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Other Genetic Principles

I just read about that bacteria that can live in arsenic. This is kind of cool but what's the big deal? It isn't really E.T. or anything.

-A curious adult from California

December 3, 2010

Scientists did recently announce that they had discovered a bacterium that can live in arsenic. What is cool is that it can replace its phosphorous with arsenic. This was thought to be impossible.

UPDATE 2/2012: Looks like it may still be impossible. A recent study are suggesting that these bacteria contain no arsenic in their DNA. It was a cool story while it lasted...

Scientists have long held that all life on Earth was absolutely dependent on six different chemicals: carbon, hydrogen, oxygen, phosphorous, sulfur and nitrogen. These chemicals were deal breakers. Without them, no life.

In this study, researchers claim that a bacterium found in Mono Lake in California can substitute arsenic for phosphorous. These bacteria don't do as well on this poison as they do with phosphorous, but they do better than any other life form found so far. Certainly better than you or I would!

One way they might pull this off is by replacing phosphorous with arsenic. In some ways this makes sense because chemically they are a lot alike.

The big difference between the two is that arsenic makes everything it is attached to unstable. So if these cells really do replace their phosphorous with arsenic, then they have DNA that falls apart easily. Same thing with a lot of its proteins and many other important molecules.

To survive, a cell needs certain of its molecules to be stable. Let's take DNA as an example.

DNA has all the instructions for making and running a cell. If the DNA gets too damaged, a cell can't run right and any offspring can't get made properly. Eventually this cell and its offspring would die. Except this bacterium didn't.

What this means is that it must have figured out a way to make its arsenic laced molecules more stable. And how to get the cell's machinery to use arsenic instead of phosphorous for all kinds of cellular reactions.

Maybe these bacteria have special proteins that keep arsenic nice and calm. Or they change the inside of their cell to make arsenic less reactive. These cells do have big sacs inside of them called vacuoles that may affect the environment inside the cell.

While the researchers haven't discovered life on another world, these studies do have something to say about E.T. When NASA has looked for life out there in the universe, they have focused on the big six chemicals. This may be way too limited.

If arsenic can replace phosphorous, then perhaps silicon could replace carbon. Or maybe even these are too constrained. Who is to say that life couldn't be built on an entirely different chemistry from that found on Earth?

Of course, the more outlandish other life gets compared to life on Earth, the harder it will be to identify it. A scientist would need to look at a glob of something and say that it is life without knowing how to test that fact. Very tricky.

Dilutions and Radioactivity

While the experiments in this study were simple and straightforward, they weren't 100% conclusive. Basically the researchers took the bacteria and grew them with arsenic but without phosphorous. They did this for many, many generations.

With each passing generation, the bacteria had less phosphorous to work with. Any other living thing would have petered out at this point. In fact, if these bacteria had no arsenic or phosphorous, they petered out too.

But in the presence of arsenic, they kept dividing. By the end of the experiment, there should have been very little phosphorous left in the cell. The obvious conclusion is that these bacteria can use arsenic instead of phosphorous.

Of course obvious doesn't always mean right! There could be contaminating phosphorous in the experiment and these cells could be dealing with the arsenic in another way.

The next step was to try to show that arsenic was actually in the molecules that usually had phosphorous. This was not easy and the results are not written in stone at this point.

The molecules are way too small to just have a peek. So the researchers turned to one of their favorite tricks -- radioactivity.

They grew the bacteria in the presence of radioactive arsenic and checked to see if the bacteria's DNA and proteins were radioactive. They were. The easiest interpretation is that these bacteria replaced their phosphorous with arsenic in these molecules. But it isn't the only interpretation.

The radioactive arsenic could have just ended up stuck to the outside of the DNA and proteins. The researchers tried to rule this possibility out by washing the bejeezus out of these molecules but it is a possibility.

To really nail down that these bacteria use arsenic for phosphorous, scientists are going to have to do some biochemistry. They are going to need to break open the cell and have the cellular machinery add arsenic to DNA in a test tube. Or show that cells can use ATA instead of ATP to drive a chemical reaction.

Replacing phosphorous with arsenic.


Arsenic using bacteria
dwell in the muck of
Mono Lake.


The orange phosphorous has
been replaced with arsenic.