Bacterial Batteries?

A schematic of the bacterial flagellum.
Image in the public domain.

The bacterial flagellum has become a symbol of the intelligent design movement, and rightly so. After all, bacteria are commonly recognized as the “simplest” organisms on the planet. Nevertheless, these “simple” organisms can make an amazingly well-designed locomotive system. Well, it turns out that the flagellum isn’t the only example of the amazing things that bacteria can construct. It seems that they can construct batteries as well!

I saw a blurb about this in the March 1, 2010 issue of Chemical and Engineering News, so I went online to find more information. I found this article on Nature News. According to the article, Lars Peter Nielsen of Aarhus University (in Denmark) did some experiments to see how bacteria are able to consume organic compounds and hydrogen sulfide in sediments that have very little oxygen. You see, in order to use these compounds, the bacteria have to oxidize them, which means that have to remove electrons from them. In order to remove electrons from the chemicals they consume, however, the bacteria have to “put” those electrons somewhere else. In most organisms, the electrons go to oxygen molecules. This process, reasonably enough, is called oxidation, and it is the reason you and I breathe. We take in oxygen so that we can oxidize our food, which produces energy for us to live.

It is very easy to understand how most organisms oxidize their food, because most organisms are exposed to a reasonable amount of oxygen in the air they breathe or the water in which they swim. However, there are lots of sediments (on the sea floor, for example) that are low in oxygen underneath the surface of the sediments. Nevertheless, bacteria in those oxygen-poor sediments seem to oxidize organic compounds and hydrogen sulfide just fine. Nielsen wanted to know how they accomplish this feat.

Most scientists aware of this situation assume that oxygen from the water above the sediments slowly diffuses downward into the sediment, and the bacteria use that oxygen just as quickly as it gets down to their depth. Thus, the sediment is oxygen-poor not because oxygen doesn’t get down to those depths, but because the bacteria use it all as soon as it gets down there. Nielsen showed that this explanation is probably not correct.

In his experiments, Nielsen took sediments from from Aarhus Bay and put them in oxygen-poor water. Not surprisingly, the amount of hydrogen sulfide in the deeper portions of the sediment built up, because there was no oxygen. As a result, the bacteria that wanted to consume hydrogen sulfide couldn’t do it, because there was nowhere for them to “put” the electrons they would need to take from the hydrogen sulfide in order to consume it. He then introduced oxygen into the water, and within less than an hour, the hydrogen sulfide concentration in the lower sediments started decreasing, indicating that hydrogen sulfide was being oxidized there.

Now the fact that introducing oxygen to the water resulted in hydrogen sulfide being oxidized is not surprising, but the timeframe in which it happened is very surprising. Nielsen was looking at the hydrogen sulfide concentration in the sediments that were two centimeters deep. Now that doesn’t seem very deep, but it would take a lot longer than an hour for oxygen to diffuse two centimeters into the sediment. Thus, his experiments make it clear that the bacteria aren’t “waiting” for oxygen to make it down to them before they start consuming hydrogen sulfide.

What does Nielsen propose? He proposes something that has been promoted by Andre Revil at the Colorado School of Mines. Nielsen and Revil think that bacteria deep in the sediment are connected to bacteria near the surface of the sediment through tiny nanowires. These wires allow electrons to flow from the bacteria deep in the sediment to the bacteria on the surface of the sediment. Remember, to oxidize a chemical, you simply need somewhere to “put” the electrons you take away from it. The bacteria deep in the sediments take electrons away from hydrogen sulfide and send them along nanowires to the bacteria near the surface, and those bacteria give the electrons to oxygen. So the bacteria near the surface are the ones using oxygen, and the bacteria below the surface are the ones consuming hydrogen sulfide. As the article says:

This means that throughout the entire system, the top layers of sediment ‘breathe’ for the whole, and those at the bottom ‘eat’ for the whole.

Now if you know anything about batteries, this ought to sound really familiar. A battery is essentially two chemical systems that are separated. One of the systems (the one on the negative side of the battery) has chemicals that would “like” to be oxidized. The other system (the one on the positive side of the battery) has chemicals that would “like” to take the electrons from that oxidation. When chemicals take electrons, we say that they have been reduced. So…the chemicals on one side of the battery “want” to be oxidized, and the chemicals on the other side of the battery “want” to be reduced. However, since they are separated, nothing happens. But when the positive and negative sides of the battery are connected by a wire, the electrons can travel through the wire from the system that wants to be oxidized to the system that wants to be reduced.

So in essence, Nielsen and Revil propose that these bacteria work together to form batteries. They grow tiny wires between them, allowing electrons to travel from the area where there are plenty of chemicals to be oxidized to the area where there is plenty of oxygen to be reduced. They suggest that these wires span distances of two centimeters or perhaps more. Of course, two centimeters doesn’t sound like a lot, but Nielsen puts it in perspective:

We’re talking about a couple of centimetres, which for bacteria is 20,000 times their body size

This would be like you and me producing wires that are more than 20 miles long.

Now it is important to note that these nanowires have not been found yet. Thus, it is not certain that this is the correct explanation for how bacteria can digest hydrogen sulfide and organic materials in oxygen-poor sediments. However, Nielsen’s experiments clearly demonstrate that the previously-believed explanation is not correct.

Regardless of whether or not the nanowire explanation is correct, it is clear that bacteria near the surface of these sediments are working together with bacteria deeper in the sediments to transport electrons in the direction they need to go so that all the bacteria benefit. This is yet another indication of the fact that bacteria aren’t “simple” organisms at all. They are mind-bogglingly complex organisms that continually remind us of how amazing the Creator really is.