Arsenate-Tolerant Bacteria from Mono Lake. Part 1: Another Perspective
Michigan Skeptics with a great followup to the stories on Arsenic based life a while back. It’s looking less likely, but the jury is still in deliberation.
Arsenic (As) is widely distributed in the lithosphere, hydrosphere and biosphere. Discovered in 1250 by Albertus Magnus, it has since become synonymous with poison. As, the 33 element on the periodic table, falls just below phosphorus (P), meaning that As shares physicochemical properties with that of P, and hence explains some of the toxic characteristic of arsenic species, including arsenite, H2As(III)O3-, and arsenate, H2As(V)O4-. Arsenate (analogous to phosphate, H2PO4-) is taken up via the phosphate transport system and is incorporated into phosphorylation reactions. The mode of toxicity of arsenate is to partially block protein synthesis and interfere with protein phosphorylation. Arsenite inactivates many enzymes because of its high affinity for thiol (-SH) groups (e.g. the amino acid, cysteine) on proteins. Microorganisms typically deal with arsenic compounds by means of methyltransferases to convert the inorganic arsenic compounds into the methylated compounds (see Figure 1). These methylated compounds can then recognized by transport pumps and excreted outside of the microorganism. This type of metabolism of arsenic compounds effectively allows microorganisms to be tolerant of elevated arsenic levels in its surrounding environment.
Last December, the journal Science, one of the most influential scientific journals, published an article by Dr. Virginia Wolfe-Simon et al. reporting the isolation of an arsenic-tolerant bacterium from the lake bottom of Mono Lake, California. Mono Lake has no outlet to the ocean and therefore has a high pH (alkaline), salt and mineral content including a relatively high arsenic concentration (~0.015 grams/liter). In a 2009 publication, Wolfe-Simon et al. predicted that
As-based life could even survive today in restricted pockets where As is present in abundance, such as shallow and deep-sea hydrothermal systems or at Mono Lake.
As far as predictions go, this was curiously accurate. Discovering an arsenic-tolerant bacterium is not novel as a number of them are known. What caught the attention of the scientific community and news media was the claim that this bacterium, named GFAJ-1,