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Friday, December 03, 2010

Its An Alien!

Or not. When the news broke yesterday that a new form of life was discovered in a lake in California, I was intrigued.

In a bombshell that upends long-held assumptions about the basic building blocks of life, scientists have discovered a a whole new type of creature: a microbe that can live on arsenic.
It is unlike every other lifeform on the planet - from the simplest plant to the most complex mammal.

Looking a bit deeper, it appears that this bacterium has just replaced phosphorus with the chemically similar arsenic in its chemistry, and it isn't a radically different "new form of life."

Now, I'm very interested in science, and I am probably better read on the subject than the average Joe, but I'm no biologist, so trying to wrap my head around what's actually occuring in this critter isn't the easiest thing to do, especially with the media freak-out. Luckily, PZ Myers rides to the rescue!

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...

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.

On an unrelated note, PZ also gets some interesting e-mail from loving Christians.

Here's another take from an actual biologist.

Look, it's an exciting discovery, but everyone is over-hyping it. This bacteria is not an arsenic-based life form in the sense that we are carbon-based life forms. It does not use arsenic as a source of fuel. It does not exclusively build its DNA backbone using arsenic. It doesn't even really like to do that at all in the wild - it incorporates arsenic under laboratory conditions that force even higher concentrations of arsenic upon it. It is not a different type of life that arose separately from phosphate-using lifeforms.What it is is an excellent example of evolution. While coming from a phosphate-using ancestor, this bacteria has somehow adapted to an extreme environment that would kill most other organisms. I'm more interested in how it avoids death by this toxin than the fact that it incorporates a molecule extremely similar to phosphate into its DNA.

More here from an evolutionary biologist.

The truth is hugely less earth-shattering, but still very interesting. Rather as Lenski did with different chemical insults, these researchers kept bacteria in an arsenic-rich environment and selected out a strain that could tolerate arsenic. They went on to show that the bacteria evolved the ability to substitute arsenic for phosphorus (the two elements are an octave apart in the periodic table, and share many properties, which is precisely why arsenic plays such a starring role in our murder stories: the body can't easily distinguish arsenic from vitally needed phosphorus and then discovers its mistake too late).

So what we have here is a bacterium that adapted to survive in a very hostile enviroment, and utilize a resource in that enviroment that other organisms cannot use. Its a very cool example of evolutionary theory in both the lab and the field, but it is not a completly new form of life as the media implied.

(BTW, if you are unfamiliar with the Lenski experiment, it is fascinating in its own right)

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