The Sole Survivor
In a previous post I examined whether it’s plausible or not for bacteria to survive inside space rocks for long enough to travel between worlds. A new study by Dylan Chivian of Lawrence Berkeley National Laboratory and coworkers (Chivian D. et al., 2008, Science v. 322, pp. 275-278) has found a microbe that might be able to achieve such a feat of survival. Three kilometers inside the Earth’s crust, Chivian found a new variety of bacterium that can survive using only the energy from natural radioactivity in surrounding rocks. It lives in total isolation from the rest of the biosphere, making copies of itself from local raw materials and not relying on any other life forms for survival.
This is exactly the kind of von Newmann microbe you’d need, if you wanted to seed the universe with life on slow-moving rocks in space.
Life needs energy, specifically chemical energy, to keep going. Living cells are little machines, and they need a ready source of electrochemical energy to drive their metabolism. Plants capture sunlight energy to drive tiny photovoltaic machines made of protein inside their cells, and they use that energy to build their bodies from carbon dioxide and other nutrients as feedstock. Other bacteria can use chemical energy directly, such as this new deep-rock microbe. Unlike other known microbes, however, this one relies solely on the oxidizing power of natural rock radioactivity to provide them with chemical power.
Deep inside the fractured rocks of a South African gold mine, where this microbe was found, natural trace amounts of uranium emit radioactivity, as they have for eons. Each time a uranium atom decays, it fires a bundle of particles out to strike some other atom in the rock. When that radiation strikes a sulfur atom in pyrite or some other local sulfur mineral, it can oxidize the sulfur to a different form, such as sulfate.
The key here is that sulfate formed in this way – the process is called radiolysis – can be used by these bacteria as an energy source. The bugs can’t use the radioactivity directly – nothing known on Earth can do that – but the bugs simply wait until some radiolytic sulfate is generated, then they turn it back into sulfide (its original form in pyrite rock) and keep a percentage of the released energy to fuel their metabolism. It’s a slick survival method, but it’s a meager existance. There’s barely enough chemical fuel produced by the radioactivity down there – 2.8 km down inside fractured metal-rich rock – to keep many bacteria alive. Each one just sits there, waiting, able to replicate only once every few thousand years on average. It’s not much of a life… but it is life.
The bacterium, named Candidatus Desulforudis audaxviator by its discoverers, is unique in other ways. It can fix its own nitrogen, make all its own nutrients from raw mineral matter, and can even subsist off the corpses of its own kind. All of these are strategies necessary when living alone, in a community of one species, imprisoned in rock for millions of years with no source of energy but a thin drip of radiolytic sulfate. These bacteria have even lost their ability to defend against corrosion from atmospheric oxygen; it’s probably been a few million years since they’ve encountered any. They exist in a closed system, a sealed micro-world of their own.
These C. D. audaxviator are almost perfect survival machines. Trimmed down by evolution to the bare minimum of genes and biochemistry necessary to exist in their weird, spartan Tartarus, they have everything they need to endure the ages totally alone. If we wanted to find a lean, rugged microbe to plant inside a planetoid and send it on a deep space mission to another star, this is the one we’d use. A colony of these bugs could survive for geologic ages inside any asteroid, any moon, as long as there was enough raw carbon and trace elements present in the rock, and enough background radioactivity to keep their fluids from freezing and to oxidize a continuous, minimal supply of sulfate.
Bacteria like this could live happily inside most planets of the Solar System, many asteroids, perhaps even inside Halley’s Comet, if we decided to plant them there. Bio-replicators, it seems, cannot easily be stopped.