By ROBERT LEE HOTZ
Researchers on Thursday said they had bred microbes that absorb the notorious poison arsenic into their DNA, in an experiment that could rewrite the rules of life.
Until now, all organisms were thought to have six essential elements in common: oxygen, hydrogen, nitrogen, sulfur, carbon and phosphorus. The researchers say the bacteria they grew in the lab now "very likely" use arsenic instead of phosphorus in critical parts of their working biology, including their spiral backbone of DNA. If the new findings hold up, scientists will have to add noxious element 33 to the construction kit for existence.
Moreover, by depending on an element so toxic to normal life, the microbes are a living demonstration of the exotic substances that biochemistry might, at least in theory, use in other worlds.
"It is building itself out of arsenic," said geo-microbiologist Felisa Wolfe-Simon at NASA's Astrobiology Institute and the U.S. Geological Survey, who led researchers from eight federal and university laboratories conducting the experiment. "All life we know is the same biochemically, and this is a little different. It is suggesting there is another way to be alive."
The researchers conceded that the odd microbes, in and of themselves, don't prove yet that there is a fundamentally different basis for life on Earth. "It is beginning to open the door a crack to possibilities," Ms. Wolfe-Simon said.
Several independent experts were convinced that these unusual organisms weren't so far out of the ordinary. "This is an interesting curiosity, a novel discovery but not a paradigm-breaking one," said New York University chemist Robert Shapiro, an authority on DNA and the origin of life who was not involved in the project. "It is a cousin of known living things that has some peculiar habits."
To be sure, life on Earth knows few bounds. Microbes can be found in rocks a mile underground and in clouds overhead. Some bacteria thrive in toxic waste or survive in brine five times as salty as the sea. Many species of single-cell creatures readily grow in the absence of oxygen, warmth and light.
Until now, however, they were all thought to share the same biochemistry, based on the Big Six, to build proteins, fats and DNA. Even the synthetic cells made in the laboratory, as announced earlier this year, rely on the same six elements. Phosphate is an essential building block for various macromolecules present in all cells, including nucleic acids, lipids and proteins.
"They made their case, and it is quite remarkable," said geo-microbiologist Clara Chan at the University of Delaware, who has studied the findings. "But it remains to be seen how common it is."
The scientists published the research in the journal Science and discussed their work at a NASA news conference Thursday.
"This will fundamentally change our definition of life and how we look for it," said astrobiologist Pamela Conrad at NASA's Goddard Space Flight Center in Greenbelt, Md. "This is a huge deal."
Their finding comes as the hunt for Earth-like planets accelerates. With 22 space-based observatories and 100 ground telescopes, researchers are scanning tens of thousands of stars for evidence of a planet that could support life like that on Earth.
Scientists have speculated that an alternative scheme of biochemistry could give rise to other kinds of life in the solar system or on one of the 504 planets astronomers have discovered so far around other stars. In forms too strange to readily recognize, exotic life might thrive in seas of liquid methane, in plumes of exotic gases or in caverns of nitrogen ice.
Indeed, rumors about the pending announcement this week fueled a fever of anticipation among space enthusiasts, with rounds of speculation about the discovery of life elsewhere in the solar system. But the researchers made it clear that their discovery was entirely down-to-earth.
The bacteria were dredged from the briny sludge of California's Mono Lake, where the water is richly laced with arsenic and with bacteria that can survive in it. In the lab, the researchers grew the bacteria in Petri dishes in which phosphate salt normally essential for life was gradually replaced by arsenic, until the bacteria could grow without needing phosphate. They confirmed their finding through a battery of tests with mass spectrometers, radioactive tracers, X-rays and conventional genetic screening.
The researchers weren't able to entirely eliminate all traces of phosphorus, leaving open the possibility that these bacteria were still eking out their existence in a normal way, the researchers said. "There does seem to be a low level of impurity," Dr. Wolfe-Simon said. But the traces of phosphorus were so minute, the researchers said, that no normal microbes could have survived."It probably has large implications for the evolution of life on Earth," said USGS microbiologist Ronald Oremland, a senior scientist on the project. "We want to see if there are more kinds of microbes that can do this, and we are trying to find that out.