All life on Earth is composed of six key elements: carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. These materials are fundamental to the survival of all life forms, and although it is possible to replace one of the minor elements with something chemically similar, no organism has been able to survive when one of the major six is missing. Until now. A collaboration between researchers in California and Arizona has found the first life form that can survive and thrive after phosphorus has been substituted for arsenic [Wolfe-Simon et al., Science, (2010) doi:10.1126/science.1197258].
Arsenic is almost chemically identical to phosphorus, being located directly below the latter in the periodic table. They have similar atomic sizes and electronegativities, but it is this similarity which makes arsenic so dangerous to most organisms. Arsenates can replace phosphates in some biological processes, but not all, due to the shorter lifetime of arsenate compounds. Researchers led by Dr Wolfe-Simon therefore hypothesized that arsenic could successfully be substituted for phosphorus in an organism that was capable of coping with the arsenate instability. The hunt was on.
The team isolated a strain of bacteria (now known as GFAJ-1) from Mono Lake; a body of water known for being hypersaline and containing high concentrations of arsenic. An enrichment procedure was used to reduce the amount of phosphorous within the bacterial environment, while simultaneously increasing the arsenic concentration. The growth rate of a single colony of bacteria in an arsenic rich environment was then closely monitored. Over a period of six days the cell count was found to increase by a factor of twenty.
Unsurprisingly, when phosphates were added, the growth rate increased, indicating the expected preference for phosphorous. However, this was not the only difference, as the bacteria grown using arsenates were approximately one and a half times larger, and contained “large vacuole-like regions”. Also, the arsenate bacteria did not contain as much arsenic as the phosphate bacteria contained phosphorous.
By using radiolabeled arsenic the team found that while around ten percent of the arsenic was found in the nucleic acids, the majority was found in proteins. Nevertheless some of the arsenates replaced the phosphates within DNA. This was confirmed using high-resolution secondary ion mass spectrometry on a purified DNA sample. Quite what this discovery means for our understanding of evolutionary biology remains to be seen, but there can be no denying the excitement surrounding these findings, and the potential for even more astonishing breakthroughs in the future.
Stewart Bland