NASA發現奇特地球細菌 拓展地外生命搜尋範圍

幾天前，NASA（美國航天局）在其官網上發佈與會通知，聲稱“將宣佈天體生物學重大發現”。在北京時間12月3日凌晨3點的新聞發佈會上，NASA宣佈研究人員在加利福尼亞莫諾胡進行的試驗首次發現，地球上的微生物也可以利用有毒的化學元素砷生長、繁殖。使地球外尋找生命的範圍得以拓展。

NASA官方原文：

NASA-Funded Research Discovers Life Built With Toxic Chemical

NASA-funded astrobiology research has changed the fundamental knowledge about what comprises all known life on Earth.

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 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."

This finding of an alternative biochemistry makeup will alter biology textbooks and expand the scope of the search for life beyond Earth. The research is published in this 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?"

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 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 was 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 results of this study will inform ongoing research in many areas, including the study of Earth's evolution, organic chemistry, biogeochemical cycles, disease mitigation and Earth system research. These findings also will open up new frontiers in microbiology and other areas of research.

"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."

The research team included scientists from the U.S. Geological Survey, Arizona State University in Tempe, Ariz., Lawrence Livermore National Laboratory in Livermore, Calif., Duquesne University in Pittsburgh, Penn., and the Stanford Synchroton Radiation Lightsource in Menlo Park, Calif.

NASA's Astrobiology Program in Washington contributed funding for the research through its Exobiology and Evolutionary Biology program and the NASA Astrobiology Institute. NASA's Astrobiology Program supports research into the origin, evolution, distribution, and future of life on Earth.相關中文新聞轉自cnbeta：
美國宇航局網站稱，這項由美國宇航局資助的研究項目將改變科學家對地球上所有已知生命構成的基本認識。新的生命構成成分的發現將改寫生物教科書，使地球外尋找生命的範圍得以拓展。這項研究成果將發表在本週出版的《科學快訊》上。

11月29日，美國宇航局發佈公告稱將於北京時間12月3日凌晨3時舉行發佈會，宣佈一項天體生物學的新發現，該項發現將影響對外星生命的搜尋。隨後即引發網絡上關於美國宇航局發現外星生命體的猜疑。

對於今天宣佈的這項生命新發現，美國宇航局負責科學任務的副局長埃德•維勒爾在華盛頓表示：“生命的定義因此擴展了。由於我們正在太陽系尋找生命跡象，所以，我們的思維必須更開闊一些，角度更多一些，對未知的生命進行研究。”

碳、氫、氮、氧、磷和硫是地球有所有已知生命形式的六大基本構建元素。其中，磷是攜帶生命基因的DNA和RNA的主要化學成分，被認爲是所有活細胞的最重要的元素。磷還是所有細胞(三磷酸腺苷)中攜帶能量的分子以及構成所有細胞隔膜的磷脂的核心成分。

但是，新發現的細菌並不是這樣，它完全不同，其細胞成分中以砷取代了磷。這種情況通常被認爲是完全不可能的，因爲砷對地球上多數生命有毒。這一發現表明人類對生命本身的理解還存在許多未知領域，同時也暗示完全有可能在其他行星上發現與地球生命不同的生命形態。

這項研究的首席科學家、美國宇航局研究員費利薩•沃爾夫•西蒙表示：“我知道有些微生物可以呼吸砷，但我們發現的微生物與衆不同，它的部分機體組成由砷構成。如果地球上的某些東西可以做這樣一些出乎我們意外的事情，那麼生命還能做哪些我們還未發現的事情呢？”

新發現的微生物----菌株GFAJ-1是一個常見細菌羣Gammaproteobacteria的成員，研究人員用一種非常依賴於磷的食物成功地培育出這種微生物，但砷在其中發揮了巨大的協助作用。當研究人員將磷去掉，換成砷，這些微生物仍可以繼續生長。隨後的研究發現，砷被用於產生新GFAJ-1細胞的“砌塊”。研究人員研究的最重要的問題是，當這種微生物依靠砷生長時，砷實際上已經成爲這種生物重要的生物化學體系(諸如DNA、蛋白質和細胞膜)的一部分。

這項研究成果將促使科學界重新檢視許多領域目前正在進行的研究，其中包括對地球演化的研究、有機化學、生物地球化學循環、緩解疾病以及地球系統研究。這些發現也將在微生物學和其他研究領域新的戰場。

美國宇航局天體生物學研究所所長卡爾•皮切爾表示：“替換生命的化學成分這樣的想法在科幻作品中很常見，但在此之前，將砷作爲砌塊，形成生命，只在理論上存在，但現在我們知道莫諾湖中就有這樣的生命存在。”