Extreme Bacteria in the Earth's Crust

Chris - Could you tell us a bit about the bacteria that you've been looking at?

Lisa - Well this is an organism that we sampled from water that was intersected in a very deep gold mine in South Africa. It requires a whole team of people to collect these samples and a lot of cooperation with the mine owners and the mine operators. Mining operations have a water intersection, often this water is under very high pressure and is very hot. They get in touch with scientists who are interested in studying that water and studying the possibility of organisms living there. We come in once the water flow has slowed down enough to be safe. We take the samples, we concentrate the organisms on filters and we ship those samples out to labs around the world and have a look at them. In this case, a water sample collected almost 2.8 kilometres below the surface yielded a very interesting community of organisms but with a single dominant bacteria that is making its living doing a sulphate reduction. Which in many ways is just the opposite of the chemical reaction that Crispin was talking about on the sea floor.

Chris - So how do you think these bacteria got there? Because it's not trivial for bacteria to be living 3 kilometres underground in water at high temperature and pressure.

Lisa - We assume they get there in much the same way as organisms in the present day move into the sub-surface. They gradually move downward with descending ground water, the circulation is very slow so by the time they get down several kilometres below the surface they've probably been in transit for tens of millions of years and have been isolated from their surface relatives for an extended period of time.

Chris - How long do you think these guys were isolated from the rest of the world?

Lisa - Well in this particular water sample, we've estimated its age by looking at the concentration of various noble gases and it appears to be something around 16-25 million years old.

Chris - So this water's been cut off from the outside world for 16-25 million years but it's still got loads of bacteria thriving in it?

Lisa - Yes

Chris - So what's powering them?

Lisa - Well that's a tough question because when we realised that these were sulphate reducing organisms we of course then went looking for a source of sulphate. Now that's a very common ion in seawater but there's no way we can imagine for sea water to get into this deep part of the basin. It could also come from dissolving ancient salt deposits but the rocks that these bacteria are in do not contain evaporitic salt-like minerals. So we started looking for another chemical pathway. And because these deposits are both rich in gold and rich in uranium, we came to the rather startling conclusion that most likely this was sulphate that resulted from radiolysis of water, which is the spitting of water molecules, and then the reaction between those fragments of water and the mineral pyrite, which is an iron sulphide mineral.

Chris - So what splits the water molecules in the first place, Lisa?

Lisa - Well of course we all hear a lot about the radioactive decay of uranium and when this process goes on it can be concentrated in the core of a nuclear reactor or dispersed as uranium-bearning minerals in the sub-surface. So here the uranium is present as a natural part of the sedimentary deposits and undergoing radioactive decay. It is releasing high energy particles and that ionising radiation is actually tearing through the water and breaking water molecules apart and creating hydrogen peroxide and hydrogen gas. And we think it's the hydrogen peroxide that then reacts with the pyrite.

Chris - Do you think that these organisms can teach us anything about: a) the possibility that life could have evolved independently on other planets which have a radioactive core or radioactive elements in their core, a bit like the earth does, and b) about ways to get energy out of things like this?

Lisa - Well I think the answer to both questions is yes. We have never before thought in terms of radiolysis of water as a source of energy to sustain organisms. So that's a very exciting discovery. And it suggests that even if you had a planetary body very distant from it's associated star, too distant to really rely on photosynthesis, you might have organisms that instead rely on this chemical energy caused by radioactive decay. In terms of what it tells us about earth, I think it reminds us that we know very little about the extreme environments on earth and we still have much to learn about our own planet.

Chris - How do you know that these bacteria that you've isolated were genuinely in the water that came out of this crack? How do you know that the water you've collected wasn't just contaminated from elsewhere in the mine when you broke open that sealed off bit of water?

Lisa - Always a tough question when we're working in these deep sub-surface environments, especially when we're working in mines. But in this case, the types of organisms that are present in the intersected water are distinct from the organisms that are present in the water that is utilised by the mine from surface sources. So it has a distinct phylogeny; its genes are different from its nearest surface relatives. And the amount of organisms that are there are consistent with their being sourced from these deep sequestered waters.