I'm enrolled this quarter is a fascinating geomicrobiology course taught by the eminent Roger Buick at the University of Washington. As we're winding down the quarter, we've begun to discuss some interesting side topics that are relevant to astrobiology, including unusual metabolisms employed by microbes. ('Metabolism' is the process by which organisms obtain energy and/or synthesize their organic material). One that struck me as particularly remarkable was the ability of some bacteria to respire radionuclides like uranium and plutonium in order to obtain energy. One of them is this little dude, Desulfovibrio desulfuricans:
Credit: Electron Microscopy Core of the University of Missouri-Columbia
Respiration is the process of combining an oxidant (for us humans, other eukaryotes, and some bacteria this is oxygen, O2) with organic matter (say, sugar) in order to obtain energy. Basically, an electron is transfered from the organic material (the reductant) to the oxidant. Schematically the reaction looks like this for oxygen respiration:
C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l) + Energy (as ATP)
Translating into words:
Glucose (sugar) + Oxygen → Carbon dioxide + Water + Energy (as ATP)
(We often think of getting our 'energy' from eating food, but it is really the combination of oxygen with the food we eat that is our primary avenue to obtaining energy. This is why we must breath to live!).
Respiration can also be accomplished with oxidants other than oxygen, such as oxidized iron or sulfur species. These types of reactions yield much less energy than oxygen respiration, but are useful to organisms who live in environment without oxygen, such as deep under ground. Environments with no oxygen are called 'anoxic'. (The entire Earth was anoxic for a large part of its early history, so these metabolisms were much more important for life at those times). Some of the organisms that primarily reduce sulfur or iron for energy can also do the same for soluble radionuclides like uranium and plutonium (in the forms of UO22+ and Pu(IV,V,VI)), combining them with organic matter to produce energy. (They are not in anyway using the energy released from radioactive decay!).
Because the reduction of these radionuclide species makes them less soluble in water, and thus a less dangerous environmental toxin, bacteria that can engage in this metabolism have been of much interest to those who work on bioremediation efforts at contaminated sites. Bioremediation is the very cool process of using biology to remove pollutants and ameliorate the environmental damage caused by harmful chemical spills or leakage. Here is a peer-reviewed journal article that reviews bioremediation of radioactive waste (warning: somewhat dense).
What does this have to do with astrobiology? We know that radioactive elements decay with time. The Earth has a certain inventory of radioactive elements, some with half lives measured in billions of years. In the past, Earth had a lot more of these elements (because there was less time to decay). So in the past, the radionuclides these organisms can use to generate energy were much more abundant and these metabolisms would have been much more important. In fact, a few scientists think that natural plutonium reactors are a plausible site for the origin of life. This is by no means a popular view, but it is intriguing nonetheless. (Natural fission reactors are cool in their own right, and while we haven't found any fossilized Pu reactors, we have found Uranium ones). How cool!