Methane, It’s What’s For Dinner… In The Deep Ocean
About 1,800 meters below the ocean surface off the western coast of Costa Rica, methane seeps dot the seafloor. These are places where methane and other hydrocarbons slowly escape from beneath the earth’s crust. Like more well-known hydrothermal vents, methane seeps are home to an unusual array of wildlife, relying on the seeps’ enriched chemistry for energy and nutrients.
Writing this week in the journal Science Advances, researchers describe two species of tube worms that live in a symbiotic relationship with methane-oxidizing bacteria that live on their crowns. The researchers collected some of the worms via deep-sea submersibles and then exposed them to carbon-13-labeled methane, showing that the worms were able to assimilate the methane into biomass. The team believes that the symbiosis allows these worms to rely on methane for much of their nutrition.
Shana Goffredi, an associate professor of biology at Occidental College in Los Angeles and one of the authors of the report, explains the research and what remains to be learned about the environment around these undersea methane seeps.
Invest in quality science journalism by making a donation to Science Friday.
Shana Goffredi is an associate professor of Biology at Occidental College in Los Angeles, California.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. About 1,800 meters down off the Western coast of Costa Rica, there’s a feature called a methane seep on the seafloor. It’s a place where methane and other hydrocarbons slowly escape from within the Earth’s crust. And like the more well-known hydrothermal vents, methane seeps are home to an unusual array of wildlife.
Writing this week in the journal Science Advances, researchers described two species of two worms that live in a symbiotic relationship with bacteria. And that relationship allows them to rely on methane for their nutrition. Sci Fri director Charles Bergquist spoke with Shana Goffredi, an associate professor of biology at Occidental College in Los Angeles, one of the authors of the report.
CHARLES BERGQUIST: Thank you for joining me today.
SHANA GOFFREDI: Thank you, Charles.
CHARLES BERGQUIST: Set the scene for us. What do these worms look like?
SHANA GOFFREDI: When you descend on the submersible, it’s pitch dark, and you land on the sea floor. And when you flip on the lights, you see that there are these amazing almost thicket-like congregations of these worms. And they make these really beautiful white calcium carbonate tubes. And so they’re quite obvious. And you also see a lot of other animals that live only in these habitats all piled on top of each other. It’s actually quite a stunning scene on the sea floor.
CHARLES BERGQUIST: So there’s this mineral-looking tube with the worm inside it. And then at the top is a fern frond-like structure that’s the top of the worm?
SHANA GOFFREDI: Yes, right. So these are all tube-dwelling worms. And so they either make a tube out of minerals, or they can make a tube out of mud. And in all cases, they stick out this really frilly, beautiful head that they use primarily for respiration or breathing, but also in some cases feeding. And then in the case of the worms that we reported on, they’re actually using them as a surface to grow bacterial symbionts.
CHARLES BERGQUIST: And that’s special to these worms in particular? Or do other worms do this, too?
SHANA GOFFREDI: Well, there are many examples of worms that have formed beneficial partnerships with bacteria that are unrelated to these worms. And sometimes, they keep them on the outside of their body. And sometimes, they provide a special organ or tissue inside their body to host them. But this is the first time that we’ve seen this kind of worm that hosts methane oxidizing bacteria on the outside of their heads essentially.
CHARLES BERGQUIST: Is it as simple as bacteria eats the methane, worm eats the bacteria? Or is it a more complicated relationship?
SHANA GOFFREDI: Yeah, I’d say that, in simplicity, that’s a reasonable statement to make. However, there is a lot of negotiation that goes on with these relationships. So you can think of them as beneficial reciprocity, or beneficial exploitation, mutual exploitation, however you want to see it. But there is a lot of complicated communication that goes on between the partners.
So this worm does not have any other bacteria on its body besides the special bacteria. And so there must be some kind of what we would call a molecular handshake, that they can recognize each other from all the other possible partners down there. And so I suspect that the relationship is much more complicated, and that there is a lot of subtle things going on in the background that we don’t yet have a good idea about.
CHARLES BERGQUIST: Would it be accurate to say that they’re somehow forming the bacteria? Or is that not the right term to use in this relationship?
SHANA GOFFREDI: No, I think that’s a– farming is actually a great term to use. And whenever an animal teams up with bacterial partners, they do one of two things in order to get nutrients from them. They can either milk them, so just slowly encourage them to leak nutrients to them. But in this case, they’re probably farming. And many animals do do that. And we might even do that in our own digestive systems. We digest a lot of bacteria that are in our digestive system for nutrients. And these animals are doing the same thing, these worms in the deep sea.
CHARLES BERGQUIST: Are they actually eating the bacteria in the way that we would think of, with a mouth and a gut? Or is there some other process going on here?
