Evidence exists for oxygen-releasing photosynthesis evolving much earlier—perhaps as early as 3 billion years ago. However, the oxygen-rich atmosphere we take for granted today has existed for only about 10 percent of Earth's 4.5-billion-year history. Why did oxygenation of the atmosphere occur so much later than the evolution of oxygen-releasing photosynthesis?

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Modern photosynthesizers consume water and release oxygen. Primitive ones instead consume dissolved iron ions—which would have been abundant in the oceans of early Earth. They produce rust as a byproduct instead of oxygen.

Using experimental microbiology, genomics, and large-scale biogeochemical modeling, "we found that photosynthetic bacteria that use iron instead of water are fierce competitors for light and nutrients," says Ozaki, the paper's first author and now an assistant professor in the Department of Environmental Science at Toho University, in Japan. "We propose that their ability to outcompete oxygen-producing photosynthesizers is an important component of Earth's global oxygen cycle."

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