NASA scientists have just announced the discovery of life fundamentally different from anything else we've ever seen before. Here's what this discovery means for our understanding of biology, the search for extraterrestrial life, and even how this could revolutionize bioenergy.
What is the discovery?
The discovery, made by NASA scientist Felisa Wolfe Simon and her team, is straightforward enough. We often think of carbon as the crucial element for life, but actually there are six elements that work together as the basis of every last organism we've ever found. These are carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus. Phosphorus is part of the structural framework of DNA and RNA, essentially acting as the molecular girders that hold everything else in place.That makes phosphorus essential to the stability of DNA and, in turn, the existence of life.
Wolfe Simon investigated whether a different element could be substituted in the place of phosphorus. The obvious place to start is with arsenic, which is directly below phosphorus on the periodic table and thus shares many of the same properties. Her team headed to California's Mono Lake near Yosemite National Park. Mono Lake is an incredibly unusual ecosystem, with three times the amount of salt as seawater and, crucially, it's poor in phosphorus and rich in arsenic. Despite this, life thrives in Mono Lake, and so the team collected some microbe-rich mud and took it back to the lab.
There, they placed mud in a setting where the microbes would have everything they needed to live, such as sugar and vitamins. Crucially, however, they created a phosphorus-free environment and pumped the test area full of arsenic. Nothing should have survived in those conditions - indeed, arsenic is notoriously toxic. But the microbes didn't just survive, they actually thrived in the seemingly impossible conditions.
The scientists then studied the microbes, and they discovered arsenic was found on a band of the genomic DNA. They isolated this section and found that arsenic wasn't just stuck on top of the DNA - it had actually replaced the role of phosphorus. Arsenic had substituted for phosphorus as the backbone of the microbe's DNA, fulfilling one of life's most critical functions.
It's difficult to overstate the importance of this discovery - these microbes are doing something fundamentally different from all other life on Earth. Wolfe Simon puts this discovery in context, explaining what it means for life on Earth...and beyond:
"We've cracked open the door for what's possible for life in the universe. And that's profound. What else might we find? What else might we want to look for?"
What does this mean for the search for extraterrestrial life?
Heading into this announcement, there was a lot of excitement and speculation about the possible discovery of extraterrestrial life. I even asked, "Is NASA about to announce the discovery of extraterrestrial life?" I had consciously avoided using the term "alien", but I probably should have used that all along. This arsenic-based life (for lack of a better term) is just as terrestrial as you or me, but it's very alien in terms of how it's put together. NASA has, in a very real sense, discovered a form of alien life.
So then, what about life on other worlds? This new discovery greatly expands the possible scope of life on other planets. In all our previous scenarios, we had assumed that we needed to find an off-world environment that had sufficient quantities of the six essential elements - carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus - in order to have any chance of discovering life. But now we know it's possible that life could exist in conditions with lots of arsenic but next to no phosphorus.
That's the big takeaway from geobiologist Pamela Conrad, an expert on the geology and life on Mars and one of the major players in the upcoming Mars Science Laboratory mission, which will run a number of experiments looking for life on Mars. She explains that the best way to predict the habitability of off-Earth environments is to examine our own planet and create metrics of habitability, which describe the extremes (and everything in between) of where life can conceivably exist. This discovery greatly expands those metrics of habitability, and she says that's very exciting.
Conrad points out that one thing we still don't know is how the presence of arsenic might affect the evolution of organic chemistry. It might function as a near-perfect substitute for phosphorus - although that's chemically rather unlikely - or it might cause life to evolve in very different directions, or it might even represent a chemical dead-end, capable of supporting only the most absolute basic forms of life. For now, she says, we just know that arsenic appears to be tolerated by these microbes, and this raises the possibility that other exotic elements might also be tolerated or even essential to certain forms of organic chemistry.
Dr. Conrad also addressed the extraterrestrial rumors:
"Discovery of extraterrestrial life would be an incredible announcement. [But] this is a phenomenal finding. We are talking about taking the fundamental building blocks of life and replacing it with another compound."
She closed by rather awesomely comparing these microbes to the silicon-based Horta seen on Star Trek. She said the story of life isn't just carbon, and that this is a huge deal. Indeed, she said we might be able to find extraterrestrial life more easily now because we have a wider palette of possibilities.
But what can arsenic-based life do for me?
