Science Interviews

Interview

Sun, 28th Jan 2007

Fossilisation at Hydrothermal Vents

Dr Crispin Little, University of Leeds

Part of the show Extreme Organisms and Hydrothermal Vents

 

 

Chris - You're interested in hydrothermal vents, but some people may not know what they are, so could you just tell us a little bit about the geology of what these things are?

 

Crispin - Well it's a pretty simple system really. What you have is a huge underwater range of mountains on the sea floor where new ocean crust is being formed, and these are longer than any mountain chain on the surface. At these sites, because you have new ocean crust being formed, you have heat. Basically you have magma under the ground that rises via lava to form new ocean crust. And at the same time you have water, because you have ocean sitting on top of these huge underwater volcanic mountains. So what happens is that sea water seeps down and gets heated up by the magma chambers underneath the ground and as it gets heated up it reacts with new volcanic rock. It takes on all the different chemical compounds including hydrogen sulphide, which is very important, and lots of metals as well including iron, copper, lead and zinc. Because it's hot and buoyant, it rises back onto the sea floor and it jets out in certain places at these mid-ocean ridges to form big deposits of sulphide minerals in particular. The water that gushes out, which you said was near boiling, well if that water was gushing out at the land surface, it most certainly would be boiling because it's up to 400 degrees centigrade. It doesn't boil on the surface of the sea floor because it's under extreme pressure. In some places such as the cruise I was on recently, we were diving in to 2500 metres of water, and that's a lot of pressure on top of this vent fluid and it stops it boiling. So it's a pretty simple system in terms of its geological process of formation.

 

Chris - And so this is a massive source of energy; both chemical energy and thermal energy on the sea bed. So there are now organisms that can exploit that. I say now, but I mean that organisms have evolved over time to exploit that.

 

Crispin - Indeed and in fact there's evidence and some people suggest that life itself may have started at hydrothermal vents in the first place. These things are actually pressure cookers of organic compounds so it's a system that's been going on for billions of years in the world's oceans. Life may have started there and certainly it's an amazing site for all sorts of different life today.

 

Helen - What sort of life would we see down there? What sort of animals, plants and creatures? What lives down there?

 

Crispin - Well there are certainly no plants because remember it's completely black; as black as black can be. What you see are very specialised communities that can live at these hydrothermal vent sites and very challenging fluids, because not only is it very hot, but there's no oxygen in it and it can be quite acidic too. So pretty unpleasant stuff, but it does have a huge number of reduced chemical compounds in the fluid and if you can combine those fluids with oxygen for example, you can release a lot of energy, and that's the basis of these communities.

 

Chris - Are they just microbes, Crispin, or are we talking bigger animals?

 

Crispin - No there are bigger animals. The first thing that you'll see if you dive on the vent communities on the East Pacific Ocean for example, are these very big tube worms. These tube worms called Riftia, it's the giant tube worm and the most charismatic beast, is up to three metres long and not only can they grow three metres long but they can grow that big in a couple of years, so they're very very fast growing indeed.

 

Chris - But what are they eating?

 

Crispin - The intriguing thing about them is that as adults, they don't have a functioning gut at all. They don't have a mouth, they don't have a digestive tract and they don't have an anus. What they have is a body that's composed of what is essentially a bacteria farm, so they have billions and billions of little symbiotic bacteria. The bacteria are using hydrogen sulphide from the vent fluid for their energy source, and the tube worms bring hydrogen sulphide from the vent fluid and oxygen in its blood to the bacteria farms in the body. The bacteria then combine these two things, the oxygen and the hydrogen sulphide, to make energy. The animal then uses the bacteria, probably by eating them directly.

 

Chris - So where do the bacteria live in relation to the worm? Do they live on the surface or does the worm have special pouches to keep them in?

 

Crispin - It's actually an organ called the trophosome, and it's formed from skin cells. It's internal to the animal's body; not on the outside.

 

Chris - Reminds me a bit of the leaf cutter ants that farm fungi and they have special pores each fed by its own sweat gland, which nourishes a certain strain of bacteria that pump out a form of antibiotic. These ants can then distribute this around their nest to get rid of the fungi. So similarly you have worms that have structures that can accommodate these hydrogen sulphide-loving bacteria, and in return for accommodation, they can give something back to the worms.

