree and moved to Woods Hole to work at the Marine Biological Laboratory there. During the next two years, she abandoned horseshoe crabs in favor of bacteria, creatures that impressed her for their ability to live anywhere. She thought that a life of research and teaching would be ideal.

Gutless Creatures

Cavanaugh applied to several graduate schools and was accepted at Harvard, her first choice.

During her first year as a graduate student, Meredith Jones, curator of worms at the Smithsonian Institution, gave her a giant tube worm captured by a research submarine working on the bottom of the Pacific. These worms lack both a mouth and a gut, and she wanted to figure out how such an animal lives.

The biggest clue involved sulfur crystals packed in its long, thin body. Cavanaugh believed, correctly, that bacteria inside the worms use sulfur to fix carbon into edible molecules. The bacteria nourish their hosts, who in turn provide them with the chemicals and oxygen they need to survive, as well as a stable place to live. Hot water pouring out of the tears in the ocean floor and undersea currents would, otherwise, float them away.

Such vents spew mineral-laden waters through many rifts in the bottom of the world ocean. Cavanaugh discovered that all the major animals living on or near these subsea springs live in partnership with bacteria who convert the minerals to food. In the middle of the Atlantic, for example, lives a shrimp whose body is covered with bacteria. The little symbionts provide nourishment for both the shrimp and other bottom dwellers.

Cavanaugh found the same kinds of partnerships among bacteria and clams living in shallow water. She also discovered a new species of deep-dwelling mussel in the Gulf of Mexico that shares its body with methane-utilizing bacteria. Methane, or marsh gas, is of no use to the mussels until bacteria convert it to carbon. The shellfish then munch the bacteria.

Such research earned Cavanaugh an M.A. in 1981, a Ph.D. in 1985, a Junior Fellowship in the Society of Fellows in 1986-89, an assistant professorship in 1989, then an associate professorship in 1993.

Cavanaugh worked on marine life in the laboratory and onboard ships for 12 years before she won a place on the deep-diving submarine Alvin. She went to the bottom of the Gulf of Mexico off Florida for the first time in 1992.

Last April, she dove to a depth of 8,200 feet off the west coast of Mexico and came face-to-no-face with the giant tube worms at last. Through a small porthole, she watched their brilliant red plumes sweeping the oxygen and sulfur from the black sea for the benefit of their body guests.

Some people think life on Earth began in these deep, hot springs. Cavanaugh thinks that "the idea makes sense because some of the oldest forms of free-living bacteria show signs of being heat-loving organisms."

She now focuses on understanding the nature and evolution of the partnerships that allow these host-guest combinations to live in otherwise inhospitable places. The host cannot survive without the food produced by its guest; and the guest cannot survive without a stable place to live. One place to look for such understanding is in the variation of genetic sequences (RNA) in the ribosomes, intercellular centers where bacteria produce vital proteins.

"The ribosomal RNA framework has absolutely revolutionized the study and exploration of microbial diversity," Cavanaugh claims. "From what I've been able to determine so far, symbiosis arose many times over hundreds of millions of years of evolution. Today, there are tens of thousands of different bacterial species living in soil, water, and even thousands of feet below the ocean bottom, but we've only identified about 4,000 of them. Ribosomal RNA studies will give us a start in categorizing this dizzying variety of life, to find out what they do, and to uncover the relationships between free-living and symbiont species."

Cavanaugh passes on her knowledge to students in undergraduate and graduate courses about environmental microbiology, bacterial diversity and symbiosis. In addition, she is thinking about starting a new undergraduate course on basic microbiology, a subject not now available to undergraduates.

Cavanaugh's husband, Phillip, is now a professor of civil and environmental engineering at M.I.T., and they have a son, Matthew, who is almost 3. When the three get together, they enjoy walking, hiking, and bicycling.

A sign at the entrance to Cavanaugh's office applies equally well to her professional and personal life. It reads: "It's the little things in life that matter."

 


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