The hunt for what eats the ocean's most abundant life

In a lab on Portland State's campus, Anne Thompson keeps two microbes alive so she can watch one hunt the other. One is a predator much smaller than a speck of dust. The other, its prey, is the most abundant light-harvesting life form on Earth. What passes between them, who gets eaten and who escapes, is a question ocean science has mostly left unanswered. Thompson wants to answer it.

The prey is called Prochlorococcus - Thompson and colleagues call it "Pro" for short. It is hard to see with even the best microscopes, yet it is the most numerous light-harvesting cell on the planet, drifting through the open ocean by trillions upon trillions. It sits near the base of the marine food web, and its life and death help shape how the ocean stores carbon and feeds everything above it. Scientists have spent decades learning what helps Prochlorococcus grow. What kills it, apart from viruses, has stayed murky.

This month, Thompson, an associate professor of biology, won a Faculty Early Career Development (CAREER) Award from the National Science Foundation to change that. The award, worth nearly $1.2 million over five years, is among the most prestigious the agency gives to early-career scientists. For Portland State, a federally designated Emerging Research Institution working to win more top-tier federal grants, it is a marker of momentum.

Thompson's approach is to bring the chase into the lab. Her predator is a choanoflagellate, a single-celled ocean hunter that feeds by whipping its tail to draw water and prey toward a collar of tiny, finger-like structures, then swallowing the prey whole. Her lab has worked out how to measure which Prochlorococcus cells get caught and which get away. Over five years, her team will test about 40 kinds of Prochlorococcus, asking whether the predator plays favorites, whether a cell's size or health changes its fate, and whether the prey can sense danger and react.

"We still cannot say very much about what other cells kill the most common cell in the ocean, or why some cells escape," Thompson said. "This gives us five years to find out, and to bring students along for the discovery."

Students are central to the plan. Thompson will fold the experiments into two microbiology lab courses, where about 200 undergraduates - many bound for nursing, pharmacy, and other health careers - will run the predation experiments themselves and generate real data for the project. Some will have the opportunity to move into paid summer research in her lab, and some into graduate study. It is the kind of early, hands-on research experience that Portland State has made a priority for its students.

Thompson is also writing a popular-science book about Prochlorococcus and the messy, human process of scientific discovery, in development with Princeton University Press. She wants readers who may never peer into a microscope or spend time on the ocean to view the open ocean not as empty water but as a crowded, living system worth protecting.

The work is basic science, and Thompson is careful not to promise more than it can deliver. But understanding what eats the ocean's most abundant microbe could sharpen how researchers predict the sea's role in a changing climate. For now, the hunt has funding, a five-year runway, and a lab full of students about to meet the smallest predators in the sea.