The Buddy System

January 8, 2014 (posted January 9, 2014)
by David Levin

Watching the images that come back from Jason's control van, it's hard to believe that any of it is real. Huge tubeworms, as thick as a rolling pin, line the rocks. Giant mussels coat the seafloor, and towering structures spit out clouds of jet-black fluids.

At first glance, it seems as if these ecosystems just shouldn't be there. The deep ocean exists in complete darkness, and the water pressure is more than 3,000 pounds per square inch. But even with these harsh conditions, hydrothermal vents host an explosion of life. How?

When hydrothermal vents were first discovered in the late 1970s, Horst Felbeck says he ran into that question wherever he went. “I got into a cab, and when the driver found out I was a marine biologist, he turned around and asked me, ‘So what's up with those giant worms?’ It was fantastic.’ ”

Felbeck's cabbie had good reason to be excited. These worms are unlike any other creature around. They don't have a mouth or a stomach, so they rely on a secret weapon to survive: bacteria.

Kings of the ecosystem

Tubeworms may be the most spectacular organisms at these vents, but in vent ecosystems, bacteria are the undisputed kings. They live on nothing but the chemicals coming out of the vents. In a process called chemosynthesis, they use minerals and gases dissolved in the fluids that bubble up from below the seafloor to generate energy they use to survive. In a world with no light and no plants, they still thrive. So other organisms, like tubeworms, have formed an alliance with them.

Inside each tubeworm is a spongy organ called a trophosome, a virtual “hotel” for microbes that live inside it. Using a red, feathery plume at the top of its tube, the worm absorbs chemicals such as hydrogen sulfide from the water, pulls them into its bloodstream, and passes them along to the bacteria. The microbes get access to all the chemicals they can eat. In return, they make nutrients for the worm. Everybody wins.

It’s a strange system, but even stranger is how the microbes get there in the first place. Tiny, just-born worm larvae haven’t yet grown a hard tube shell and they haven’t yet attached to the seafloor. As they travel through the water, certain species of microbes burrow into their skin. Gradually, they work their way into the center of the worm, where the trophosome forms as the worm grows older.

This sort of relationship is a form of symbiosis, and Felbeck is here to study how it works in tubeworms. He’s taking samples of trophosomes and sending them back to colleagues in Germany who will study the bacteria’s genetic code and the proteins they make. They will try to figure out the relationship between worm and microbe. In the process, Felbeck said, they may be able to answer some lingering questions about tubeworms.

“How do they control the growth of the bacteria inside them, for instance? Why doesn’t it just grow like crazy? How do the bacteria find the larval worm in the first place?” he said. “Even 35 years after finding these worms, we’re still only starting to scratch the surface when it comes to knowing how they work.”

Bacteria everywhere

Those questions apply to more species than just tubeworms. Symbiotic relationships pop up all over the vent ecosystem. Consider the giant mussels that Ruby Ponnudarai is studying, for instance.

Mussels, she said, attract bacteria sort of like tubeworms do. They absorb them into their bodies when the mussels are still larvae and let the microbes make a home inside their gills. The mussel filters water that’s rich in chemicals from the vents. It passes those chemicals along to the microbes, which use them for chemosynthesis. The bacteria use the energy they get from chemosynthesis, and carbon dioxide or other chemicals in the water, to produce nutrients for themselves and the mussel.

Unlike tubeworms, though, “the mussels have a digestive system, so they can feed themselves if they need to. But it’s pretty inefficient, so they’ll still starve after a while,” Ponnudarai said.

It’s uncertain why the mussels still have a gut at all, especially since they rely so heavily on the microbes inside them to survive. Ponnudarai thinks this could give them a slight advantage over other vent animals—a survival mechanism in case of emergencies. If the vent goes dead and the flow of hydrothermal fluid stops—which can happen during a volcanic eruption—chemosynthetic bacteria lose the chemicals they need to generate energy. Animals like tubeworms, which have no other source of nourishment, will die. But the mussel will survive long enough to spread more of its eggs into the water, producing more offspring that have a chance to find another vent.

“Nobody is really sure if that’s the case,” said Ponnudarai, “but then again, there’s a lot we don’t know about these animals. That’s part of what makes studying them so exciting.”

You eat my trash, I’ll eat yours

Even microbes themselves can forge a sort of symbiotic relationship at the vents, according to Chief Scientist Stefan Sievert. Many of them need specific chemicals to survive, but they can only get those chemicals from other microbes that create them.

He said one type of bacteria uses ammonium (NH₄), a common chemical at vent sites, as food. As it converts that ammonium into energy, it spits out a second chemical called nitrite (NO₂). If levels of nitrite get too high, though, it could kill the bacteria—so the microbe lives only in the presence of a second species that can use the nitrite as food and limit the level of that chemical in the water.

Confused? Ileana Pérez-Rodríguez put it nicely: “Think of it like this: Imagine you were living somewhere where your trash accumulated. If the trash pile got too big, it’d start to be pretty nasty. But if you were living with an animal that could eat your trash, the situation would be great for both of you. You get rid of your trash, and they get lots of food.”

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