Re: How does Thiobacillus ferrooxidans 'digest' rock?

Let's start by looking at the information available to us - the scientific name Thiobacillus ferreoxidans. Thio refers to sulfur, bacillus to the shape of the organism - elongated; ferre refers to iron and oxidans to oxidizing. So put it together we have an organism that oxidizes sulfur and iron.

We, as humans, often get things confused because we base the metabolism of other creatures on our own, but there are two distinct processes to life: energy and mass. We consume carbon compounds (fats and sugars etc.) to supply both needs but other organisms do not; they use different ones for each. In the case of ferreoxidans, sulfur and iron are used in energy production and carbon compounds in producing the "mass" of the organism. So what happened? Well over billions of years the organism has adapted to oxidize sulfur and iron.

What they do is basically this reaction

2FeS + 7H₂O ---> 2Fe(OH)₂+H₂SO₄ (sulfuric acid)

The sulfuric acid causes the general environment to become more acidic and the rocks/minerals dissolve in the acid.

How can the bacteria use this to produce ATP, the molecule of energy in the cell? Well, just like you and I, they generate something called a chemiosmotic gradient. The process involved is not about sugars or fats but about pH. pH is actually a measurement of the amount of H+ (protons or free hydrogen ions) in a system. What cells do is they use a difference in pH like a dam and reservoir. High pH (lots of H+) on the upside of the dam (outside the cell membrane) and less on the downside (inside the cell). The H+ by 2nd Law of thermodynamics wants to be the same and so wants to enter the cell, we have a force called the pmf (proton motive force). So just like a hydroelectric plant the cell lets the H+ enter but only through a turbine to capture the energy, the turbine in your cell is the ATPase that makes ATP. And so by moving H+ we can capture energy.

However, this reactions only releases a little energy for the organism. Like a car that gets bad gas mileage they have to consume lots of FeS (iron sulfide). As you can see this reactions leads to the production of sulfuric acid and so the pH of the system (water) drops rapidly, often to below pH 2.

If you would like more info on this biochemistry of the process look in Brock's Biology of Microorganisms (Prentice Hall Press), Madigan et al, The book covers this topic very well, and in far more detail than I have described here.