Up until a few days ago, scientists believed that all life forms on Earth were composed of six elements: carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. Then, on December 2, 2010, NASA researchers made a discovery that forced scientists everywhere to reconsider this belief: a bacterium that can replace phosphorus with arsenic.

Typically, arsenic is regarded as toxic. Many famous humans (and animals) have been killed by arsenic poisoning, including the Turkish Grand Duke Francesco I de’ Medici, China’s second-to-last emperor Guangxu, and the racehorse Phar Lap. Arsenic was also used as a pest control, in the form of chromated copper arsenate, until 2003 when it was banned in the United States by the Environmental Protection Agency.

Arsenic (element 33) is very similar to phosphorus (element 15), which is what makes arsenate—the arsenic ion—so toxic to living organisms: it can enter cells via the same channels as phosphate (the phosphorus ion), but then interrupts cell metabolism and other life processes.2 However, research has found that some bacteria can use forms of arsenic as metabolites, and one particular strain—recently investigated by NASA—can incorporate arsenic into its DNA.

Felisa Wolfe-Simon, a geomicrobiologist at NASA, and her colleagues took mud samples from Mono Lake, California, which has an extremely high arsenic concentration. After removing all traces of phosphate from their samples, they isolated one bacterium that continued to grow rapidly in the absence of phosphate. Using radio-labeled arsenate, the researchers were able to track the distribution of arsenic throughout the bacterium, finding it in the microbe’s proteins, lipids, and ATP, glucose, and nucleic acids. Using mass-spectrometry, they also confirmed the presence of arsenic in the bacterium’s DNA.

These findings imply that the bacterium is using arsenate in the same way other “normal” cells use phosphate: to provide structure for DNA and RNA, form cell membranes, and supply energy via the molecule adenosine triphosphate (ATP). The next steps Wolfe-Simon and her team at NASA will take are to:

1. Examine whether bacteria replace phosphate with arsenate naturally (without being forced to perform the substitution in a lab), particularly since arsenate bonds are considerably weaker than phosphate bonds, and
2. Sequence the microbe genome, in order to demonstrate that, while arsenic was found in the DNA, it really has replaced phosphorus in the backbone of DNA.

Resources from Wiley on This Topic
	Arsenic Pollution: A Global Synthesis by Peter Ravenscroft, Hugh Brammer, Keith Richards
	Arsenic: Environmental Chemistry, Health Threats and Waste Treatment by Kevin Henke

1. Wolfe-Simon, F., Blum, J., Kulp, T., Gordon, G., Hoeft, S., Pett-Ridge, J., Stolz, J., Webb, S., Weber, P., Davies, P., Anbar, A., & Oremland, R. (2010). A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus Science DOI: 10.1126/science.1197258
2. Hughes, M. (2002). Arsenic toxicity and potential mechanisms of action Toxicology Letters, 133 (1), 1-16 DOI: 10.1016/S0378-4274(02)00084-X