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March 13, 2009

Minding the Oxygen Gap

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The origin of oxygen-producing photosynthesis was a major event in Earth's history, not least because it paved the way for air-breathing animals like us. Nailing down the timing of this innovation would help researchers clarify the evolution of bacterial metabolism and better understand how oxygen released by the early bugs transformed almost every habitat on Earth.

So far, though, researchers fall into two camps whose answers differ by hundreds of millions of years, as discussed in Science’s Origins essay this month. The first widely accepted geological evidence for significant amounts of atmospheric oxygen is the great oxidation event (GOE) of 2.4 billion years ago. For some researchers, that means bacteria didn’t begin releasing oxygen until a little before the GOE. But according to other researchers, some clues—such as ancient oil thought to be the remains of oxygen-making photosynthetic bacteria—suggest that microbes turned on the gas several hundred million to more than 1 billion years before the GOE.

It’s no surprise that this is a tough question to answer. The evidence—whether it’s embedded in ancient rocks or inscribed in the DNA of modern bacteria—is hard to find and difficult to read. But scientists who argue for an early start to oxygen-releasing photosynthesis have another hurdle to overcome: explaining why it took so long for oxygen to accumulate in the atmosphere. Researchers are mulling several competing hypotheses.

You could call one possibility the distraction hypothesis. Oxygen released into the ocean by early photosynthesizers bubbled into a "reducing atmosphere" containing hydrogen gas and methane spewed out by volcanoes similar to Mount Pinatubo, as seen in the photo above. Moreover, electron-rich minerals that are targets for oxygen abounded on Earth's surface. Under this hypothesis, oxygen couldn't build up in the air because it would first react with and oxidize these tempting compounds.

According to some researchers, diminishing volcanic activity finally allowed the switch to an oxygen-containing atmosphere. Alternatively, in a 2001 Science paper, astrobiologist David Catling, who's now at the University of Bristol in the United Kingdom, and colleagues proposed that the reducing power of the atmosphere fell because early Earth gradually shed hydrogen. High in the atmosphere, their argument goes, sunlight fractured methane molecules, freeing lightweight hydrogen atoms that floated off into space.

Another hypothesis for the rise of oxygen ties it to sinking carbon. Over the long term, the amounts of carbon and oxygen in the atmosphere equilibrate. As photosynthetic organisms add more oxygen, other organisms absorb it for metabolism, releasing carbon dioxide. Shifting the balance toward more oxygen requires taking carbon out of circulation. That happens when organic matter, such as dead photosynthetic bacteria, settles to the ocean bottom and is buried by sediment, eventually getting smushed into sedimentary rock that becomes part of the continents. Some scientists argue that shortly before the GOE, the burial of carbon increased, possibly because rising mountains boosted sedimentation and because the separating continents created larger ocean basins where organic matter could amass. As more carbon was entombed in Earth's crust, more oxygen could accumulate in the air.

A related explanation contends that the early photosynthesizers were victims of their own success. The oxygen the microbes emitted could have disrupted the nitrogen cycle, favoring bacteria that transform usable forms of nitrogen into nitrogen gas, which most organisms can't handle. That would leave photosynthetic microbes short of the fertilizer they require for growth. In 2005, simulations by geobiologist Paul Falkowski of Rutgers University and colleagues showed that under such nitrogen-starvation conditions, the atmosphere gets stuck in a low-oxygen state. What eventually lifted oxygen levels, they propose, is carbon burial. As published in the American Journal of Science, the simulations revealed that a surge in atmospheric oxygen results from an increase in the size of the continental shelves, which capture large amounts of organic matter sifting down through coastal waters. Researchers are still digging for evidence that will allow them to determine which of the explanations—if any—is correct.

—Mitch Leslie

PHOTO CREDIT: USGS/T. J. Casadevall

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