Science 321, 967–970 (2008)

Credit: L. G. MILLER & S. BAESMAN

Arsenic-fuelled photosynthetic bacteria may have helped shape the arsenic cycle on ancient Earth.

The newly discovered bacteria use electrons from arsenite to drive photosynthesis in the absence of oxygen, producing arsenate in the process. Ronald Oremland of the US Geological Survey in Menlo Park, California, and his co-workers found the microbes growing in red and green mats in arsenite-rich brine pools (pictured right) at Mono Lake, California. A pure culture of one purple bacterium isolated from the red mats required both light and arsenite to thrive.

Although other bacteria are known to rely on arsenic, this is the first example of the photosynthetic use of arsenite in the absence of oxygen. The process could have been important on Earth before oxygenic photosynthesis reshaped the atmosphere.