This week there was extensive news coverage on a recent finding that phosphorous is substituted by the poison arsenic in the genetic material, DNA, of a micro-organism found in a Carlifornia Lake.
The fundamental building material of life is carbon. Carbon forms compounds called nucleosides with hydrogen, nitrogen and oxygen that are linked by phosphates to form long chains of nucleic acids, DNA and RNA. Furthermore, carbon, oxygen, hydrogen, nitrogen and sulphur make aminoacids and proteins that form the structure of every cell and these proteins perform most of the functions in a cell, tissue or organism. However, phosphorous is a critical component in DNA and RNA, forming the backbone structure of these nucleic acids, linking nucleotides, without which no new proteins will be created or no cell will be divided.
In the absence of DNA, tissues cannot differentiate or grow, they die. So phosphorus plays a critical role in the origin and sustenance of life. Until now, there has been no place for an element such as arsenic in DNA.
Photo courtesy NASA
NASA scientists identified a microbe, which lives in California's Mono Lake and substitute arsenic for phosphorus in the backbone of its DNA and other cellular components. What is not known is the energetics of arsenic based DNA/RNA backbones, which replaces phosphorous backbones. Both phosphorous and arsenic belong to the same group, Group six, in the Periodic Table, though arsenic is in the period below phosphorous. This means arsenic (chemical symbol, As) is bulkier and its chemical properties are slightly different from phosphorous. Periodic Table is a tabular display of chemical elements grouped together in the increasing order of their atomic number (number of protons or electrons) and therefore grouped based on chemical properties. Arsenic has one shell of electrons more than phosphorous and therefore chemically reactive electrons are more easily removed than from phosphorous. It is argued that the arsenic-containing compound arsenate is much more unstable than phosphate in water, and no cell would be able to cope with that.
Let us look at the implications of the new finding on the fundamentals of life chemistry and extraterrestrial life. The observation by NASA scientists is important in the context of both origin of life and existence of life on other planets. It has been unequivocally proved that living organisms sustaining near hydrothermal vents under the ocean can survive on the minerals emitted from these vents and live in the absence of solar energy. It is also known that hydrothermal vents can introduce large amounts of potentially toxic elements, such as arsenic. Such hydrothermal vents are believed to be present in several planets such as Mars. The current observation give credence to the hypothesis that life might have originated and existed under water from energy and minerals obtained through hydrothermal vents. If arsenic can substitute for phosphorous, one block of this puzzle is solved. However, the probability for arsenic-DNA to sustain complex life forms is unknown at this point.
According to Dr Wofe-Simon of the U.S. Geological Survey in Menlo Park, California, who was also lead author on the paper, "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," and added, "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.
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.
The research was published in this week's edition of Science Express. The authors on the article are: Felisa Wolfe-Simon, Jodi Switzer Blum, Thomas R. Kulp, Gwyneth W. Gordon, Shelley E. Hoeft, Jennifer Pett-Ridge, John F. Stolz, Samuel M. Webb, Peter K. Weber, Paul C. W. Davies, Ariel D. Anbar and Ronald S. Oremland.
1. A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus Science DOI: 10.1126/science.1197258.