Thriving on Arsenic - Rethinking the Chemistry of Life
Chris - This week, a team from Arizona State University have announced the discovery of bacteria that can thrive in an environment laced with arsenic. This is a chemical that's normally very toxic, but not only can this bacteria tolerate it, they can even use arsenic instead of phosphorus which is normally a critical element in DNA. Professor Paul Davis from Arizona State University is one of the authors on that paper which announces the discovery this week in Science, and he is with us now. Hello, Paul...
Paul - Hello. Welcome from Arizona.
Chris - Thank you. First of all, what actually is the bacterium that you've been studying and how did you come to isolate it?
Paul - It's a common garden bacteria - it's not a weirdo. It didn't stand out as being anything odd. And if it wasn't for the brilliant insights of my colleague Felisa Wolfe-Simon, who incidentally is now working at the US Geological Survey in Menlo Park, then we would never have known that there was any amazing arsenic capabilities, but she had a hunch some years ago which we then developed here at ASU into a full hypothesis. There could be organisms that can replace phosphorus with arsenic, these would be, as it were, arsenic life, and she went to look for them in Mono Lake in California which is heavily contaminated with arsenic. So, it was a shrewd place to look but we would never have known if she hadn't fed them on diets with huge amounts of arsenic and zero phosphorus.
Chris - So what happened was she initially got the samples of bacteria from the lake which are tolerating a degree of arsenic in the environment and then by forcing them to live in an environment that's very arsenic-rich with no phosphorus, they were able to substitute arsenic into their actual behaviour, biochemically, in terms of DNA, lipids, and everything else that keeps their cells going, and use arsenic in place of phosphorus.
Paul - Absolutely right. They took the arsenic into their vital innards - this was an important point. Of course ideally, we wanted to go somewhere where there was much more arsenic and much less phosphorus, and there are places on Earth, like deep ocean volcanic vents but they're expensive to get to, Mono Lake is convenient. Also, there has been work down there by Ron Omland at the US Geological Survey studying organisms that flirt with arsenic, but nobody other than Felisa and a handful of us had really expected to find anything that did more than that flirtation, that would actually take the arsenic into their innards and use it to substitute the phosphorus. And so, this is the first because all along, biologists have assumed that all life is built out of the basic toolkit of six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur. And here, we have an organism that departs from that basic tool kit.
Chris - Chemically, why is it possible for this bacterium to substitute arsenic for this - what we previously thought of as absolutely critical element - phosphorus.
Paul - Well, arsenic is a poison precisely because it looks chemically like phosphorus and so, that's the reason that led us to think that this might happen, but we don't understand the mechanism. We don't know who's shifting the gears inside these little bugs, what actually is going on. All we could tell is the phosphorus is getting replaced by the arsenic. Ideally of course, we'd like to find an organism that right from the outset is arsenic through and through, and for which phosphorus is a poison. This is not it. This has dual capability. It likes phosphorus. It likes arsenic. It can deal with both and so, it can sort of mix and match, but the Holy Grail would be to find one that was an arsenic organism by obligation, not by choice.
Chris - But I think the point is, and the point that you make very well in your paper, is that this shows that if we complacently think that all life has to be based around these six building blocks that you've mentioned, one of them being phosphorus - actually, this is not true and we have an example here on Earth of how an organism can substitute a completely different element into its life. And therefore, this suggests that the opportunities for life to exist in an entirely different way than the way we understand here on Earth could well exist in outer space.
Paul - That's right. Well not perhaps outer space, but on a planet or moon that was rich in arsenic, but I think it proves an even more exciting point and that is, that you could have radically alternative forms of life, hiding in plain sight, right under our noses, just looking like any common or garden microbes. You can't tell by looking with microbes - what they're made of, what makes them tick. This study is part of the broader context to look for what we call a 'shadow biosphere,' or the search to see whether Earth hosts more than one fundamentally different form of life. Is there just one Tree of Life with lots of interesting branches, or might there be more than one tree? Now this particular organism is clearly on the same Tree of Life as you and me, but it does show because you can't tell by looking, that there may be even bigger surprises in store and if we follow the arsenic, see where that goes, we might find that we have an organism which simply can't even be fitted on the same Tree of Life as you and me, and that would show that life on Earth has started more than once and the implications of that are literally cosmic.
Chris - I was going to say that the life we see on Earth today is life which is adapted to the planet as it is now. If we were to wind the clock back 4 ½ billion years to the very early Earth, it was a very different place. It's possible that there were organisms like this abounding, and they were replaced by the ones that suit the planet as it is today.
Paul - Yes. The more conservative interpretation of this is that this is a sort of latter day adaptation to tolerate high arsenic conditions. But the more exciting possibility as you mentioned is that maybe life started out going down the arsenic route, for the simple reason that the favourite place among astrobiologists for life to begin are the deep ocean volcanic trenches where there's a sort of chemical brew being stirred around by the heat of the volcano. If that's the case, well, it's laced with arsenic down there, it's a very arsenic-rich environment and so it makes sense to think that maybe life started out with arsenic and only when it spread, then it started making use of phosphorus. So these things could be like living fossils, a hangover from those ancient days. It's too soon to say yet because we need other examples. If we have a whole collection of arsenic microbes, we could begin to do a phylogenetic tree, we can begin to see how ancient they are, how ancient the genes are, but it's early days yet.
Chris - And just to finish this off Paul, you've done this for arsenic, but what about the other elements that we know are critical for life? Is it possible that other neighbours in the periodic table of elements that are again chemically similar in the same way that arsenic is to phosphorus could be substituted in life, and therefore, we have other organisms that are using entirely different chemicals instead of those carbon, hydrogen, nitrogen, oxygen, sulphur, and phosphorus?
Paul - Well it's rash to rule anything out in this game of course, but the favourite is carbon being replaced by silicon, and that's so popular, it even made it to an episode of Star Trek. So, we have to take it seriously. But my chemist friends tell me that that's a pretty tough one and I think carbon would be the last to go, but we do have to take a look say, at sulphur, and also the possibility of the fact that possibly, phosphorus could be replaced with something other than arsenic. So, I think this is going to open the eyes of microbiologists and encourage people to look in a much wider range of locations, a much wider suite of chemistry than we have dealt with either two.
Chris - Should always take Star Trek seriously. Paul, thank you very much. That's Paul Davis from Arizona State University.