Since the 1990s, scientists have discovered about 20 species of bizarre bacteria that "breathe" arsenic. They are typically found in environments where oxygen is scarce and have been forced to survive on whatever strange substance is easily available. "Just like you and I inhale oxygen and breathe out carbon dioxide, these bugs breathe in Arsenic (III) and breathe out Arsenic (V)," said researcher Ronald Oremland.
Arsenic (III) is the most poisonous form of the element, because it acts much like phosphorous. Most life on Earth needs phosphorous to build the molecule called ATP, which supplies cells with energy. Arsenic can substitute for phosphorous and upset the ATP molecule, essentially starving cells to death.
In the arsenic-rich, oxygen-poor waters of Mono Lake, Oremland and team found that bacteria turn the lethal toxin to their advantage through photosynthesis. With the help of sunlight, the microbes oxidise Arsenic (III) ions into Arsenic (V) ions, stripping away electrons in the process. The electrons are then used as the energetic push needed to build ATP.
More than a mere biological oddity, the discovery adds weight to Oremland's theory that the bacteria's ability evolved billions of years ago, when the first life was just getting started on earth. At the time, the planet's oceans were devoid of oxygen, but hydrothermal vents spewed elements such as sulphur, iron and arsenic into the water column. In this ancient stew, arsenic may have been an important nutrient to life. The life-forms would have used whatever they could to survive these noxious waters, and sunlight and arsenic were probably plentiful. As the life-forms found they could make a living off of these odd bedfellows, one of the first forms of photosynthesis was born.
Modern photosynthesis is thought to have evolved between 2.3 and 2.7 billion years ago. The arsenic-based form may be much older.
File this one under fascinating!