Don't worry, this discovery isn't just about rewriting our understanding of organic chemistry or helping the quest for extraterrestrial life - it could also mean cheaper gas. Dr. James Elder, one of the world's foremost experts on phosphorus, says arsenic-based life could be used to help in the cleanup of toxic waste, where the buildup of arsenate is frequently an issue. These organisms would be perfectly suited to break down such waste.
He also said that the discovery of life that doesn't need phosphorus could have some very intriguing applications in areas where phosphorus is in short supply. Beyond its role in organic chemistry, phosphorus has a bunch of industrial applications. Perhaps its most important role has been as a fertilizer in the Green Revolution, the massive increase in agricultural yields in the last seventy years that's often credited with saving billions of lives.
The problem is that phosphorus is in increasingly short supply, and that could mean trouble for the continued success of the Green Revolution. Elder says that it will be interesting to examine organisms that have evolved to survive without phosphorus, and this might provide some unexpected solutions to the phosphorus shortage. In particular, he says arsenic-based life could be used to help recover and recycle phosphorus that has already been used in fertilizers.
But the most intriguing idea is to create an entirely new bioenergy technology that's built around arsenic. Ethanol, an alternative to fossil fuels created from crops, has struggled with efficiency concerns because it takes a lot of phosphorus-based fertilizers to grow the crops in the first place, which often results in a rather troublesome net energy loss. Arsenic-based ethanol, on the other hand, could prove much more efficient, and the fact that it doesn't have any phosphorus would make it a very unattractive target for outside contaminants.
This is still just science fiction, says Elder, but it's all within the realm of possibility. It's entirely conceivable that, thirty years from now, people's main interaction with arsenic life will be when they go to gas up their cars.
How skeptical should I be of this discovery?
Whenever a discovery of this magnitude is announced, you should always reserve some room for skepticism, and the NASA press conference included the dissenting voice of renowned chemist Dr. Steven Brenner. He introduced himself as the panel's resident curmudgeon and wet blanket, and he made it very clear what he wanted to accomplish:
"My next three minutes will be successful if I explain why chemists think this is an exceptional result and why, as Carl Sagan said, this requires exceptional evidence."
He explained that we still need to understand how arsenic is tolerated and phosphorus is limited in these microbes. In terms of their role as the structure of DNA, he compared phosphorus to steel and arsenic to aluminum foil - the former makes for strong links in the chains, while the latter is nothing if not a liability. Indeed, enzymes that would attempt to use arsenic to build DNA chains would encounter what he calls "a demon wolf in sheep's clothing", a seemingly useful material that continually fails and causes the enzymes to waste a lot of energy in repeatedly making the same links.
He also pointed out that we know similar compounds that involve arsenic along with elements like carbon or oxygen tend to be very unstable. Taken together, there are two possibilities here - the enzymes might evolve to get very good at not being fooled by the arsenic, and so only take what little phosphorus there is available to build the DNA chains, or it might conceivably find a way to deal with the weak links. He said there's a lot of old chemistry that says the latter is impossible, but he also readily admitted:
"Old chemistry can be wrong. As Richard Feynman would say, ‘Science begins when you distrust experts,' and I'm an expert."
He said he would want to see a lot more data before he's convinced, but he did admit this is a fascinating finding regardless and this microbe has a lot to show us, even if the idea of arsenic-based chemistry doesn't ultimately hold up.
He also raised a rather fascinating possibility, pointing out that his measurements of the weaknesses in the links are based on room temperature. However, if you look at extreme temperature situations, like that of Saturn's moon Titan where the temperatures hover around -200 degrees Celsius, then a very reactive element like arsenic might actually be useful because it's more stable there. In fact, he said, it's possible that arsenic would be more useful than phosphorus on Titan, because you might need the increased reactivity to make biopolymer chains.
So what's next?
Ultimately, Dr. Wolfe Simon says this isn't about arsenic, it isn't about the microbes, and it isn't about Mono Lake - this is all about looking at life and asking questions. She says this cracks open the door to the potential of a huge range of other possibilities of further substitutions in the elements of organic chemistry. More importantly, this discovery offers a proof of the concept that we can actually experimentally test the substitution of elements.
These microbes are, she says, a part of Earth's tree of life, albeit the most wayward branch we've ever found. We may find even more alien substitutions in time, although she resisted the urge to speculate, saying that's something she'll leave for future scientists (herself included) to find out experimentally over the next few decades. For now, she says, the important things is that "It has solved the challenge of being alive in a very different way," and she believes this has greatly aided our search for extraterrestrial life: "We will find it, one day, in the universe."
[Science]