Researchers have discovered an isolated, self-sustaining, bacterial community living under extreme conditions almost two miles deep beneath the surface in a South African gold mine. It is the first microbial community demonstrated to be exclusively dependent on geologically produced sulfur and hydrogen and one of the few ecosystems found on Earth that does not depend on energy from the Sun in any way. The discovery, which appeared in the October 20 issue of Science, raises the possibility that similar bacteria could live beneath the surface of other worlds, such as Mars or Jupiter's moon Europa.
|About two miles below the ground in a South African gold mine, co-author Duane Moser stands next to the fracture zone (white area) where the one-of-a-kind bacteria were found.
Image Credit: Li-Hung Lin.
"These bacteria are truly unique, in the purest sense of the word," said lead author Li-Hung Lin, now at National Taiwan University, who performed many of the analyses as a doctoral student at Princeton and as a postdoctoral researcher at the Carnegie Institution's Geophysical Laboratory.
As Lin explained: "We know how isolated the bacteria have been because our analyses show that the water they live in is very old and hasn't been diluted by surface water. In addition, we found that the hydrocarbons in the local environment did not come from living organisms, as is usual, and that the source of the hydrogen (H2) needed for their respiration comes from the decomposition of water (H2O) by radioactive decay of uranium, thorium, and potassium."
Analysis of hydrocarbon gas at the site showed that carbon and hydrogen isotopic values were similar to abiotic gas from water-rock interactions at other locations previously studied by the research team. The abiotic gas was not recognized early because it mixes with gas produced by methanogenic bacteria that live in the subsurface habitat.
Humans and most other land-dwelling organisms ultimately get their energy from the Sun, with photosynthetic plants forming the base of the food web. But in dark places where sunlight doesn't reach, life has to depend on other energy sources. Other communities of "chemoautotrophs"-a word chained together from Greek roots meaning "chemical self-nourishment"-have been found in exotic places such as aquifers, petroleum reservoirs, and vents linked to deep-sea volcanoes. Yet these communities all depend at least in part on nutrients that can be traced back to photosynthetic plants or bacteria.
|The diagram shows where the photograph was taken with respect to the discovery.
Image Credit: Duane Moser
The international team led by T. C. Onstott of Princeton University, which also includes Carnegie staff scientist Douglas Rumble and former Carnegie postdoctoral researcher Pei-Ling Wang, also now at National Taiwan University, found the community in a rock fracture that intersects the Mponeng gold mine near Johannesburg, South Africa. Water trapped in the fracture is home to the otherworldly bacteria.
Using genetic tools, the team discovered that there is very little species diversity in the rock fracture community. Compared with bacteria in the water used for mining, the fracture water is dominated by one type of bacteria related to Desulfotomaculum, which is known to get energy from the reduction of sulfur compounds.
"We also believe that the sulfate used by these creatures is left-over from ancient groundwater mixed with ancient hydrothermal fluid. We can detect that because the chemical signature arises from interacting with the fracture's wall rock," commented Rumble. "It is possible that communities like this can sustain themselves indefinitely, given enough input from geological processes. Time will tell how many more we might find in Earth's crust, but it is especially exciting to ponder whether they exist elsewhere in the solar system."
One of the major problems researchers in subsurface biology face is access to the subsurface environment itself. Todd Steven, a geobiology researcher in Mosier, Oregon, has faced the same difficulty in his previous work with the Mars Analog Drilling Project in the Rio Tinto region of Spain.
"We now have good reason to think that very interesting things are going on in fractured rock environments all over the planet," Steven noted. "Unfortunately, one problem with all of these studies is that nobody has mustered the technology to observe these fractures without grossly disturbing or contaminating them. Everyone has to extrapolate back to what the environment must have looked like before we contaminated it with drilling fluid, or oxygen, or surface microbes, or started extracting water from the system at thousands of times the normal rate."
In spite of the difficulties, subsurface research can yield clues about the history of life on our planet.
The findings in South Africa may have important implications for numerous scientists working on life in extreme environments and the search for life on other worlds. "The research provides interesting data for astrobiologists working to expand our knowledge of how life survives in unique environments," said Steven. "The results coming out of the South Africa mines project are certainly exciting. The more we examine Earth's subsurface, it seems, the more energy fluxes we find that microbes can use. Like many of the other ecosystems that have been examined, however, this work points out large discrepancies between the geochemical rate estimates and what laboratory studies have taught us about microbial metabolism. There is certainly a lot more to be learned about how these systems work."
When looking at the surface environments of planets in our solar system, only Earth is able to support life as we know it. However, scientists cannot yet see into the interiors of planets like Mars, and this new research indicates that there might be a chance for life to survive deep below a planet's surface, independent from energy provided by the Sun.
"This work advances our understanding of how life can survive in the deep subsurface over geological time scales and provides important further evidence for ecosystems completely independent of the surface photosynthetic biosphere," commented Charles Cockell of the Planetary and Space Sciences Research Institute at the Open University in the United Kingdom. "It gives further optimism for the idea of deep subsurface life on other planets."