A challenge for microbiologists is conveying how organisms that are so small as individuals can act collectively to influence planetary scale processes. I study bacteria that eat iron; iron is their food. Here is an attempt I made to give a sense of scale to these bacteria and the process they carry out, the precipitation of rust from an oceanographic expedition to the Loihi Seamount in the Pacific Ocean.
There are actually quite a few reasons, but this is kind of a convoluted explanation that I think gets at scale and planetary processes. Our ship is made from steel and steel is mainly iron. First, consider if there was no iron to produce steel for building ships, or that steel cost as much as aluminum(at least 5 times the price), which is used to make airplanes. Aluminum is actually the third most abundant element in the Earth’s crust, while iron is the fourth. While it might be possible to build a medium sized ship like the R/V Thompson from wood, or fiberglass, or aluminum, it would be challenging and expensive, and it would not be possible to build the really large ships like freighters, oil tankers, air craft carriers or big cruise ships. Now consider the effects on society and the global economy if we could not ship large amounts of goods cheaply on steel ships across the world’s oceans, or have them load and unload at steel docks with cargoes handled by steel cranes, etc, etc. So why is steel pretty cheap? An important reason is because the iron that it is produced from is cheap to mine. The reason for this is that the ore from an iron mine is highly concentrated in iron oxides. Several hundred pounds of iron can be extracted from every ton of iron ore that is removed from the ground, making it quite efficient, hence cheap. By contrast, it takes several tons of ore to produce an ounce of gold. So why is iron so concentrated in the places it is mined?
We know that for at least the first half of Earth’s history there was very little oxygen in the atmosphere. One result of this was that there was much more soluble iron (ferrous iron) in the ocean than there is today. This is because oxygen reacts rapidly with ferrous iron causing it oxidize to ferric iron (rust) and precipitate as an iron oxide. But the bacteria we are after do the same thing, except without much oxygen.We believe that there were episodes in Earth’s past (mostly between 2 and 3 billion years ago) when the ferrous iron in the ocean was oxidized and precipitated quite rapidly to form concentrated ores over significant areas, perhaps like shallow ocean basins. These deposits are called banded iron formations(BIFs) and have a unique stratified appearance containing different iron oxides. It is these iron-rich ores that we mine today. These BIFs are huge, hundreds of feet thick and can cover thousands of acres. The mines where the ore is extracted are really big.
So bringing the story back to our tiny bacteria that live by eating iron. They gain energy from that oxidation of ferrous to ferric iron, and need lots of iron to grow. We think they may have been responsible for the accumulation of the concentrated iron oxides in the banded iron formations that we mine today. This is the iron goes into making the steel that goes into making the hulls of the ships that we sail out into the ocean to study these bacteria. It is a source of amazement to think these tiny organisms by virtue of their large numbers, unique metabolism, and preserverance over millions of years could create the massive deposits of iron ore that we exploit today. And it is a source of some humility to think that the iron our ship is made of, may have, at one time, been food for bacteria like the ones we are hunting at Loihi.