Update: a further look at the details of this paper is in a later post.
So: arsenic for phosphorus? That’s the big news from NASA today. I listened to much of the press conference, and I’ve read the paper in Science. Is this real – and if it is, what does it tell us?
Let’s do the second part first. Phosphorus is an extremely important element for every living thing on Earth. It’s mostly found as phosphate, and phosphate groups are found all over the place: decorating proteins, carbohydrates, and lipids, as the invariable outside of DNA helices, and as the key part of the ultimate energy currency of every living cell, ATP. Phosphate’s no bit player.
This is a good time to emphasize that (as far as we can tell) all life on Earth shares the same chemistry and the same kinds of biomolecules. Humans, frogs, fruit flies, fungi, tube worms on the ocean floor, lichens in Antarctica, and weirdo single-celled creatures living in boiling hot springs: we all have cells full of proteins, carbohydrates, lipids, and nucleic acids. We use DNA and RNA to pass on our genetic information, and the enzymes we use to manipulate them and to power our cells are all similar enough that we just have to share a common ancestor. (Either that, or life only gets going in a very specific way indeed).
One thought about today’s press conference was that it might be announcing “alien life on Earth”. That’s been a subject of argument for quite a while. Even though everything we’ve ever found is of the same family tree, that doesn’t rule out (logically or practically) the possibility that some other form of life, with different chemistry entirely, might be hanging out in its own environment. A good deal of searching has failed to turn it up, but (if it’s such different stuff) we might be looking for it in the wrong ways, or might even have trouble recognizing it when we see it.
That’s not what today’s work has turned up, though – but it’s probably the next best thing. What this group was looking for were hypothetical organisms that have learned to use arsenic instead of phosphorus. There are environments that are much richer in arsenic (and its corresponding arsenate salts) than they are in phosphorus. And arsenic is right under phosphorus in the periodic table, and forms similar sorts of compounds (albeit with rather different behavior), so. . .maybe it could substitute? Well, they didn’t find any native arsenic-users – but they did force some into existence. They took a strain of bacteria from such an environment (Mono Lake sediments) and starved it of phosphate while providing it plenty of arsenate. The colonies that grew under these conditions were picked out and grown under even higher arsenate concentrations, and the process was continued stage after stage.
The end result appears to be bacteria that have incorporated arsenate into their metabolism. They still have phosphate in them, but not enough to keep everything running on a phosphate basis. Some parts have switched over to arsenate, without gumming up the works completely. That surprises me quite a bit – I really wouldn’t have thought that things could be pushed that far. After all, in higher organisms, it’s that arsenate-for-phosphate switch that’s responsible for arsenic’s reputation as a poison. Eventually, some key enzyme systems can’t handle the switch and cease to function.
But not in these bacteria. They look different and grow more slowly than their phosphate-saturated brethren, and they’d clearly like ditch the arsenic at the first opportunity (add phosphate and they start growing more vigorously). But they’re getting by, presumably with just enough phosphate to hold things together. (Have they hit the wall, one wonders?) A number of physical methods all point in the same direction, to arsenate being incorporated into their biomolecules. We still don’t know where most of it goes, or how the various phosphate-manipulating enzymes manage to still work, but working out those details will keep a lot of people busy for quite a while. Personally, I’d love to see some X-ray structures of aresenate-containing proteins or nucleic acids, and I’m sure that the people who reported this are trying to get some.
So what does this mean? Well, you can apparently bend the most basic chemistry of life as we know it quite a bit before it breaks. As I said, I really would not have thought that this could be possible – we’re all going to have to keep rather more open minds about what biochemical systems can handle. This makes the arsenic-from-the-ground-up idea look a lot more plausible, too, and you can be sure that the search for such organisms (using arsenate naturally, without having to be forced in the lab) will intensify.
It also makes you wonder about what other directions the biochemistry we know of can be stretched. Selenium for sulfur is my best guess – there, you have the advantage that selenium already has a small but real role in biochemistry as it is. I don’t know of any environments that are higher in selenium than sulfur, but it would be worth trawling the closest candidates, culturing some bacteria, and giving them the same forcing treatment that was used here. If you really wanted to go wild, you could try pushing down to tellurium and down to antimony in the phosphorus column. Now, I really don’t think those have much of a chance, but you never know. It’s a lot more plausible to me than it was yesterday.
And the implications for extraterrestrial life are. . .what? Well, we keep finding the sorts of chemicals that we live with (amino acids, simple carbohydrates and the like) out in space. Our type of biochemistry might be fairly common – and if it is, it’s good to know that it has a lot of wiggle room in it. It’s hard for me to imagine a planet that’s loaded down with arsenic and is short of phosphorus, but hey, it’s a big universe. Big enough, it appears, for all kinds of weird things. It’s great.