Startling discovery debunked: bacteria can't survive on arsenic
A two-year old report suggested that one type of bacteria could survive by assimilating arsenic – a finding that held implications for the search for life in the cosmos. But new research contradicts those conclusions.
Nearly two years ago, a team of biologists claimed to have discovered new bacteria that not only could survive in an environment rich in arsenic, it could fold the toxic element into the very heart of its biochemistry – substituting to small extent arsenic for phosphorus – and survive.
Some astrobiologists were tantalized, suggesting that the results held the potential to broaden the range of habitats for life in the cosmos. But the research also generated intense push-back from other biologists, who said the results flouted well-established recipes for biologically critical molecules. And, they said, they found serious flaws in the experiments that led the team to its conclusions.
Now, an international team of scientists says it has found that the bacteria – discovered in the arsenic-rich mud of California's Mono Lake – not only have a much stronger preference for phosphorus than arsenic, but they also preferentially cull even tiny amounts of phosphorus from surroundings significantly enriched in arsenic. In other words, what may have looked like bacteria thriving on arsenic actually was bacteria efficient at feeding on the relatively low levels of phosphorus.
Phosphorus, along with oxygen, hydrogen, sulfur, nitrogen, and carbon, constitute the main ingredients for the complex molecules from which organic life is built.
The results represent the "nail in the coffin" for the idea that these bacteria can incorporate arsenic into their proteins, enzymes, and even into their DNA as a phosphate stand-in, as the initial study claimed, according to Dan Twafik, a researcher at the Wiezmann Institute of Science in Israel in an interview with the journal Nature. Nature is publishing the results in Thursday's issue. Dr. Twafik was part of the team undertaking this latest study.
The study comes on the heels of two other papers, published in July in the journal Science, which tried to replicate the original team's experiment and came up empty.
In essence, the teams concluded that the bacteria, prosaically dubbed GFAJ-1, had adapted well to living in a high-arsenic environment, but that the organisms still depended on phosphates to survive. The results prompted the editors of Science to issue a statement noting that the studies showed "GFAJ-1 does not break the long-held rules of life."
The three latest studies appear to mark the bacteria's transition from a potentially novel life form to one more addition to the list of extremophiles that intrigue scientists but that adhere to the basic chemistry of life.
Dr. Twafik and colleagues were interested in unraveling the mechanism the bacteria use to distinguished between arsenic-based compounds and phosphate and to control their uptake.
The team studied so-called binding proteins the bacteria use, a kind of biological trailer hitch that attaches one molecule to another. In this case, these proteins hitch phosphates to molecules that transport them into the cell so the bacteria can use them. The team found that the presence of arsenic weakened and distorted the hitch, while phosphates didn't. This gave the bacteria a preference for phosphate even in environments where concentrations of arsenic compounds were more than 3,000 times higher than phosphates.
Still, the team acknowledges that this strong preference for phosphates can allow some arsenic compounds to enter the cell.
For all the flack they've taken for their initial paper, Dr. Wolfe-Simon, with the Lawrence-Berkeley National Laboratory, and colleagues are continuing to probe the workings of GFAJ-1.
The Nature study has "helped us understand molecular-level discrimination between arsenate and phosphate" in GFAJ-1 and other microbes, writes Felesia Wolfe-Simon, who led the original study, in an e-mail exchange.
Indeed, the original work revealed arsenic in the cells, adds colleague John Trainer, with the Scripps Research Institute in La Jolla, Calif., and the new work doesn't rule out its migration into the bacteria, "so microbes evidently have multiple levels of adaptation to arsenic."
The team now is focusing its efforts on how the bacteria accommodate arsenic in their cells and live to tell the tale.