In December 2010, stories began hitting the news wires saying researchers had discovered the first-known microorganism on Earth able to reproduce using the toxic chemical arsenic according to a Science/AAAS (American Association for the Advancement of Science) report. The GFAJ-1 microorganism – part of the Gammaproteobacteria family — substitutes arsenic for phosphorus in its cell components and was discovered in California’s Mono Lake.
But how could that be? Phosphorus was considered essential to life, while arsenic, while chemically similar, is a poison. The discovery that arsenic might comprise a seventh building block was almost too good to be true. And, according to two recently published papers, it might well have been.
“The definition of life has just expanded,” Ed Weiler, NASA’s associate administrator for the Science Mission Directorate at the agency’s Headquarters in Washington, was quoted as saying at the time. “As we pursue our efforts to seek signs of life in the solar system, we have to think more broadly, more diversely and consider life as we do not know it.”
Felisa Wolfe-Simon of NASA’s Astrobiology Institute published a 2011 paper that said the bacteria at Mono Lake could grow by substituting arsenic for phosphorus. The researchers had looked at the salt-based Mono Lake, once part of a vast inland ocean, because of its high arsenic levels, and they reported their conclusions from lab experiments. In the past, NASA researchers have frequently come to Mono Lake to simulate Mars in various experiments.
“We know that some microbes can breathe arsenic, but what we’ve found is a microbe doing something new: building parts of itself out of arsenic,” Wolfe-Simon wrote.
Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur are the six basic building blocks of all known forms of life on Earth. What Wolfe-Simon’s findings suggested were that life, a very different kind of life, could possibly exist on some other planet. If the bacteria can break the rules like this, some argued, who knows what kinds of life may be possible beyond Earth?
Wolfe-Simon’s research led to rampant speculation about how life might exist elsewhere, and quickly much dissent about her findings. This past weekend, that same journal, Science, released two papers that dispute the basic premise of the original research, charging that the bacteria can’t use arsenic, at least not in the way the Wolfe-Simon’s research purported.
For both new papers, scientists did their own tests of the bacteria. One team led by Rosemary Redfield of the University of British Columbia in Vancouver, Canada, reports that arsenic does not contribute to the bacteria’s growth. “Basically, it doesn’t present any convincing evidence that arsenic has been incorporated into DNA,” Redfield said.
“The bacterium seems to survive in tiny concentrations of phosphate, a concentration that was interestingly deemed as an ‘impurity’ in the original Wolfe-Simon studies,” Redfield said.
The other paper, from Swiss researchers, finds the bacteria to be highly resistant to the poisonous effects of arsenic, but still dependent on phosphorus to grow. They concluded that in the original experiment, trace contamination with phosphorous may have let the bacteria grow.
As the Science statement summarizes the results, the new work shows the bacterial species “does not break the long-held rules of life, contrary to how Wolfe-Simon had interpreted her group’s data.”
Nevertheless, Wolfe-Simon, now at Lawrence Berkeley National Laboratory, stands by her research. In an email to the Associated Press, she said, “There is nothing in the data of these new papers that contradicts our published data.”
She said her team continues to build upon its finding of the extreme resistance to arsenic poisoning.