Arsenic and Old Lakes
What does the discovery of arsenic eating microbes really tell us about finding life elsewhere?
What did they expect? NASA can’t plant expectations of a major astrobiology story without also releasing seeds of wild speculation. The actual discovery announced on December 2 was tame compared to the rumors that had mutated across the internet like alien microbes: “Arsenic-based life” is cool, but its not aliens on a moon of Saturn.
A brush fire of criticism soon erupted, slamming the specific claims made in the Science paper, and the perceived arrogance and overreach of the press conference. Did the researchers really demonstrate that arsenic is replacing phosphorus in DNA and other crucial molecules, such as adenosine tri-phosphate or ATP, the chemical battery that stores power in every known cell on Earth? Did these microbes really evolve to do this in the wild, or, in a case of biological entrapment, were they merely made to go along with the plan in an unnatural laboratory situation? What about the fact that arsenic bonds are so much less stable than phosphate bonds in the internal environs of living cells? Further experiments will tell what arsenate tricks the suspects are actually capable of performing, and divulge more about the evolution of this capability. When did they diverge from our branch of the tree of life, and how long have these oddities inhabited the arsenic rich waters of Mono Lake?
The now infamous press conference, while dispelling rumors of Martians and Titanians, encouraged sensational news story about “arsenic based life”, a “fundamentally new form of life”, and even a “second genesis” or a “shadow biosphere” on Earth. Lets assume for the moment that these critters can really use arsenic where the rest of us need phosphorus. Does this really have huge implications for the search for alien life?
Yes and no. It is certainly a significant finding that expands life’s known limits and chemical bag of tricks. But these strange carbon-based microbes do not represent “arsenic based life”, and they do not reveal a shadow biosphere or a distinct origin. No, these are clearly our relatives and, like the rest of us, they use giant carbon molecules to build cells and carry information. They do imply that the edges of this biosphere, our biosphere, might be a bit wider and more porous than we imagined. If anything this shows we will have to look farther afield for any true shadow biosphere, for life that exists in conditions so different from ours that carbon or at least DNA cannot rule.
The major value in this may actually be to help the astrobiology community resist an intellectual trap. At the press conference there was discussion of how to alter future astrobiology missions to Mars or elsewhere to search for Arsenic as well as the “known biogenic elements” of CHONS and P. But, that is the wrong lesson. We don’t need to start looking specifically for arsenic elsewhere. Rather, the take home should be that we cannot assume we know what the biogenic elements are. Any of them. We still have no idea if life elsewhere will even be based on organic (carbon) chemistry. It is too easy to assume that all life must be just like life here. I don’t believe we’re capable of thinking clearly about the prospects for life, which is built on fundamentally different chemical or physical systems. When we do astrobiologcal exploration, we have to pretend we know more than we do. We are always looking for ourselves out there. We can’t think of anything better, but that may say more about us than life in the universe.
We can follow our geocentric hunches about alien biochemistry, as long as we remain alert for its general signature in anomalous chemistry and disequilibrium. What we do know is that life takes advantage of excess energy and redistributes it in ways that affect the local equilibrium. Whatever it eats or breathes, it will produce waste and exhaust that will change its environment. While we search for what we think life is, we should keep in mind that all we really know is what life does.