Aug. 14, 2008 -- A deadly toxin, arsenic is known for its ability to end life. But new organisms found floating in Mono Lake in California depend on the poison to survive, even using it along with sunlight to make food through photosynthesis.
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 +3 and breathe out Arsenic +5," said Ronald Oremland of the United States Geological Survey in Menlo Park, Calif.
Arsenic +3 is the most poisonous form of the element, because it acts much like phosphorous. Most life on Earth needs phosphorous to build the molecule adenosine triphosphate (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 a team of researchers found that bacteria turn the lethal toxin to their advantage through photosynthesis. With the help of sunlight, the microbes oxidize Arsenic +3 ions into Arsenic +5 ions, stripping away electrons in the process. The electrons are then used as the energetic push needed to build ATP.
The research will be published in the Aug. 15 issue of Science.
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 sulfur, 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.
Photosynthesis may be the most important biochemical machine on the planet. Modern plants, from the biggest trees down to microscopic algae floating in the ocean, use sunlight, water and carbon dioxide to fuel photosynthesis, and they in turn supply the oxygen that nearly every animal on the planet breathes.
Felisa Wolfe-Simon of Harvard University said that's what makes the team's discovery so fascinating -- in effect they've uncovered a prototype of photosynthesis.
Modern photosynthesis is thought to have evolved between 2.3 and 2.7 billion years ago. The arsenic-based form may be much, much older.
"The move to the modern form of breaking up water molecules was the biggest move" in the evolution of photosynthesis, Wolfe-Simon said. "But along the way the innovations in the machinery are very interesting."
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