Discovery of Arsenic-Based Life on Earth Could Widen Search for Extraterrestrials
Astrobiology research announced by NASA Dec. 2 may change our fundamental concept of what constitutes all known life on Earth—and is causing astronomers to broaden their thought of what life elsewhere may be.
Researchers conducting tests in the harsh environment of Mono Lake in California have discovered the first known microorganism on Earth able to thrive and reproduce using the toxic chemical arsenic. The microorganism appears to substitute arsenic for phosphorus in its cell components.
“The definition of life has just expanded,” said Ed Weiler, NASA’s associate administrator for the Science Mission Directorate at the agency’s Headquarters in Washington. “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.”
If further research confirms the NASA team’s work, this finding of an alternative biochemistry makeup could alter biology textbooks and expand the scope of the search for life beyond Earth. The research is published in last week’s edition of Science Express.
Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur are the six basic building blocks of all known forms of life on Earth. Phosphorus is part of the chemical backbone of DNA and RNA, the structures that carry genetic instructions for life, and is considered an essential element for all living cells.
Phosphorus is a central component of the energy-carrying molecule in all cells (adenosine triphosphate) and also the phospholipids that form all cell membranes. Arsenic, which is chemically similar to phosphorus, is poisonous for most life on Earth. Arsenic disrupts metabolic pathways because chemically it behaves similarly to phosphate.
“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,” said Felisa Wolfe-Simon, a NASA astrobiology research fellow in residence at the U.S. Geological Survey in Menlo Park, Calif., and the research team’s lead scientist. “If something here on Earth can do something so unexpected, what else can life do that we haven’t seen yet?”
“Toxicity, I think, is in the eye of the beholder,” she added.
The newly discovered microbe, strain GFAJ-1, is a member of a common group of bacteria, the Gammaproteobacteria. In the laboratory, the researchers successfully grew microbes from the lake on a diet that was very lean on phosphorus, but included generous helpings of arsenic. When researchers removed the phosphorus and replaced it with arsenic, the microbes continued to grow. Subsequent analyses indicated that the arsenic was being used to produce the building blocks of new GFAJ-1 cells.
The key issue the researchers investigated was this: When the microbe was grown on arsenic, did the arsenic actually become incorporated into the organisms’ vital biochemical machinery, such as DNA, proteins and the cell membranes? A variety of sophisticated laboratory techniques were used to determine where the arsenic was being incorporated.
The team chose to explore Mono Lake because of its unusual chemistry, especially its high salinity, high alkalinity, and high levels of arsenic. This chemistry is in part a result of Mono Lake’s isolation from its sources of fresh water for the past 50 years.
The results of this study could inform ongoing research in many areas, including the study of Earth’s evolution, organic chemistry, biogeochemical cycles, disease mitigation and Earth system research.
What about the impact on astronomy? To date, more than 500 planets have been found orbiting stars other than the Sun, and more are being announced every week. These possible habitats are being intensely studied to find conditions hospitable to life similar to our own. “The biggest impact the Mono Lake findings may have is to remove a bit of limiting prejudice from our thinking about life in the Universe,” said Laurance Doyle, who studies potential habitats for extraterrestrial life with the SETI Institute in Mountain View, Calif.