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Sallie Watson Chisholm is planting the seeds of science literacy

AAAS Fellow Sallie (

She's a professor at the Massachusetts Institute of Technology, a National Medal of Science award winner, and co-discoverer of the most dominant photosynthetic organism in the ocean. With so many accomplishments, who would think the greatest challenge Sallie (\Penny\") Watson Chisholm has faced in her career would be writing children's books?

"It's the hardest thing I've ever done," Chisholm says, \"but it's also been really fun."

Chisholm is currently working on the third book in the "Sunlight Series" with co-author and illustrator Molly Bang. The first two books, Living Sunlight: How Plants Bring the Earth to Life and Ocean Sunlight: How Tiny Plants Feed the Seas, both received the AAAS/Subaru Science Books & Films Prize for Excellence in Science Books.

"Photosynthesis is the foundation of essentially all life on our planet," Chisholm says, \"and I was frustrated that so many people go through their lives not understanding this fundamental fact about living on Earth."

She believed that reaching out to kids would get her message to a broader audience: While adults may not read an adult book about photosynthesis, they might read a children's book about it to their kids.

Chisholm graduated from Skidmore College, in Saratoga Springs, NY, in 1969, when the school was still a women-only institution. She says it was the encouragement of her advisor that inspired her to pursue a career in biology. "It had never occurred to me to go to grad school until my advisor told me I could get a Ph.D. if I wanted," Chisholm says.

As a graduate student studying ecology at the State University of New York at Albany, Chisholm remembers being impressed by the power of the model systems that molecular biologists used. "I always thought we should be able to do that in ecology: pick one species and understand it at all scales," she says.

For the past 20 years, Chisholm's research has revolved around a single organism, the ocean microbe Prochlorococcus. Although Prochlorococcus is a bacterium, it performs photosynthesis like plant cells, turning sunlight and carbon dioxide into living biomass. Chisholm calls Prochlorococcus an organism of superlatives: It's the smallest and most abundant photosynthetic organism on the planet. The bacteria may measure less than one micrometer, but they dominate the seas. It's estimated that there are 1027 of the microbes in the ocean, the equivalent of roughly 100 million Prochlorococcus per liter of seawater. 

Chisholm was part of the team that first discovered Prochlorococcus in 1988. The scientists were searching the Atlantic Ocean for another microbe. Chisholm's former postdoc, Rob Olson, of the Woods Hole Oceanographic Institute, decided to bring a biomedical instrument called a flow cytometer onto the research vessel. Flow cytometers count cells individually as they pass through a sensor, and they're mainly used in hospitals to count the cells in blood samples, but Chisholm and Olson figured that they could also count cells in samples of seawater.

When Chisholm and her team first saw the results of the flow cytomer, they weren't sure what they had found. At the time of their discovery, scientists didn't know of any ocean microbes that small that contained chlorophyll. It soon became clear the scientists had discovered a tiny new photosynthetic organism, and Chisolm's research career took off in a new direction.

"When I saw the beauty of the data, I was hooked," Chisholm says. "Seeing how you could study this one species—in controlled experiments in the lab and in its larger ecological context in the wild—inspired me to pursue Prochlorococcus as a simple model system for the rest of my career."

In the years since the discovery of Prochlorococcus, Chisholm has shown how these tiny organisms play a significant role in keeping ocean ecosystems healthy and the Earth habitable. Through photosynthesis, they produce as much as 20 percent of the oxygen we breathe. They serve as the base of the food web in the ocean. They even play a role in modulating climate by consuming and sequestering carbon dioxide from the atmosphere.

Prochlorococcus is part of the biological pump in the oceans, drawing carbon dioxide out of the atmosphere and sequestering some of it in the deep ocean. A better understanding of this system will help studies on climate change. Chisholm points out that scientists are just beginning to understand the complexity of the ocean ecosystem. "Basic research on the carbon cycle and the organisms that play a role in it is absolutely essential for any kind of policy decisions about climate change," she says. "The oceans are an essential part of regulating our climate and the biology of the oceans is an essential part of that."

Chisholm's third children's book will focus on fossil fuels and the carbon cycle. "In order to understand why burning fossil fuels so rapidly over 200 years is such a dramatic shock to the earth, you need to understand the natural carbon cycle," Chisholm says.

When she and Molly Bang started the "Sunlight Series," they considered taking a cute approach, like following the adventures of a cartoon carbon atom. But they ultimately decided to go for drama and awe at the majesty of the natural world. "We need to respect and understand life on Earth in order to use its resources in ways that are responsible and sustainable," Chisholm says. "That's really what the books are about."

Representative Image Caption
AAAS Fellow Sallie ("Penny") Watson Chisholm next to her two books about photosynthesis for children. (Photo: James Long)
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Mary Bates, Ph.D.