Andrew Thurber is a self-described “connoisseur of worms.” He finds these wriggling, sinuous creatures, many with jaws and enough legs to propel an army, to be “enticing.” In the Antarctic, where he dives through the ice in the name of science, a type of worm known as a nemertean can reach 7 feet long.
Giant worms aren’t the only extreme feature of the seafloor next to the Ross Ice Shelf. Voracious sea stars and sponges the size of a person dot a muddy, rock-strewn landscape. At nearly 2 degrees below zero Celsius, sea water in the Southern Ocean is as cold as it can get without freezing. And it’s stunningly clear. Although sunlight filters dimly through surface ice, visibility can reach 500 feet on a bad day. On a good day, a diver can see underwater mountain ranges in the distance.
What attracts scientists like Thurber to this eerie, forbidding place is a riddle. Here, where darkness prevails for much of the year, the density of some species is higher than anywhere else on the planet. Colonies of worms, Thurber’s favorite animals, have five times the number of individuals, up to 150,000 per square meter, as one would predict and twice more than any other known location. In attempting to understand what’s going on in this remote habitat, Thurber is revealing fundamental processes that fuel deep-sea ecosystems worldwide. His work could also refine estimates of how carbon is sequestered in the deep sea, a critical question in climate change.
Diving Through the Ice
Over the last decade, Thurber has made the often turbulent trip to the frozen continent four times. Near the United States base at McMurdo, he and his team drill a hole through as much as 10 to 15 feet of ice to reach the water. They place a warming hut over the opening, as though they were preparing for a day of ice fishing.
Not surprisingly, divers take extraordinary care in this harsh environment. They wear extra layers, including three hoods, and cover nearly every inch of skin. “The only thing that is exposed is my lips, and when you get in the water, they go numb immediately,” says Thurber.
Divers avoid breathing into their scuba apparatus until they’re submerged. In the frigid Antarctic air, moisture in the breath can freeze the regulator and cause the entire air supply to discharge at once. And if vapor accidentally hits the inside of a facemask, it can rapidly turn into a sheet of ice and obscure vision.
Nevertheless, for Thurber, the sea is actually a relief from the bitter Antarctic wind. “The water is so much more pleasant than the air; it’s wonderful,” he says.
Once underwater, Thurber spends time exploring his surroundings and collecting samples of seafloor sediment to take back to his lab. In 2012, he and Rory Welsh, an Oregon State graduate student in microbiology, investigated the 100-foot face of a glacier that ended in the Ross Sea. Streaming out from the bottom of the ice onto rocks were mysterious filaments of microorganisms. “We have no idea what it is,” says Thurber. “It’s one of the things we hope to study in upcoming years.”
Thurber has set his sights on understanding the relationship between microorganisms and marine animals. In 2011, he reported on a type of crab that “farms” bacteria on its claws and lives off the harvest. “There’s an idea that bacteria don’t do well in the cold and play a minor role in these ecosystems compared to animals,” he says. “The general idea is that the worms bury their food in the mud and eat it throughout the year, sort of like putting their food in a refrigerator. This is called the ‘food bank hypothesis.’ I don’t know that I buy that, so that’s one of the things I’m testing.”
By “food,” Thurber means the algae that grow on the bottom and edges of the sea ice. For a brief period during the Antarctic summer, algae “rain down onto the seafloor” after they die, he says. Thurber is testing the possibility that worms and microorganisms feast on this abundance of organic matter. “By the end of the winter, the easily available food is gone, and the worms switch to eating their competitors. They are living off bacteria as a food source,” says Thurber.
To find out which idea — whether the worms store their food in the mud or dine on microorganisms — is closer to the truth, Thurber collects tubes of sediment during his dives. Within an hour, he can have them, complete with worms and other animals, back in a well-stocked biology lab. He analyzes some for microbial fingerprints to see how abundant the bacteria are and who’s eating whom. He conducts experiments on other tubes to see how the organisms process nutrients.
In one experiment on the seafloor, Thurber placed transparent tubes vertically into the sediment. He put some in the dark by covering them with black electrical tape. The next day, he took them back to the lab — including the muddy, wriggling contents — to see if organisms on the seafloor were actually producing food through photosynthesis. “It turned out that diatoms on the seafloor were producing about 25 percent of the daily energy for the community, the whole community, including the bacteria, worms and other animals,” Thurber says.
“That may be an additional food source during the light time of the year. Since there may be more food available than scientists thought, that means the worms have even a greater swing between feast and famine over the course of the year.”
Almost two-thirds of the planet is covered by a vast expanse of dark, muddy seafloor where life thrives despite extreme conditions. These mechanisms — how animals compete with and eat bacteria, how seasonal pulses of nutrients stimulate growth — may control the long-term productivity of the marine environment as well as long-term carbon sequestration, a critical step in the global carbon cycle. Since most of the seafloor is thousands of meters deep, well beyond the range of divers, the Antarctic happens to be the most easily accessible place to find out how these systems work.
In an Antarctic research lab, Andrew Thurber became enamored with worms. “Worms are incredibly diverse. That was one of the most amazing things to me,” he says. “They don’t all look like earthworms. They have feet and these crazy breathing structures. I found them kind of enticing.”
Ecologists, Thurber says, have spent a lot of time studying how large animals interact — wolves and moose, for example, or lions and gazelle. In contrast, science has largely ignored how animals compete with and prey on microorganisms. “Since bacteria and archaea perform most of the important chemical reactions on the planet, that’s a real shortcoming in our understanding of the globe,” he adds.
See Thurber’s blog for more photos and last winter’s reports from the field.