Biogeochemical Sleuth


Following a trail of chemical clues through the ocean

Susan Henrichs has picked a tough puzzle to solve. She tracks organic compounds in the biggest, fiercest, most inscrutable ecosystem on Earth—the oceans. And she tries to piece together these small, sparse chemical clues to reconstruct the vast, complex, cyclical food chain that is the engine of life in the oceans.

The cycle starts with plants harnessing sunlight, carbon dioxide, nutrients, and water to produce the organic building blocks they need to grow, said Henrichs, a 1980 MIT/WHOI Joint Program graduate and now a Professor of marine science at the University of Alaska’s Institute of Marine Sciences.
“The plants are consumed by animals,” she said, “and when the animals are eaten or die, some organic building blocks are released to the water to be used again by plants. Bacteria and other organisms break down the organic substances, recycling them in a process similar to that which occurs when leaves and grass decompose in a compost pile. However, a small fraction of the organic substances survives and sinks to the seafloor to accumulate, layer by layer over thousands of years, in muds at the bottom.”
Tracking the fate of organic substances in the ocean is a key to unraveling this complicated biogeochemical cycle and to understanding the ocean’s productivity, Henrichs said. Some organic substances, produced only by specific plant or animal groups, act as “biomarkers” to track specific marine life way into the past.

“The record is imperfect, though, because bacteria are generally so effective at destroying organic matter,” she said. “Only about one in a million molecules originally making up the cells of marine organisms survives to end up in sediments, and, even there, bacteria are constantly breaking down molecules and changing what we find. It is sort of like trying to read a history book with five, or even nine, out of every ten words blacked out. Knowing more about the decomposition process, and why some organic substances escape and are preserved for millions of years, helps us to read the record better.”

Henrichs is using the biomarker approach to learn why polluting hydrocarbons from petroleum remain in sediments long after they have disappeared from the water above. She also leads several programs investigating productivity in the Bering Sea and the decline of fisheries there.

Henrichs divides her time nearly evenly between research and teaching—both graduate-level courses and an introductory oceanography course for freshman and sophomore non-science majors.
“I find the latter enjoyable, though challenging, because it is difficult to interest some of these students in scientific issues,” Henrichs said.

A native of Anchorage, Henrichs was exposed to the sea early on, enjoying fishing, beachcombing, and clamming along the Alaskan coast. A high school teacher had sparked an interest in ocean science, but she entered the University of Washington to study chemistry—until a fateful chemistry lab held in the basement of an older campus building.

“The ventilation was poor, it was about 110 degrees in the room, and we were using benzene, a smelly and rather toxic organic solvent, in our experiment,” she said. “During that lab I decided I did not want to be an organic chemist under those conditions.”

The next day she was an oceanography major, and upon graduation she traveled to the East Coast for the first time to study in the Joint Program.

“WHOI had a very active research group in marine organic chemistry at the time,” Henrichs said. “There was a huge variety of outstanding research going on.” She was intellectually stimulated by WHOI scientists who spanned all academic generations—from older pioneers in the field and young professors to post-docs, visiting scientists, and even her own classmates. “There are few other oceanographic institutions where you could go to a seminar given by a leading researcher, either local or visiting, several times per week,” she said.

The Joint Program requirement that students develop their own research “was a great experience and excellent preparation for the real world,” she said. She received support all down the line—from her advisor, John Farrington, who “helped a great deal with both the broad aspects of research design and data interpretation and the practical aspects of getting supplies, ship time, and other resources,” from Assistant Scientist Cindy Lee, and from “an excellent technical staff, who made it easy to do a lot of things in the lab that are very difficult, I found later, without such support.”

After the Joint Program and a postdoc at Scripps Institution of Oceanography in California, Henrichs returned home to Alaska, completing a cycle reminiscent of the one she has been studying.

“Most days I enjoy my work,” she said, “especially the fun of finding a piece in the larger puzzles I am trying to solve, or explaining something difficult to students, so that they finally understand.”

—By David M. Lawrence