Early Earth rich in carbon monoxide

U. CHICAGO (US) — A volcanic crater is offering clues to the composition of Earth’s early atmosphere, when carbon monoxide levels may have been tens of thousands of times higher than current concentrations.

A team of American-Russian scientists studying the microbiology and geochemistry of Uzon Caldera in eastern Siberia, focused in part on microbes called anaerobic carboxydotrophs—that use carbon monoxide instead of oxygen—mostly for energy, but also as a source of carbon for the production of new cellular material.

“We targeted geothermal fields,” says Albert Colman, assistant professor of geophysical sciences at University of Chicago, “believing that such environments would prove to be prime habitat for carboxydotrophs due to the venting of chemically reduced, or in other words, oxygen-free and methane-, hydrogen-, and carbon dioxide-rich volcanic gases in the springs.”

The carbon monoxide-based physiology produces microbial hydrogen, a component of certain alternative fuels, so biotechnological applications for cleaning carbon monoxide from certain industrial waste gases, and for biohydrogen production was also examined.

Most of the carbon monoxide at the Kamchatka site was not bubbling up with the volcanic gases; instead “it was being produced by the microbial community in these springs,” calling into question the implications of a strong microbial source of carbon monoxide, both in the local springs but also for the early Earth.

The great oxidation event

Earth’s early atmosphere contained hardly any oxygen but relatively large amounts of carbon dioxide and possibly methane. During the so-called Great Oxidation Event about 2.3 to 2.5 billion years ago, oxygen levels in the atmosphere rose from vanishingly small amounts to modestly low concentrations.

“This important transition enabled a widespread diversification and proliferation of metabolic strategies and paved the way for a much later climb in oxygen to levels that were high enough to support animal life,” Colman says.

The processing of carbon monoxide by the microbial community could have influenced atmospheric chemistry and climate during the Archean, an interval of Earth’s history that preceded the Great Oxidation Event.

Previous computer simulations rely on a primitive biosphere as the sole means of removing near-surface carbon monoxide produced when the sun’s ultraviolet rays split carbon dioxide molecules. This theoretical sink in the biosphere would have prevented substantial accumulation of atmospheric carbon monoxide.

“But our work is showing that you can’t consider microbial communities as a one-way sink for carbon monoxide,” Colman says. The communities both produce and consume carbon monoxide. “It’s a dynamic cycle.”

Calculations suggest that carbon monoxide may have nearly reached percentage concentrations of 1 percent in the atmosphere, tens of thousands of times higher than current concentrations, exerting influence on concentration of atmospheric methane, a powerful greenhouse gas, with consequences for global temperatures.

Carbon monoxide influenced atmospheric chemistry and climate during an interval that preceded the Great Oxidation Event.

Furthermore, such high carbon monoxide concentrations would have been toxic for many microorganisms, placing evolutionary pressure on the early biosphere, Coleman says.

“A much larger fraction of the microbial community would’ve been exposed to higher carbon monoxide concentrations and would’ve had to develop strategies for coping with the high concentrations because of their toxicity.”

“This fantastic field site has a wide variety of hot springs,” Coleman says of the Siberian site. “Different colors, temperatures, chemistries, different types of micro-organisms living in them. It’s a lot like Yellowstone in certain respects.”

Springs in California’s Lassen Volcanic National Park have a narrower range of acidic chemistries, yet microbial production of carbon monoxide appears to be widespread in both settings, he says.

Some of the microbial life within the caldera’s complex hydrothermal system may survive in even more extreme settings than scientists have observed at the surface, Colman says.

“One thing we really don’t know very well is the extent to which microbial communities beneath the surface influence what we see at the surface, but that’s possible as well.

“We know from culturing deep-sea vent microbes that they can live at temperatures that exceed the temperatures we’re observing right at the surface, and some of the turn out to metabolize carbon monoxide.”

The National Science Foundation and the National Aeronautics and Space Administration’s Astrobiology Institute funded the research.

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