SHANA GOFFREDI: Well, we do see that these worms have a digestive system. But it is not filled with these bacteria. And we suspect, based on microscopy, that these animals are actually digesting them or engulfing them– you can think of it as a better term– across their skin essentially. And it’s the skin or the epidermis of their crowns, which are their heads.
And so we can see in microscopy that they’re actually engulfing these cells, and that those bacterial cells end up further down in the tissues. And we suspect that’s how they’re being incorporated. And we know that this happens rather quickly, because, in our shipboard experiments, within 24 hours, we could trace carbon that comes methane molecules that we offered to them into their bodies. And so within 24 hours, there’s a substantial signature of methane in their tissues.
CHARLES BERGQUIST: How do you tell that it’s your methane that’s going in? How are you tracing its process?
SHANA GOFFREDI: That’s a very good question. So we actually label the methane with a special isotope. So most carbon exists in the carbon-12 form. And we gave them carbon-13 methane. And we so we can track the amount of carbon-13 into the animal tissues.
CHARLES BERGQUIST: Are they getting all their nutrition from the bacteria and the methane? Or are they consuming other things, too?
SHANA GOFFREDI: Yeah, so we currently believe that this is what’s called a facultative relationship. So the worms benefit from it. But they’re not completely dependent on it. And there are many examples of animal microbe symbiosis that are facultative. But there are many that are obligate, meaning that the animal must have their bacterial symbionts to survive.
So we think, in this case, that the worms do retain an ability to feed. We don’t think that that happens very often in the cold sea environment, because they’ve got plenty of methane around to fuel their symbionts. But for example, we transplanted some of these worms about a kilometer away from the active seeping area. And they survived for 16 months without methane. And we suspect that, in that case, they were feeding on their own. And their tissue suggested that. So we do think that they retain some ability to feed on their own.
CHARLES BERGQUIST: I feel like people might be familiar with the idea of hydrothermal vents, but maybe less so about these cold methane seeps. Can you describe them for us? What’s going on there?
SHANA GOFFREDI: Most continental margins around the world have these habitats close by. And those are called methane seeps. And we think of them as a cold water analogy of hydrothermal vents, in that there’s a lot of reduced chemicals, like methane and hydrogen sulfide, that are fueling the communities, whether they’re microbial or animal communities and they exist along the continental margins, because there’s a lot of primary productivity that occurs there.
And that organic matter gets buried. And it gets converted into these reduced chemicals. So methane and hydrogen sulfide results from the activity of other microbes that are living in the sediments. And then there’s geological phenomena along the continental margins where there is subduction of the Earth’s plates under the continents. And that squeezes these chemicals out from the seafloor, or shaves off whole sea mounts, and then reveals those chemicals as a possible source that’s more at the surface.
CHARLES BERGQUIST: When we talk about methane down there, is it a large reservoir that potentially a petrochemical company would be interested in drilling for? Or is it more of a gradual process coming out of my compost pile or something?
SHANA GOFFREDI: I think there are probably a number of ways to think about that. So there is methane seeping out, like might be out in your compost pile. But also, there are methane hydrates. So this is methane tied up in an ice-like molecule, molecular structure. And that has been discussed as being mined.
And there are some estimates that the methane hydrates on the planet equal all the fossil fuel reserves that still exist. And then, of course, there is oil and gas in these areas as well. So they do coexist in the same habitat as these methane seeps and these very unique and diverse biological communities.
CHARLES BERGQUIST: Do you see actual bubbles of methane? Or is it not in that quantity? It’s more dissolved in the water process?
SHANA GOFFREDI: Oh yeah, you see streams of methane bubbles. It is very forcefully coming out. It’s not like a hydrothermal event, like volcanism. But it’s definitely streams of bubbles. And when we take samples of sediment by something we call push core, we often liberate a whole bunch of bubbles that will emerge. And those are all filled with methane.
And I should make a point to say that it is possible. So not much methane ends up in the water column. A lot of methane is being produced in the sediments by microbes. But there’s also many more microbes that are consuming methane in the sediments. So very little of it reaches the water column. And even less reaches the atmosphere.
And we have some of these animals may to thank for that, because they’re hosting methane oxidizing or methane eating bacteria that are essentially consuming that methane before it has a chance to reach the atmosphere. And methane is quite a potent greenhouse gas. And so we consider them to be providing an ecosystem service for the rest of us, for sure.
CHARLES BERGQUIST: Where are these communities geographically? Are they in one specific place? Or can you find these all over the world?
SHANA GOFFREDI: Well, methane seeps are found all over the world along continental margins, so along the west coast of the Americas, but also on the east coast, and then all around, basically all continental margins. But if you are asking about these worms in particular, we discovered them off of Costa Rica. But we suspect that they are worldwide, based on reports of the same genus of worms in other habitats, not just cold seeps, but also hydrothermal vents. And so we suspect this phenomenon of using methane eating bacteria as a beneficial partner probably extends beyond the seeps in Costa Rica.
CHARLES BERGQUIST: And how far down do they live?
