Some bacteria from Mono Lake in California do photosynthesis the old-fashioned way — really old-fashioned: They use arsenic instead of oxygen.

Arsenic photosynthesis may be an ancient form of metabolism, dating from the earliest days of life on Earth before oxygen filled the atmosphere, says Ronald Oremland, a microbiologist and geochemist with the United States Geological Survey in Menlo Park, Calif. Oremland and his colleagues describe the two types of newly discovered bacteria in the Aug. 15 Science.

One of the first steps most organisms perform in photosynthesis is to split water molecules, creating oxygen. Oxygen donates energy in the form of electrons to other molecules, setting off a chain reaction that eventually results in the building of sugars for the organism’s own food. For the red and green bacteria found in Mono Lake, arsenic plays the role of oxygen.

Both of the newly discovered bacteria seem to lack an enzyme that other arsenic-metabolizing organisms use to convert arsenite, also called arsenic (III), to arsenate, known as arsenic (V). Instead, the newly discovered bacteria use an enzyme that normally converts arsenic (V) to arsenic (III), but in reverse. Other organisms use this conversion of arsenic for other metabolic purposes, such as respiration.

No other organisms are known to use arsenic for photosynthesis, although some other photosynthetic bacteria substitute sulfur compounds, iron or nitrous oxide for oxygen in the process. Oremland has previously found other bacteria in Mono Lake that use arsenic in respiration, the reverse of photosynthesis.

Not only does the discovery give hints about early life on Earth, says Joanne Santini, an environmental microbiologist at University College London in England, but it also “gives us a bigger picture of how arsenic is being moved around in the environment.”

Bacteria such as these may play a role in releasing arsenic into the water supply, she says. If so, the bacteria may be somewhat helpful in that the arsenic (V) they produce is less toxic and easier to remove than arsenic (III). But these particular bacteria are unlikely to help in cleaning up arsenic contamination, Santini says. The Mono Lake bacteria live in very salty water (nearly three times saltier than the ocean) and can only grow in environments lacking oxygen. It’s hard to imagine how they might be used in removing arsenic from freshwater.

Arsenic-respiring and arsenic-photosynthesizing bacteria help form a biological cycle of the chemical. Even though arsenic is poison for humans and many other organisms, some, like the newly discovered bacteria, thrive on the energy-rich chemical.

“More and more we’re discovering that this is a very useful element,” says Oremland.

• Print
• |
• Comment

Found in: Life