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January 18, 2011

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Microbes made partly of “toxic” chemical found

Dec. 2, 2010
Courtesy of NASA
and World Science staff

Re­search­ers stu­dy­ing the harsh en­vi­ron­ment Cal­i­for­nia’s Mon­o Lake say they have dis­cov­ered the first known mi­crobe that thrives and re­pro­duces us­ing the tox­ic chem­i­cal ar­se­nic. 

The find­ing il­lus­trates how the chem­is­try of life can take sur­pris­ing forms, and ex­pands the range of in­gre­di­ents we might ex­pect life on oth­er plan­ets to use, re­search­ers said. The new­found crea­ture sub­sti­tutes ar­se­nic for phos­pho­rus in its cell com­po­nents.

Ge­omi­cro­bi­o­log­ist Fe­lisa Wolfe-Simon, col­lect­ing lake-bottom sed­i­ments con­tain­ing bacteria in the shal­low wa­ters of Mon­o Lake in Cal­i­for­nia. (©2010 Hen­ry Bort­man )
“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.” 

The study is pub­lished in this week’s ad­vance on­line edi­tion of the re­search jour­nal Sci­ence.

Car­bon, hy­dro­gen, ni­tro­gen, ox­y­gen, phos­pho­rus and sul­fur are the six bas­ic build­ing blocks of all known forms of life on Earth. The el­e­ment phos­pho­rus is con­sid­ered es­sen­tial. It is part of the chem­i­cal back­bone of DNA and its chem­i­cal cous­in RNA; a cen­tral com­po­nent of the energy-carrying mol­e­cule in all cells, aden­o­sine tri­phos­phate; and a key in­gre­di­ent of the cell mem­brane, the skin-like cov­er­ing that en­closes each cell.

Ar­se­nic, which is chem­ic­ally si­m­i­lar to phos­pho­rus, is poi­son­ous for most life on Earth. Ar­se­nic dis­rupts met­a­bol­ic path­ways pre­cisely 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?” 

The mi­crobe, known as strain GFAJ-1, is a mem­ber of a com­mon group of bac­te­ria, the Gam­ma­pro­teo­bac­te­ria. The re­search­ers said they suc­cess­fully cul­ti­vat­ed the mi­crobes on a di­et that was very lean on phos­pho­rus, but in­clud­ed gen­er­ous help­ings of ar­se­nic. When scien­tists re­moved the phos­pho­rus and re­placed it with ar­se­nic, they said, the mi­crobes con­tin­ued to grow. Anal­y­ses in­di­cat­ed that the ar­se­nic was be­ing used to pro­duce the build­ing blocks of the new cells.

The key is­sue the re­search­ers in­ves­t­i­gated was: when the mi­crobe was grown on ar­se­nic, did the ar­se­nic ac­tu­ally be­came in­cor­po­rat­ed in­to the or­gan­isms’ vi­tal bio­chem­i­cal ma­chin­ery, such as DNA, pro­teins and the cell mem­branes? Various tech­niques were used to de­ter­mine where the ar­se­nic was 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 50 years.

“The idea of al­ter­na­tive bio­chem­istries for life is com­mon in sci­ence fic­tion,” said Carl Pil­cher, di­rec­tor of the NASA As­tro­bi­ol­o­gy In­sti­tute at the agen­cy’s Ames Re­search Cen­ter in Mof­fett Field, Ca­lif. “Un­til now a life form us­ing ar­se­nic as a build­ing block was only the­o­ret­i­cal, but now we know such life ex­ists in Mon­o Lake.”

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Researchers studying the harsh environment of Mono Lake in California say they have discovered the first known microbe that thrives and reproduces using the toxic chemical arsenic. The finding illustrates the way that the chemistry of life can take surprising forms, and expands the range of ingredients we might expect life on other planets to use, researchers said. The microorganism, which substitutes 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.” The study is published in this week’s advance online edition of the research journal Science. Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur are the six basic building blocks of all known forms of life on Earth. Phosphorus is considered an essential element. It is part of the chemical backbone of DNA and its chemical cousin RNA; a central component of the energy-carrying molecule in all cells, adenosine triphosphate; and a key ingredient of the cell membrane, the skin-like covering that encloses each cell. Arsenic, which is chemically similar to phosphorus, is poisonous for most life on Earth. Arsenic disrupts metabolic pathways precisely 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?” The newly discovered microbe, strain GFAJ-1, is a member of a common group of bacteria, the Gammaproteobacteria. The researchers said they successfully cultivated the microbes 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. 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 when the microbe was grown on arsenic did the arsenic actually became 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 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 50 years. “The idea of alternative biochemistries for life is common in science fiction,” said Carl Pilcher, director of the NASA Astrobiology Institute at the agency’s Ames Research Center in Moffett Field, Calif. “Until now a life form using arsenic as a building block was only theoretical, but now we know such life exists in Mono Lake.”