Arsenic eaters and alien life

A buzz of excitement spread through the scientific world in December 2010, in the run-up to a closely guarded press conference by NASA (the US National Aeronautics and Space Administration) on a reported new discovery in astrobiology, which is concerned with the search for life on other planets. Scientific journals speculated that new signs of life had been discovered elsewhere in space.

The announcement, when it came, was anticlimactic to some, but in biological terms it could be ground-breaking. Scientists have found bacteria in Mono Lake in California in water that is highly saline and alkaline and rich in arsenic — conditions toxic to almost any other organism. But these bacteria not only survive the environment, they actually thrive.

In most other organisms, the metabolism of arsenic plays an important role in determining the degree of its toxicity, with the trivalent and pentavalent states in particular causing disruption to cell proteins, especially the energy-producing processes in the cell mitochondria. They react with critical thiols within the proteins, especially pyruvate dehydrogenase, an enzyme that breaks down pyruvates in preparation for their processing for energy production.

It has long been considered that the six elements most essential for life are carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus. However, within their DNA the bacteria of Mono Lake appear to have substituted arsenic for phosphorus — the two elements are chemically similar — and have developed the ability to use arsenic in their basic metabolism.

While this discovery does not alter our understanding of the origins of life on earth, it challenges the perception of the biochemistry of life. Until now, astrobiologists have tended to search for conditions in other planets based upon life on earth, but now environments previously considered too hostile to support life may have to be considered.

If alternative biologies can happen here, say the astrobiologists, there is no reason to suppose they cannot happen elsewhere.