Pharyngula

It’s not an arsenic-based life form

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Oh, great. I get to be the wet blanket.

There’s a lot of news going around right now about this NASA press release and paper in Science — before anyone had read the paper, there was some real crazy-eyed speculation out there. I was even sent some rather loony odds from a bookmaker that looked like this:

WHAT WILL NASA ANNOUNCE?

NASA HAS DISCOVERED A LIFE FORM ON MARS +200 33%
DISCOVERED EVIDENCE OF LIFE ON ONE OF SATURNS MOON +110 47%
ANNOUNCES A NEW MODEL FOR THE EXISTENCE OF LIFE -5000 98%
UNVEILS IMAGES OF A RECOVERED ALIEN SPACECRAFT +300 25%
CONFESSES THAT AREA 51 WAS USED FOR THE ALIEN STUDIES +500 16%

[The +/- Indicates the Return on the Wager. The percentage is the likelihood that response will occur. For Example: Betting on the candidate least likely to win would earn the most amount of money, should that happen.]

I think the bookie cleaned up on anyone goofy enough to make a bet on that.

Then the stories calmed down, and instead it was that they had discovered an earthly life form that used a radically different chemistry. I was dubious, even at that. And then I finally got the paper from Science, and I’m sorry to let you all down, but it’s none of the above. It’s an extremophile bacterium that can be coaxed into substiting arsenic for phosphorus in some of its basic biochemistry. It’s perfectly reasonable and interesting work in its own right, but it’s not radical, it’s not particularly surprising, and it’s especially not extraterrestrial. It’s the kind of thing that will get a sentence or three in biochemistry textbooks in the future.

Here’s the story. Life on earth uses six elements heavily in its chemistry: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur, also known as CHNOPS . There are other elements used in small amounts for specialized functions, too: zinc, for instance, is incorporated as a catalyst in certain enzymes. We also use significant quantities of some ions, specifically of sodium, potassium, calcium, and chloride, for osmotic balance and they also play a role in nervous system function and regulation; calcium, obviously, is heavily used in making the matrix of our skeletons. But for the most part, biochemistry is all about CHNOPS.

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Here’s part of the periodic table just to remind you of where these atoms are. You should recall from freshman chemistry that the table isn’t just an arbitrary arrangement — it actually is ordered by the properties of the elements, and, for instance, atoms in a column exhibit similar properties. There’s CHNOPS, and notice, just below phosphorous, there’s another atom, arsenic. You’d predict just from looking at the table that arsenic ought to have some chemical similarities to phosphorus, and you’d be right. Arsenic can substitute for phosphorus in many chemical reactions.

This is, in fact, one of the reasons arsenic is toxic. It’s similar, but not identical, to phosphorus, and can take its place in chemical reactions fundamental to life, for instance in the glycolytic pathway of basic metabolism. That it’s not identical, though, means that it actually gums up the process and brings it to a halt, blocking respiration and killing the cell by starving it of ATP.

Got it? Arsenic already participates in earthly chemistry, badly. It’s just off enough from phosphorus to bollix up the biology, so it’s generally bad for us to have it around.

What did the NASA paper do? Scientists started out the project with extremophile bacteria from Mono Lake in California. This is not a pleasant place for most living creatures: it’s an alkali lake with a pH of close to 10, and it also has high concentrations of arsenic (high being about 200 µM) dissolved in it. The bacteria living there were already adapted to tolerate the presence of arsenic, and the mechanism of that would be really interesting to know…but this work didn’t address that.

Next, what they did was culture the bacteria in the lab, and artificially jacked up the arsenic concentration, replacing all the phosphate (PO43-) with arsenate (AsO43-). The cells weren’t happy, growing at a much slower rate on arsenate than phosphate, but they still lived and they still grew. These are tough critters.

They also look different in these conditions. Below, the bacteria in (C) were grown on arsenate with no phosphate, while those in (D) grew on phosphate with no arsenate. The arsenate bacteria are bigger, but thin sections through them reveal that they are actually bloated with large vacuoles. What are they doing building up these fluid-filled spaces inside them? We don’t know, but it may be because some arsenate-containing molecules are less stable in water than their phosphate analogs, so they’re coping by generating internal partitions that exclude water.

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What they also found, and this is the cool part, is that they incorporated the arsenate into familiar compounds*. DNA has a backbone of sugars linked together by phosphate bonds, for instance; in these baceria, some of those phosphates were replaced by arsenate. Some amino acids, serine, tyrosine, and threonine, can be modified by phosphates, and arsenate was substituted there, too. What this tells us is that the machinery of these cells is tolerant enough of the differences between phosphate and arsenate that it can keep on working to some degree no matter which one is present.

So what does it all mean? It means that researchers have found that some earthly bacteria that live in literally poisonous environments are adapted to find the presence of arsenic dramatically less lethal, and that they can even incorporate arsenic into their routine, familiar chemistry.

It doesn’t say a lot about evolutionary history, I’m afraid. These are derived forms of bacteria that are adapting to artificially stringent environmental conditions, and they were found in a geologically young lake — so no, this is not the bacterium primeval. This lake also happens to be on Earth, not Saturn, although maybe being in California gives them extra weirdness points, so I don’t know that it can even say much about extraterrestrial life. It does say that life can survive in a surprisingly broad range of conditions, but we already knew that.

So it’s nice work, a small piece of the story of life, but not quite the earthshaking news the bookmakers were predicting.

*I’ve had it pointed out to me that they actually didn’t fully demonstrate even this. What they showed was that, in the bacteria raised in arsenates, the proportion of arsenic rose and the proportion of phosphorus fell, which suggests indirectly that there could have been a replacement of the phosphorus by arsenic.


Wolfe-Simon F,
Blum JS,
Kulp TR,
Gordon GW,
Hoeft SE,
Pett-Ridge J,
Stolz JF,
Webb SM,
Weber PK,
Davies PCW,
Anbar AD, Oremland RS (2010) A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus. Science DOI: 10.1126/science.1197258.