Novel Worm Community Affects Methane Release in Ocean

Image of the polychaete in its natural state where it is sitting in its tube with it head out grazing the sediment around itself. You can see the branchie (breathing tentacles) on its head gathering oxygen while it eats. Credit: Oregon State University, photo: Andrew Thurber

In 2006, a multinational crew of scientists boarded a ship of off New Zealand’s east coast and set out to discover new “methane seeps” — sites where natural gas trickles through the sea floor.

The several sites they hit upon all displayed an unusual feature — a unique food web, dominated by worm beds, that resulted in much more methane escaping into the water.

“One of the most exciting things, aside from discovering seven new sites, was that they didn’t look like seeps anywhere else,” said Andrew R. Thurber, a post-doctoral researcher at Oregon State University. “The amount of methane seeping was off the chart — as much as 100 to 300 times greater than other locations.”

The seeps also looked different. Instead of the beds of dead clam shells and microbial mats characteristic of these environments, they were covered with strange dark patches prickled with many little dots. “We called them rain drop sites,” says Thurber, “because they looked like a muddy field after a fresh drop of rain.”

On a subsequent expedition in the spring in 2007, the scientists found that the mysterious black patches were in fact filled with bright-red worms. The worms belonged to the class Polychaeta and family Ampharetidae, which are found in methane seeps and other deep-sea sites all over the globe. But the two particular species discovered there were new to science. In these worm beds, the amount of methane escaping was much, much more than previously observed in any other seep habitats.

Methane is stored as an ice-like structure in the sea floor, and slowly trickles out of these geological reservoirs. Most of the methane escaping, however, is consumed and kept out the atmosphere by the bacteria and archaea that live in the sediments. “It’s a very important ecosystem service provided by sediment microbes,” says Thurber. “Most of the time, they keep the methane from releasing into the water column — except in extreme cases. This is an extreme case.”

Thurber is the lead author of a paper just published online in the journal Limnology and Oceanography. His study showed that the worms feed on the microbes that filter the methane, and hence belong to a novel food web.

This is an image of the worm out of its tube. Its head is in the upper left and its tail at the bottom. The red structures throughout are its circulatory system that it uses to harness the overlying oxygen rich water. Credit: Oregon State University, photo: Andrew Thurber

Implication for Global Warming

Methane is 23 to 27 times more potent than CO2 as greenhouse gas. But according the researchers, most of the methane in the New Zealand seep is likely consumed by biological activity in the water. So it doesn’t make it into the atmosphere, where it could exacerbate global warming.

The findings, however, have some implications for the warming of polar regions.

One critical element for the creation of this unique habitat may be oxygen-rich water. The study sites off New Zealand are bathed in oxygen-rich water from the Southern Ocean. Methane seeps and worm communities are present in many areas around the world at mid depths of 400-1200 m, but this water often has low levels of oxygen. This could be a factor that constrains the growth of the worm populations, according to the researchers.

“Other areas of oxygen-rich water include polar habitats,” says Thurber, “and polar habitats are warming and releasing more methane than they have in known history.”

In fact more than 150,000 sites where methane is seeping into the atmosphere were recently discovered in the arctic.

“If there’s some sort of key relationship between the type of worm, high-oxygen and high methane release, understanding how the ecosystem works may help us when we start looking at high methane release in other areas,” said Thurber.

“Warming of the deep ocean raises the possibility that additional methane will be released from dissociation of gas hydrates on continental margins. The types of animal-methane interactions described in this paper will be critical to understanding the fate of methane released,” added Lisa Levin, a researcher at the Scripps Institution of Oceanography and a co-author of the study.

Implications for Astrobiology

Methane seeps belong to a type of challenging environment called “chemosynthetic ecosystems.” Organisms living there don’t get their energy from sunlight, but instead use the chemical energy released from deep within the Earth.

“You can get much stranger kinds of animal there, which can provide some insight into the origins of life,” says Thurber. “These challenging ecosystems may produce fauna that would be much more analogous to what we might find on another planet, because they are existing in what we consider ‘harsh conditions.'”

A planetary body that could potentially sustain extraterrestrial life is Titan, Saturn’s largest moon. Titan has been described as having conditions similar to those of early Earth. On its surface, scientists have discovered the first liquid lakes outside of Earth–though they’re not composed of water, but methane.

This is an image of the ampharetid beds off of of New Zealand. Each of the black dots in the image is one of the larger worm with the head of the worm out and grazing the sediment. Credit: Oregon State University, photo: D. Bowden/ NIWA Wellington

“We are just beginning to understand the interactions between methane and different microbial and higher life forms. This study reveals some unexpected roles for higher animals in consuming methane,” said Levin.

What’s Next?

Thurber’s research now focuses on figuring out what exactly causes the higher release of methane compared to other seeps.

“We think it’s the worm,” says Thurber.” But what exactly is it about the worm that drives that? Is it the tube itself that act like conduit for more methane to escape? Is it that the worm is eating too much bacteria, and the bacteria can’t keep up with the worm grazing, so there is more methane release? Or do the tubes allow for different types of microbes to inhabit the sediments, which then leads to higher methane release? These are the questions we’re now considering.”