ARSENIC-BASED BACTERIA POINT TO NEW LIFE FORMS

Breno Grisi

We could be witnessing the first signs of a "shadow biosphere" – a parallel form of life on Earth with a different biochemistry to all others. Bacteria that grow without phosphorous, one of the six chemical elements thought to be essential for life, have been isolated from California's Mono Lake. Instead of phosphorous, the bacteria substitute the deadly poison arsenic. "Life as we know it could be much more flexible than we generally assume or can imagine," says Felisa Wolfe-Simon of NASA's Astrobiology Institute and the US Geological Survey in Menlo Park, California.

Wolfe-Simon's team took mud containing bacteria from the arsenic-rich Mono Lake and grew them in ever decreasing concentrations of phosphorous. Their rationale was that since arsenic is just below phosphorous in the periodic table, and shares many of its chemical properties and is even used as a source of energy for some bacteria, the bugs would be able to swap one for the other. That is just what happened.

"After one year, they are still alive and well," says Paul Davies of Arizona State University in Tempe. Not only that, the team showed that this ability was incorporated deep into the molecular building-blocks of the bacterium, strain GFAJ-1 of the salt-loving Halomonadaceae family, right down to the DNA.
"It's the first time such a chemical substitution has been shown for DNA," says Philippe Bertin of the University of Strasbourg, France, who was not part of the team. "Possibly, it's a relic of an ancestral metabolism that was supplanted during evolution because using phosphorus was more stable and less toxic."

Despite surviving on arsenic for a year, the bacteria would still "prefer" to grow using phosphorous: biomolecules react more efficiently in water and seem to be more stable when constructed with phosphorous than arsenic. They only substitute arsenic if there is no alternative.

Steven Benner, a chemist from the Foundation for Applied Molecular Evolution in Gainesville, Florida, who works on alternative forms of DNA, is sceptical that the bacteria really do contain arsenic. "I doubt these results," he says, since in order to measure the modified DNA it has to be put into a water-containing gel, which would rapidly dissolve any arsenate molecules. Any hypothesis that arsenate might replace phosphate in biomolecules must take this into account, he says.

Shadow biosphere. Davies says that future work will address the stability-in-water issue, but argues that the discovery underlines the need to look further for the first true representatives of alternate life forms in Earth's shadow biosphere.

Breno Grisi
Professor (retired) of Ecology, at UFPB, Federal University of Paraíba - Brazil, currently teaching courses in ecology at private universities