 

Crispin - Exactly. And in fact it's not only the giant tube worms that can do this. There are two different sorts of bivalve shells, the giant vent mussel and the vent clam, which also do similar things. But here the bacteria are in the gill tissue. The gills are what bivalves usually do to filter feed, but in this case they're taken up entirely by these symbiotic bacteria in their tissues. So their gills are extremely large indeed for deep sea bivalves.

 

Chris - Why are they so big because there doesn't seem to be an obvious advantage to being so big because when you're a big animal in the sea and things find it harder to hunt you, it's an advantage. But for something like this where chemicals drive your existence, is there an advantage to being huge?

 

Crispin - Well I suppose so, firstly because you can have more symbiotic bacteria. The thing is that the energy available is almost indefinite. There's more energy than you can make use of, so the larger you are and the more bacteria you have in your bacteria farm, the better.

 

Chris - Some people have said that this is evidence that life could exist on other planets. Because if you can get a very specialised ecosystem like this on the bottom of the sea where the energy supply comes from the heat of the planet, this means that if you could find another planet somewhere in the solar system with a similar environment, you could have life there. But then others have come along and said that that's all very well, but there are still elements to that sort of ecosystem that rely on energy input from the sun, and if you took the sun away, they wouldn't survive.

 

Crispin - I think that if you look down at the bottom of the food chain here, to the bacteria and the archaea, this other kingdom of life you have amongst very small prokaryotic beasts, then I can't see why you couldn't have a similar system on other planets. Really for the starting process all you need is the heat source and some rock to react with the water, and you could have that on any rocky planet.

 

Chris - But where do you think these worms came from, because the bacteria that nourished them is one thing, but they're obviously very specialised organisms?

 

Crispin - If you have a look at their genes for example, they seem to fit quite nicely into polychaete worms, so things like the ragworm for example would be in this group. So while they are pretty specialised, they seem to have rather mundane origins. But we find fossils going back into the geological record that look very similar indeed. It's almost 430 million years, our oldest example. So similar sorts of beasts have been around for a very long period of time at vent sites.

 

Helen - Crispin, you're actually a palaeontologist, so I guess your interest is mainly what used to be there on the ocean ridges and in the process of fossilisation. I believe you've recently come back from a trip to the Pacific, and I was wondering what it was that you were doing during that trip to the ocean.

 

Crispin - The reason for going out on a cruise to modern hydrothermal vent sites is that I'm very interested in the process of fossilisation. So we find fossils of these things back in the geological record, but I don't really know how the process occurs - no-one does. The reason for going out on a modern cruise is to do some experiments on the sea floor and just see what this process is by looking at modern sites.

 

Chris - Were you exploring these vents remotely or were you in some kind of submersible that would take you down personally to see them?

 

Crispin - Unfortunately not me personally on this trip but yes, we were using the deep sea submersible Alvin, which is a very famous vehicle and was the one used to discover the Titanic, the Bismarck and hydrothermal vents were first discovered by Alvin in 1977. So we were taking Alvin down and it has three people on board: the pilot and a port and a starboard observer, who direct the science on the trip. So they were actually going down in the submersible and they were putting down experiments at the vent sites and bringing stuff up and collecting animals. They were diving each day on the research trip I was going on.

 

Helen - And what were the experiments you were doing?

 

Crispin - Well my experiments were a relatively simple system. I have a titanium mesh cage box with a base twelve centimetres across and six centimetres high. Inside that titanium mesh cage, I wire up different objects to see how they become mineralised. So I have tube worms, bits of shells from gastropods and bivalves, bits of shrimp carapace, and then some control materials as well. So each one of these titanium mesh cages is identical, and the idea is to put them onto hydrothermal vent sites as well as control sites, and just see how quickly this process occurs.

 

Helen - Because it's quite quick isn't it? That's the key that we're looking at fossilisation far quicker than anywhere else in the world really.

 

Crispin - Exactly. It's going to be nothing like our normal process of fossilisation that occurs in sediments, which we think might take hundreds or thousands or millions of years. This thing happens really quickly and we know that because if you look at some of the animal tubes of the worms that build their tubes on the outside of the mineral chimneys, the animals are still living in the tubes where part of the tubes is actually becoming fossilised. So it's a process which occurs during the life of the animal, which is really pretty extraordinary.

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