There's lots of excitement in the biology community today over the discover of a bacterium from Mono Lake in California that does something we thought life couldn't do - it uses arsenic in place of phosphorus. The discovery was made by geomicrobiologist Ronald Oremland and by Felisa Wolfe-Simon, an astrobiologist. Astrobiology is a young field that sounds made-up: it's the study of what extraterrestrial life might look like.
The tricky part about astrobiology is that we don't know whether life that arises elsewhere will follow the general rules and blueprint of life as we know it, or whether it might use different building blocks and designs altogether. Biology classes often start off by discussing what makes life, anyway? Answers usually include the ability to metabolize chemicals into energy, the ability to grow and reproduce and respond to the environment. But which chemical building blocks are necessary to carry out these functions?
The life we know on earth depends on six key elements: carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus. Phosphorus shows up in DNA, the self-copying molecule responsible for heredity and as a blueprint for protein manufacture. It also plays a key role in the energy exchange system (you could say the money) inside the cell, as part of the molecule ATP.
The reason arsenic is a poison is that it can sometimes find its way into cells and works its way into molecules in place of phosphorus, which it resembles but not well enough. Replace phosphorus with arsenic and you gum up the works, making the machinery of life grind to a halt.
But what if arsenic doesn't poison all organisms? That's the question Wolfe-Simon asked herself. She went bio-prospecting at Mono Lake, a lake without natural outlet in the foothills of the Sierras, where arsenic levels are remarkably high. Wolfe-Simon grew the bacteria she collected there on plates deprived of phosphate but supplied with plentiful arsenate. Together, Wolfe-Simon and Oremland were able to demonstrate that at least one bacterium, GFAJ-1, incorporates arsenic into a number of biomolecules, including the backbone of DNA.
Some scientists are hailing this odd bacterium as a remarkably new life form, one that suggests life may have evolved more than once on earth. To me that sounds like an overstatement. After all, the rest of the DNA and other complex molecules sound identical, so it seems to me more likely that one form of DNA design arose first, and that the ability to tolerate substitution of arsenic for phosphorus or vice versa came as a later adaptation to changing local conditions. But what's clear is that when we say to ourselves, "Life is like so; it requires these six elements, this temperature range, and this kind of atmosphere," we may well be wrong. Life is incredibly diverse and adaptable, and our ability to find it everywhere, including deep in the crust of the earth, at the bottom of a poisonous lake, in the midst of boiling volcanic vents, or even on faraway moons, is probably limited only by our assumptions and our strategies for looking.