SHANA GOFFREDI: These were at 1,800 meters depth, so approximately a mile. But there are many other examples of them living deeper than that.
CHARLES BERGQUIST: To look at these, you obviously– I mean, it’s a mile down. You weren’t free diving to look at these. How did you study them?
SHANA GOFFREDI: Yeah, so we were lucky enough to be able to have a project funded by the National Science Foundation, where we could use the human occupied vehicle, the Alvin. And we were out there on three different expeditions with Alvin over the course of three years. And it’s quite an exciting experience to be in a submersible. I liken it to being in the front seat of your car with three other people for nine hours.
And when you get to the sea floor, it’s pitch dark. And there’s all kinds of amazing bioluminescence of the animals that are bouncing off the submersible, reacting to the electricity that is given off by the submersible. And then you flip on the lights. And like I said, it’s like an incredible world down there that we hardly ever get to see.
CHARLES BERGQUIST: Is it completely alien? Or is there any environment or community on Earth that would be somehow analogous on the surface?
SHANA GOFFREDI: I’d say it’s completely alien. And the reason why I say that is not just the types of animals that are there, but the speed at which life exists. It’s just it’s probably the most peaceful place I’ve ever been, in terms of just the– yeah, the pace seems just so slow and peaceful.
CHARLES BERGQUIST: You’re listening to Science Friday from WNYC Studios. I’m Charles Bergquist talking with Dr. Shana Goffredi of Occidental College in Los Angeles about her research into deep sea tube worms. So an undersea expedition with the Alvin and all of the support crew doesn’t sound like a minor undertaking.
SHANA GOFFREDI: No, certainly not. So it’s years in the making. There’s usually a handful of scientists that are the lead principal investigators on a research proposal. Sometimes, it’s funded. Luckily, we were funded. And this was a collaboration by at least five different universities in the US, and then also some Costa Rican colleagues.
And then on top of that, about 40 different scientists came on board the ship with us over the many years, the three different expeditions and also had a role to play. And each of us has a different skill. And it’s an amazing collaboration between many different people that have these different expertise. And that allows you to do interdisciplinary science, where you can really see these problems or approach these problems from different perspectives, which then, of course, gives you the whole picture eventually. So it was a really exciting international collaboration among scientists.
CHARLES BERGQUIST: What is it that amazes you most about these worms? Is it just the methane relationship? Or is there something deeper or larger?
SHANA GOFFREDI: I’d say what amazes me the most is that these worms have been studied for a long time, or at least known to exist in these habitats. And we’ve overlooked them in terms of the possible novel nutritional strategy. And I think that’s the case for a lot of animals in the deep sea especially, as we can assume one thing about them, but they might surprise us and are doing something completely different. And so it’s important to look closely and expect to novel processes from them.
I’m also fascinated by this idea– there has been on Earth records of 200 million year old fossil seeps that have large congregations of these serpulids. And this has been a mystery to scientists for a long time. And I think we finally solved the mystery of why we see all of these fossil tubes of serpulids when they weren’t ever suspected to be tied to the seeping chemicals at all. And we have finally realized that they actually were relying on the methane that was coming out of these seeps. And they were there as a natural endemic member of those communities.
So they’ve been around for over 200 million years. And they’re down there today. It’s just that it’s pretty hard to explore the deep sea and find these new ways of life that has been occurring all this time.
CHARLES BERGQUIST: So I think we often think about the world as a survival of the fittest or exploitative relationship. Is it possible that there are a lot more of these symbiotic relationships than we know about that are just going on in the background behind us all the time?
SHANA GOFFREDI: Yeah, so you actually hit on one of my favorite things to discuss with regard to symbiosis, is that we are taught that life is competitive. And it is survival of the fittest. And I would argue that there are many more cooperative relationships that get a lot further in life than there are competitive relationships.
And I think, for the longest time, studying symbiosis between animals and bacteria was considered an exotic thing to study, maybe wasn’t that relevant for more disciplines. But now, we realize, even with the human body and the human microbiome, that actually symbiosis is probably the norm. And to not have any bacterial symbionts would be quite rare. And so it took a lot of pioneers in the research field of symbiosis to show us that.
CHARLES BERGQUIST: What’s the next thing that you want to learn, the next goal for– next big dive?
SHANA GOFFREDI: Hm, well, we are we are hot on the trail of another very interesting symbiosis that we discovered in the Gulf of California. And these are at the deepest known hydrothermal vents in the Pacific, at 3,800 meters depth. And I may not tell you much more about that. But we are currently writing the manuscript.
SHANA GOFFREDI: Well, good luck with that. And thank you for joining me today.
CHARLES BERGQUIST: Of course. Thank you, Charles,
IRA FLATOW: That was Sci Fri director Charles Bergquist speaking with Shana Goffredi, associate professor of biology at Occidental College in Los Angeles.