The iron foundations of industrial civilization were laid billions of years ago with the help of photosynthesizing microbes.
In Earth’s early days, volcanoes spewed out enormous amounts of smoke, ash, and lava, all laden with iron. The iron dissolved into the oceans and, some 3.8 billion years ago, began turning into rust. Because rust isn’t soluble in water, it rained down to the ocean floor--for 2 billion years, until the oceans were rusted out. Today that rust can be found around the globe, in banded iron formations that stretch for hundreds of miles. Each one consists of thin alternating layers of rust and silt, in a stack that may be thousands of feet tall. It’s from such deposits that we mine iron today.
But there is a mystery behind them: How did all that iron get rusted in the first place? Rust results when iron reacts with free oxygen in the presence of water. But 3.8 billion years ago, it is thought, there was virtually no free oxygen because there were no plants. Plants liberate oxygen during photosynthesis by splitting molecules of water; they use electrons from the water’s hydrogen to absorb solar energy, use that energy and the hydrogen itself to make carbohydrates, and throw the oxygen away. The oldest fossils of organisms that may have produced oxygen in this way are filament-forming microbes from western Australia. They are 3.5 billion years old--300 million years younger than the oldest banded iron formation.
Recently two different suggestions have been made for bridging this time gap between the first appearance of rust and the liberation of oxygen. Bernhard Schink, a bacterial physiologist at the University of Konstanz in Germany, says the rust could have been made without free oxygen: last year he and his colleagues announced the discovery of a new type of photosynthesizing bacterium that eats iron and spits out rust.
These bacteria live today in shallow lakes and ditches, soaking up energy from the sun. But instead of using water as an electron source, they pull electrons off iron atoms, converting the iron from its ferrous to its ferric form. The ferric iron then reacts with a water molecule to make ferric hydroxide, which later becomes ferric oxide--also known as rust. If these bacteria were present in Earth’s early oceans, says Schink, they could have worked directly on the iron around them to produce the banded formations, without help from free oxygen.
But that if is a big one: there is no evidence that Schink’s bacteria existed 3.8 billion years ago. And even if there were, says paleontologist J. William Schopf of UCLA, the banded iron formations are far too big to have been made directly by rust-depositing bacteria. Organisms are patchy, with groups found here and there, he says. They aren’t spread all over the planet. Schopf, who discovered the 3.5-billion- year-old fossil microbes in western Australia in 1986, believes that organisms produced the free oxygen that rusted the iron, but not the rust itself.
Last year Schopf reported evidence from his Australian site indicating that organisms were producing free oxygen even earlier than 3.5 billion years ago. He has now discovered 11 distinct species of fossils, at least 7 of which strongly resemble primitive types of oxygen-producing microorganisms called cyanobacteria. It’s very clear that life not only existed but was already very diverse 3.5 billion years ago, says Schopf. Nobody had guessed that. The diversity suggests that cyanobacteria--like microbes had already been evolving and releasing oxygen for some time--long enough, perhaps, to account for some of the earliest banded iron formations.
Schopf’s theory explains the global distribution of banded iron formations: unlike rust, free oxygen dissolves in water, so it can spread over large areas. The theory also explains the bandedness: the rust would have been deposited only intermittently, whenever an upwelling current brought a pulse of iron from seafloor volcanoes to the surface and into contact with the oxygen released by the floating microoganisms.
Schink’s bacteria, in contrast, would have needed a constant supply of iron to survive, and would have produced a constant rain of rust. To be honest, I don’t really have an explanation for the layers, says Schink. The only thing I can say for sure is that the appearance of oxidized iron is not necessarily evidence of free oxygen. What the banded formations are evidence of, everyone seems to agree, is that life of some kind was already having a dramatic effect on our planet nearly 4 billion years ago.