A new kind of photosynthesis that uses arsenic instead of water to harvest light promises to rewrite evolutionary history – at least that of arsenic metabolism on Earth.

Some bacteria use arsenate – arsenic with four oxygen atoms attached – as an energy source. It was thought that this form of metabolism didn’t get going until long after photosynthesis filled the atmosphere with oxygen about 2.7 billion years ago. When this happened, naturally occurring arsenite would be transformed into arsenate.

But Ronald Oremland and colleagues at the US Geological Survey in Menlo Park, California, were puzzled by the great range of arsenic-eating bacteria. If they evolved recently they must have passed the ability to metabolise arsenic to each other by lateral gene transfer, he says.

Alternatively, arsenic metabolism could have evolved much earlier, giving plenty of time for bacteria to diversify. Newly discovered bacteria from oxygen-free hot springs in Mono Lake, California, support this interpretation.


Toxic lunch

Oremland’s team isolated and bred these bacteria in the lab. By growing them with with arsenite as the only possible food source, the researchers showed that the bacteria can indeed thrive.

“It was quite tricky to culture the critters, because if you give them too much arsenic, it’s still toxic even to them. But once we had figured out the right dose and gave it to them at regular intervals, they flourished on the stuff,” says Oremland.

The results suggest that arsenic photosynthesis evolved at the same time, or even before, “normal” photosynthesis. Oremland says a similar mechanism might once have fuelled life on Mars or on Jupiter’s moon Europa.

Jon Lloyd, a geomicrobiologist at the University of Manchester, UK, says it will be interesting to see how widespread arsenic photosynthesis is. He also says that these bacteria might one day help to clean up arsenic-contaminated drinking water, but emphasises that there is a pressing need for detoxifying technologies now.

“There are existing robust approaches that can be implemented more quickly,” he says.

Journal reference: Science, DOI: 10.1126/science.1160799