Ancient Iron in Earth's Primeval Seas Rusted by Bacteria

Some of the world's largest iron ore deposits can be found in South Africa and Australia. Geological formations that are billions of years old, these ores satisfy the Earth's demand for iron. Now, though, researchers have announced that they've found how microorganisms contributed to the formation of these deposits.

In Earth's primeval seas, intensely hot springs at the bottom of the ocean regurgitated iron as dissolved, reduced ferrous iron. Yet most of today's iron is actually oxidized ferric iron in the form of "rust minerals." This begs the question of how the ferrous iron became ferric iron. It's possible that the iron was oxidized as it was deposited by the hot springs by oxygen produced by cyanobacteria. This process could have happened either chemically, or by the action of microaerophilic iron-oxidizing bacteria.

Despite these two possibilities, though, scientists have often wondered whether there was simply enough oxygen available for the process to occur chemically. The oldest known iron ores were deposited in the Precambrian period, a time when there was little or no oxygen in the atmosphere. Therefore, the most ancient banded iron formations could not have been the result of oxygen-dependent oxidation.

Further evidence seemed to point to the bacteria theory. In 1993, researchers discovered bacteria that did not need oxygen but could still oxidize iron by using energy from light. Further studies revealed that these bacteria transformed dissolved ferric iron into iron oxide (rust).

In this latest study, the researchers placed different amounts of organic material together with iron minerals into gold capsules. They then increased the pressure and temperature of the surroundings in order to simulate the transformation of the minerals over geologic time. In the end, the scientists ended up with structures of iron carbonate minerals--exactly like the ones that occur in geological iron formations.

The findings seem to suggest that, in fact, microorganisms were directly involved in the iron ore deposits on ancient Earth. The findings could allow researchers to better understand the conditions that our planet experienced early in its formation.

The findings are published in the journal Nature Communications.