Iron extracted from cores drilled in rocks similar to these in Karijini National Park, Western Australia by UW-Madison researchers determined that half of the iron atoms had originated in shallow oceans after being processed by microbes 2.5 billion years ago Clark Johnson, UWM

Almost half the iron found on Earth was processed by bacteria billion years ago, says a new study which suggests iron should probably be considered a biological element.

Geoscientists at the University of Wisconsin-Madison (UWM) and Nanjing University were examining samples from the ancient banded iron formations in Western Australia.

The 150 metre iron deposits were believed to have originated from inside the Earth and brought through ocean vents to the ocean floor. But Clark Johnson at UWM and Weiqiang Li from Nanjing now show most of the iron was metabolised by ancient bacteria living along the continental shelves.

The study is published this week in the Proceedings of the National Academy of Sciences (PNAS).

The banding was thought to represent some sort of seasonal changes.

Long-term swings in the composition along the bands however did not show variations on shorter periods like decades or centuries.

Precise measurements of isotopes of iron and neodymium using fast lasers took over three years to arrive at the conclusion.

Around 2.5 billion years ago there was little oxygen in the atmosphere and organisms like bacteria derived energy by metabolising iron instead.

North-west Australia is also the region where oldest remains of bacterial life were found dating back to 3.5 billion years.

Johnson says, "These ancient microbes were respiring iron just like we respire oxygen. It's a hard thing to wrap your head around, I admit."

Bacteria played a crucial role in breaking down dead organic matter made of cyanobacteria and releasing the carbon dioxide locked in.

But even before oxygenic photosynthesis began filling the planet with oxygen, microorganisms were using iron oxide in place of water for photosynthesise, and in the process oxidising iron.

The iron rich world meant early biological molecules may have been iron-based.

Johnson says, "In my introductory geochemistry textbook from 1980, there is no mention of biology, and so every diagram showing what minerals are stable at what conditions on the surface of the Earth is absolutely wrong."

Geomicrobiology has completely turned geoscience on its ear, he adds.