We all saw the hype. So, now, what's actually going on? In a nutshell, there are certain chemicals that are—based on everything we know about life on Earth—assumed to be necessary for the development of life anywhere. When scientists talk about which planets could harbor life, part of what they're basing that speculation on is the presence of these chemicals. At the same time, though, scientists do have active imaginations, and have long cherished the possibility that Earth life (as we know it) isn't necessarily the same thing as life, itself.
This new paper, set to be published in Science, might be the first evidence of weird life in action. (Quick clarification, though. This is not evidence of a shadow biosphere or Second Genesis on Earth. More on that below.)
A couple of years ago, scientists found bacteria in California's Mono Lake that used arsenic compounds, rather than water, as an ingredient of photosynthesis. In fact, there's been a lot of weird life research centered around Mono Lake. Hot, salty, low in oxygen, and high in lots of other useful chemicals, the Lake has been described as a here-and-now model of the old primordial soup. (It also tastes terrible, as you can see around 4:00 minutes into the video I linked above.)
This new study is different, in that it is trying to show that certain bacteria can not only eat arsenic, they can use it in their DNA—completely replacing phosphate, which is one of those chemicals we thought was necessary for life to happen.
Wolfe-Simon and her colleagues collected mud from the lake and added the samples to an artificial salt medium lacking phosphate but high in arsenate. They then performed a series of dilutions intended to wash out any phosphate remaining in the solution and replace it with arsenate. They found that one type of microbe in the mix seemed to grow faster than others.
The researchers isolated the organism and found that when cultured in arsenate solution it grew 60% as fast as it did in phosphate solution -- not as well, but still robustly. The culture did not grow at all when deprived of both arsenate and phosphate.
When the researchers added radio-labelled arsenate to the solution to track its distribution, they found that arsenic was present in the bacterium's proteins, lipids and metabolites such as ATP and glucose, as well as in the nucleic acids that made up its DNA and RNA. The amounts of arsenate detected were similar to those expected of phosphate in normal cell biochemistry, suggesting that the compound was being used in the same way by the cell.
The team used two different mass-spectrometry techniques to confirm that the bacterium's DNA contained arsenic, implying -- although not directly proving --that the element had taken on phosphate's role in holding together the DNA backbone. Analysis with laser-like X-rays from a synchrotron particle accelerator indicated that this arsenic took the form of arsenate, and made bonds with carbon and oxygen in much the same way as phosphate.
Not everybody agrees that this research proves the bacteria are capable of replacing phosphate with arsenic. You can read more about that debate in the really nicely done article at Nature News that I'm quoting above.
Also, even if this is proof that phosphate isn't necessary for life, we still don't know whether the bacteria in question actually replace their phosphate in the wild. Right now, this is something humans are convincing it to do in a petri dish. That's why it's not entirely fair to say that weird life has been discovered—all this paper does (if it stands up to the coming onslaught of scrutiny) is show that weird life is, in fact, possible.
But that's still a pretty big deal. However you slice it, this is an extremely interesting little bacterium. It isn't alien. It still has the same basic DNA structure we all know and love. It just might be able to use different chemicals to build that old, familiar structure. And that's pretty cool on its own.
There are some implications when you start thinking about how said bacterium might expand the list of planets that could potentially harbor life. That, I can only assume, is what got NASA's press release writers so excited to begin with. However, as far as astrobiology goes, what we have here isn't an answer, so much as it is a prompt for a lot of other really interesting questions.
A Bacterium That Can Grow by Using Arsenic Instead of Phosphorous in the journal Science (Not available yet, but should be later this afternoon, they tell me.)
(Thanks to David Dobbs, Lee Billings and Adam Rutherford for their help putting this together. Rutherford, by the way, is the guy drinking Lake Mono water in the video. He says, "I had to drink about 18 cups of Mono Lake. I can exclusively reveal it tastes bloody awful.")