Boetius’s big moment happened in a dark room at the Max Planck Institute late on a Saturday night, when she would have rather been out dancing. She was sitting in front of the microscope, staring at images of sediment slices. She had used probes designed to detect 20 different species of sulfate-reducing bacteria before she finally found one that would produce glowing dots on the screen. The probe from DeLong and Hinrichs, on the other hand, had worked right away: The Hydrate Ridge sediments were loaded with their methane eater, which is not a bacterium at all but a species of Archaea, an ancient group of microbes that diverged from bacteria billions of years ago and are as distinct from them now, genetically speaking, as humans are.
Boetius was trying to count those dots on her computer screen, first the archaea and then the sulfate reducers, to find out how many were in her sediments. The cells were clumped together, which made them maddeningly hard to count. Then, in her growing irritation, she noticed something. “I see these stupid clusters of archaea, and now I see these stupid clusters of sulfate reducers,” she says. “And they had a very funny shape. The archaea looked like real clumps—lots and lots of cells sitting together. The sulfate reducers were like shells, a circle of sulfate reducers with nothing in the middle. And, really, I sat there for two hours before it finally popped into my head.”
The sulfate reducers were stuck to the archaea, forming a shell around them. Tori Hoehler’s idea of a microbial consortium suddenly looked compelling. Boetius and Widdel and graduate student Katja Nauhaus at Max Planck later performed the same experiment they and other microbiologists had done so many times before in vain—injecting methane into the sediment. But this time it vanished, and sulfide appeared in its place. Each clump was less than one-thousandth of an inch across and contained hundreds of cells. There were about 900 million clumps in every ounce of sediment at Hydrate Ridge.
The archaea in Boetius’s clumps were close relatives of other archaea that live a quarter or a half mile down—the ones that make the methane in the first place. The methane makers assemble the gas from hydrogen and carbon dioxide; the methane eaters do something like the reverse—but not quite, because they don’t seem to give off hydrogen. In some way that remains unclear, they pass energy onto the sulfate reducers that surround them. What the archaea get in return is also not clear. “There’s some kind of delicate interaction that we do not understand,” says Widdel. He has a postdoctoral student and a graduate student trying to grow the consortium in a laboratory, knowing that the reason he and other microbiologists failed to do so in the past is that the microbes grow extremely slowly. “We know it will take time,” Widdel says. “We might need two or three Ph.D. theses.”
Why is it worth the trouble? Boetius and her colleagues have found the consortium in 20 or so other places around the world—everywhere they have looked, including a mud volcano in the Arctic Ocean, and at cold seeps and hydrate mounds in the Gulf of Mexico. Two years ago, in a kind of crater off the Democratic Republic of the Congo, 10,000 feet down, a team led by Myriam Sibuet of the French Research Institute for Ocean Exploitation, discovered a spectacular cold seep with a vast field of clams and mussels, blue shrimp, purple sea cucumbers, and six-foot-long tube worms growing in bushes next to mounds of gas hydrate. Boetius’s microbes were in the mud there too. Boetius thinks her consortium, or something like it, provides sulfide at cold seeps everywhere. It is at the base of the food chain for these seafloor oases.
In the Black Sea, on the other hand, the consortium is the food chain. The mats on the seafloor there, and the walls of the chimneys, are a thick patchwork of methane-eating archaea and sulfate-reducing bacteria. The carbonate cores that allow the chimneys to stand tall are a by-product of the microbes’ metabolism. (At Hydrate Ridge there are giant slabs of carbonate.) A network of microscopic channels allows water to circulate through the chimneys, supplying the microbes with the chemicals they need. “It’s astonishing that as microorganisms they build up structures like that,” says Seifert.
It’s astonishing, too, to think what Earth would be like if these microorganisms didn’t exist. All the methane that is now being converted to carbonate and biomass would instead be bubbling freely from the seafloor—everywhere. Hinrichs and Boetius have estimated that an additional 300 million tons a year of methane would escape from the mud. It’s not clear how much of a greenhouse effect that would produce, but it’s a good bet that Earth would be a lot warmer—much as it would be, say, if there were no plants drawing carbon dioxide out of the atmosphere. “Everybody knows that our planet would be a different planet if there weren’t any plants,” says Boetius. “But nobody has thought about who keeps us from having a methane atmosphere.” All of which is a lot more than a thought experiment—because in the past the microbes have not always succeeded as well as they do today.
 Graphic by Don Foley (click to enlarge 62k) |
AN UNSTABLE ECOSYSTEM
The seafloor methane cycle is a loop in the planetwide carbon cycle that governs climate—a loop that has mostly been ignored. The methane cycle is run by microbes. Hundreds of yards below the seafloor, microbes called archaea produce methane from hydrogen and carbon extracted from organic sediments. The methane bubbles up along faults and fissures (green arrows). As it approaches the seafloor, it chills, and in many places it freezes, together with water in the mud, into solid methane hydrate (white). The hydrate is extremely unstable; as it gets buried deeper by fresh sediment falling on the seafloor above, it warms enough to release its methane again. A small fraction of all the methane bubbles up from the seafloor at cold seeps, which are turning out to be extremely common along the edges of continents. But ordinarily most of the methane never makes it into the water. Instead, it gets eaten by other species of archaea, which in turn supply energy to microbial partners, bacteria that can reduce sulfate in the mud to hydrogen sulfide. It is this foul-smelling compound that provides food for the clams, tube worms, and other animals that cluster around cold seeps on the seafloor itself. The trouble with this fascinating cycle, as far as humans are concerned, is that it is extremely unstable. “You can build up enormous amounts of methane over time,” says Gerald Dickens, both in the frozen hydrates and as free gas below them. When that happens, it doesn't take much—a submarine landslide or a slight warming of the bottom water—to release potentially catastrophic burps. —R. K.
