Bacteria stir debate about 'shadow biosphere'
Friday, December 3, 2010; 9:12 PM
All life on Earth - from microbes to elephants and us - requires the element phosphorus as one of its six components.
But now researchers have discovered a bacterium that appears to have replaced that life-enabling phosphorus with its toxic cousin arsenic, raising new and provocative questions about the origins and nature of life.
News of the discovery caused a scientific commotion this week, including calls to NASA from the White House asking whether a second line of earthly life has been found.
A NASA press conference Thursday and an accompanying article in the journal Science said the answer is no. But the discovery opens the door to that possibility and to the related existence of a theorized "shadow biosphere" on Earth - life evolved from a different common ancestor from all we've known so far.
"Our findings are a reminder that life-as-we-know-it could be much more flexible than we generally assume or can imagine," said Felisa Wolfe-Simon, 33, the biochemist who led the effort.
Prompted by debate about the possible existence of a shadow biosphere, Wolfe-Simon set out specifically to see whether microbes that lived in California's briny, arsenic-filled Mono Lake naturally used arsenic instead of phosphorus for basic cellular functions, or were able to replace the phosphorus with arsenic.
She took mud from the lake into the lab and began growing bacteria in Petri dishes. She fed them sugars and vitamins but replaced phosphate salt with arsenic until the surviving bacteria could grow without needing the phosphates at all.
Her research found that some of the bacteria had arsenic embedded into their DNA, RNA and other basic underpinnings.
"If something here on Earth can do something so unexpected - that breaks the unity of biochemistry - what else can life do that we haven't seen yet?" said Wolfe-Simon, a NASA Astrobiology Research Fellow and member of the National Astrobiology Institute team at Arizona State University.
"This is different from anything we've seen before," said Mary Voytek, senior scientist for NASA's program in astrobiology, the arm of the agency involved specifically in the search for life beyond Earth and for how life began here.
"These bugs haven't just replaced one useful element with another; they have the arsenic in the basic building blocks of their makeup," she said. "We don't know if the arsenic replaced phosphorus or if it was there from the very beginning - in which case it would strongly suggest the existence of a shadow biosphere."
Theoretical physicist and cosmologist Paul Davies, director of the Beyond Center at Arizona State and a prolific writer, is a co-author on the new Science paper. He had been thinking about the idea of a shadow biosphere for a decade and had written a paper on it in 2005. The same year, philosopher and astrobiologist Carol Cleland and biochemist Shelley Copley, both of the University of Colorado at Boulder, teamed up to publish on the subject. Both asked why nobody was looking for life with different origins on Earth, and Cleland coined the phrase "shadow biosphere."