In a mat of lowly bacteria found in a foul-smelling hot spring near Mono Lake, California is a living window into Earth's early history, a time when photosynthesis was barely evolved and the atmosphere non-existent.
The bacteria, discovered by a team of American scientists, doesn't need oxygen to photosynthesize. Instead it uses arsenic, a toxic metal abundant in the volcanic stews of primordial Earth.
By using arsenic to free oxygen from carbon dioxide, say scientists, the bacteria's ancestors helped form the oxygen-rich environment in which life would eventually flourish.
"Once you spit out oxygen, all kinds of things arise," said Ronald Oremland, a United States Geological Survey biogeochemist and co-author of the bacteria's description, published today in Science. "It's Part One of the evolution of Earth."
During traditional photosynthesis, plants and some bacteria break carbon dioxide into its constituent parts. The carbon they use to feed themselves; oxygen is released into the air.
The process is fueled by electrons harvested from molecules of water -- but that ability appeared about 2.5 billion years ago, some one billion years after the first simple, single-celled organisms evolved.
During that first billion years, a rudimentary version of photosynthesis was too weak to break down water. Instead it targeted other compounds, such as hydrogen sulfide and a certain type of iron
(recently discovered, in a promising finding for extraterrestrial life fans, on Mars.)
Scientists suspected that early photosynthesizers also used arsenic: at least 20 modern types of bacteria and algae use the toxic compound.
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.
Together with its hydrogen sulfide- and iron-eating brethren, it pumped oxygen for a billion years, conditioning the air as we now know it:
temperate and breathable.
"All it needed," said Oremland, "was sunlight and a warm pool on the surface of the Earth."
Arsenic(III) Fuels Anoxygenic Photosynthesis in Hot Spring Biofilms from Mono Lake, California [Science]
Image: Science
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