© Image Source/Rex
Measurements of nickel prior to the "Great Oxygenation Event" 2.5 billion years ago suggest that hungry bacteria spewed less oxygen-eating methane.

Hungry nickel-grabbing bacteria could be to thank for the surge in atmospheric oxygen 2.5 billion years ago that made Earth hospitable to life.

Stefan Lalonde of the University of Alberta in Edmonton, Canada, and colleagues measured the concentration of nickel deposited in layered sedimentary rocks, or "banded iron formations". They found that levels had dropped by two-thirds in the 200 million years prior to the "Great Oxygenation Event".

The team speculate that this drop in nickel starved primordial ocean-dwelling bacteria called methanogens that used dissolved nickel in seawater to help turn food into energy and methane. As methane reacts with oxygen to remove it from the atmosphere, a decline in the methane produced by bacteria would have led to a build-up of oxygen.

Though it is not clear quite how much the ancient bacteria relied on the metal, "growing modern methanogens in the lab requires extremely high concentrations of nickel", says Stephen Zinder at Cornell University in Ithaca, New York.

So what could have caused the nickel shortage? A surge in the number of magma plumes just before the nickel decline removed a large amount of heat from Earth's core, say the team. In these cooler conditions, more oceanic crust was created relative to continental crust. This contains less of the nickel that the bacteria can use. The work was presented at the American Geophysical Union meeting in December.

"This study is one of the first to look at hard data about metal concentrations, which is an important new idea," says Timothy Lyons of the University of California, Riverside. But he suspects the oxygenation effect may be less than the team thinks, because the bacterial famine could have enabled other atmospheric reactions that used up oxygen.