Origins

For some microbes, nickel is a precious metal. A new paper suggests that by starving these bugs, a nickel shortage might have triggered an oxygen surge in the ancient atmosphere. The work will come as good news for researchers who favor an early start for oxygen-releasing photosynthesis, an intensely debated topic that was discussed in Science’s Origins essay for last month.

The oldest photosynthetic bacteria, which lived more than 3 billion years ago, didn’t produce oxygen. But researchers are at odds about when the first oxygen-making bacteria, known as cyanobacteria, appeared. Most experts agree that the early atmosphere didn’t contain significant amounts of O₂ until the so-called Great Oxidation Event (GOE) about 2.4 billion years ago. That’s when leftovers of reactions involving oxygen—such as rusted iron deposits—pop up in the geological record. Some researchers assume that oxygen-producing photosynthesizers didn’t evolve until shortly before the event. However, based on evidence such as the balance of carbon isotopes in preserved organic remains and ancient oil thought to be the residue of cyanobacteria, other scientists date the origin of oxygen-generating photosynthesis at 2.7 billion to 2.8 billion years ago—or even earlier.

If the bugs were emitting oxygen hundreds of millions of years before the GOE, why didn’t the gas start accumulating in the atmosphere? One obstacle might have been the microbes called methanogens. Today, these bugs hide from oxygen, sheltering in airless environments such as cows’ intestines, swamps, and landfills. However, they were widespread on early Earth, and the methane they released would have reacted with oxygen from photosynthetic cyanobacteria.

For the GOE to occur, “you need to have a significant decrease in atmospheric methane,” says geologist Kurt Konhauser of the University of Alberta in Edmonton, Canada. And what slashed methane levels by knocking down the methanogens was a shortage of nickel, Konhauser and his colleagues propose in a Nature paper published today.

The researchers scrutinized banded iron formations like these from western Australia (left) that were between 3.8 billion and 550 million years old. These striped sediments record concentrations of different elements in the ancient oceans, so they provide a good indicator of nickel availability. What the researchers found was that nickel abundance dropped by about half between 2.7 billion and 2.5 billion years ago. This decline probably resulted from the cooling of Earth’s upper mantle, which reduced volcanic activity that brings nickel to the surface in eruptions.

Nickel scarcity would have hit methanogens hard because several of their key enzymes require the metal, Konhauser notes. As the microbes’ methane output fell, oxygen levels could rise. “It’s the only explanation that accounts for the big picture,” say Konhauser.

—Mitch Leslie

Source: K. Konhauser et al., “Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event.” Nature 458, 750 (2009).

Photo credit: Mark Barley, University of Western Australia