In a salty hot spring near Mono Lake, California, researchers have found two new species of bacteria that use arsenic for photosynthesis, and require no oxygen to fuel the process. Researchers say the bacteria may be similar to those that existed on primordial Earth where oxygen was scarce, and may illustrate an important stage of how early life developed in mineral-rich waters over 2 billion years ago.
Arsenic is well-known for its toxicity; it was used so often as tool for homicide in the 1800s that it earned the nickname “king of poisons” [The Scientist]. Yet the newly discovered bacteria can not only tolerate the element, they require it to survive. 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 [Science News].
Researchers noticed the colorful “biofilms” of bacteria on rocks in the small, hot pools around Mono Lake. Lead researcher Ronald Oremland explains that the pools are an unusual environment for life: “These lakes are fed by hydrothermal waters that leach out arsenic-containing minerals from the surrounding rocks” [BBC News]. The arsenic-loving “extremophiles” that can survive in these oxygen-depleted waters may also indicate how life could survive in far harsher conditions; Oremland says a similar mechanism might once have fuelled life on Mars or on Jupiter’s moon Europa [New Scientist].
Oremland believes the new species, reported in the journal Science [subscription required], were present before the evolution of modern photosynthesis, which is thought to have occurred between 2.3 and 2.7 billion years ago. Researchers have now found more than 20 modern types of bacteria and algae that use arsenic to create energy. These are scattered so widely across the evolutionary spectrum, said Oremland, as to suggest an ancient lineage: a common ancestor hidden in the shadows of time. The as-yet-unnamed hot spring bacteria from Mono Lake, able to survive in an oxygen-depleted environment reminiscent of early Earth, appears to be a direct descendant of that first photosynthesizer of arsenic [Wired News].
Image: Science/Laurence G. Miller and Shaun Baesman