Life in Deep Spaces
Bill O’Neill
Recently I’m immersing myself in marine biology, but I’ll defer writing on life at extremes in the sea to address a more down-to-earth topic—down in the earth— subterranean life. Life didn’t appear on the earth’s surface until the early Devonian era, about 400 million years ago, but in the preceding 3 billion years subterranean (and marine) creatures were becoming established. This relates to where and how we might look for signs of life elsewhere in the solar system. It turns out that terrestrial life inhabits earthly environments only recently penetrated by science.
In the 1920s, bacteria were found in water associated with deep oil deposits, but it was impossible to ensure that samples were not contaminated in the extraction process. More than 60 years elapsed before the EPA and Department of Energy (DOE) concerned with underground wastes, funded a Subsurface Science Program. The DOE retrieved rock cores (initially near Savannah River, SC, and later in the Northwest) under conditions where contamination by drilling fluids and surface water could be detected and prevented. Microbes were recovered from depths as great as 2.8 Km (nearly 2 miles) below earth’s surface living in interstices of “solid” rock. At that depth the temperature is 75°C (170°F). We don’t know for sure how much heat is too much for life, but some organisms survive “sterilization” conditions (135°C) and thrive at over 110°C around deep-sea volcanic vents. Typically, temperature rises about 25°C per Km in continental crust (15°C/Km in oceanic crust), so life’s possible at least 4 Km down. Needless to say, there ain’t much to eat down there, so the population’s usually pretty thin. However, that varies: at 400 meters (roughly a quarter mile) in sedimentary rock it ranges from as few as 100 to as many as 10 million bacteria per gram of rock. (For comparison, agricultural soil contains more than a billion bacteria per gram of dirt.) Vital prerequisites for life include water, space, and nutrient elements (carbon, nitrogen, phosphorus, and traces of a few metals). Most important is some form of usable energy, and there’s no sunlight down there.
Diverse microbial communities thrive in sedimentary rocks, which provide a rich supply of organic nutrients— produced eons before at the surface. Microbial metabolism can draw upon the energy stored in oxidized forms of sulfur, iron or manganese, and flourish in the absence of oxygen we surface creatures require. Over geologic time, as sediments are buried they are increasingly compacted. Microbes can deal with the enormous pressure, but they do require space. (Recent laboratory experiments support the notion that bacteria are not killed by pressures far exceeding those we’re talking about.) As the rock becomes denser, the distribution of organisms is confined to spots especially rich in nutrients—which makes them harder to find. But Hanford scientists have shown that the larger the sample you collect, the more likely you’ll find life. Surprisingly, they’ve even found microbes deep in basalt—rock solidified from molten magma which contains little carbon. Conversely, two miles down in South Africa’s Driefontein mine, the carbon-rich vein that yields gold contains 100,000 to 1 million organisms per gram—some “breathing” iron oxide and others exhaling methane (natural gas). Some of these make unusual use of cobalt and uranium in their biochemistry.
The best-studied subsurface lithoautotrophic microbial ecosystems (SLIME for short) were “unearthed” in our Columbia River basin in the 1990s. In this basaltic aquifer, the organisms at the base of the food chain obtain their
carbon from CO2, and lab studies suggest they “burn” hydrogen gas derived from the reaction of water with the iron-silicate compounds in the rock. They produce methane. Verification that hydrogen from basalt-water reaction can provide enough energy to support an entire community of microbes will have profound implications for the search for extra-terrestrial life. This supports estimates that the biomass up to 4 Km deep in the earth (200 trillion tons) may vastly exceed that which we are familiar with on the surface. To any reader who might like to dig deeper into this subject, I recommend Tales from the Underground by David Wolfe (2001), which is in the Kitsap Library’s collection.