There are several lines of evidence leading us to this idea:
- Groundwaters from the basalt formation contain 10,000 to 1,000,000 microbial cells per milliliter, yet basalt contains little or no organic carbon to sustain such a population.
- The groundwaters also contain products of anaerobic bacterial respiration, including methane, and sulfide, which suggests that the bacteria have been active in the aquifers for a long time.
- On a regional scale, as groundwaters get older, they contain more methane, and less carbon dioxide, suggesting that microorganisms are making methane from the carbon dioxide. If fermentation of organic matter was the source of carbon and energy, carbon dioxide would increase, instead of decreasing.
- Naturally occuring carbon consists of two stable isotopes: Carbon-12 (98.89%), and Carbon-13 (1.11%). Different biological processes have characteristic preferences for carbon-12 over carbon-13, and change the ratio of these isotopes in biological products. The ratio of C-12 to C-13 in the CRB can be predicted exactly (in our current data set) by using the fractionation factor of methanogenic lithoautotrophic microorganisms.
- The CRB also contains high concentrations of dissolved hydrogen gas, which is an excellent energy source for many kinds of microorganisms, including methanogens. In the laboratory, we can demonstrate that interactions between water and basalt produce hydrogen gas.
- In the laboratory, we can grow microorganisms, using basalt as the
sole source of energy and carbon dioxide as the sole source of carbon.
The Subsurface Science Program has recently been replaced by the Natural and Accelerated Bioremediation Research Program
Our current research related to the SLME hypothesis is sponsored by
NASA's Office
of Space Science program in Exobiology. The goal of NASA's Exobiology
Program is to understand the origin, evolution, and distribution of life
in the universe. Research is focused on achieving this goal by tracing
the pathways taken by the biogenic elements, leading from the origin of
the universe through the major epochs in the evolution of living systems
and their precursors. These epochs are: 1) The cosmic evolution of
the biogenic compounds ; 2) prebiotic evolution; 3) the early evolution
of life; and 4) the evolution of advanced life. The SLME research
contributes to the third category by providing a potential model for primitive
ecosystems on the early Earth, and potential extraterrestrial ecosystems.
We have NOT discovered a �new type� of microorganism in the CRB. (Not yet, anyway.) The types of bacteria we�ve found in SLiMEs are quite familiar to microbial ecologists. They are typically found in anaerobic environments such as aquatic and marine sediments, water-logged soils, and intestinal tracts of animals. What is new about SLiMEs is the ecological relationship between anaerobic bacteria and igneous rocks. In �ordinary� anaerobic sediments the bacteria grow on decaying organic matter, or on hydrogen gas that is produced by fermentation of organic matter. In SLiMEs, bacteria grow on hydrogen that is produced from inorganic reactions between rocks and water.
The kinds of bacteria we�ve found in SLiMEs include methanogenic archaea (produce methane from hydrogen and carbon dioxide), sulfate reducing bacteria (produce hydrogen sulfide from hydrogen and sulfate), acetogenic bacteria (produce acetate (vinegar) from hydrogen and carbon dioxide), dissimilatory iron reducing bacteria (produce ferrous iron from ferric iron (rust) and hydrogen), and fermentative bacteria (live on the by-products of other bacteria). We do not know the species names of any of these organisms as yet. It takes a great deal of biochemical and genetic work to identify a bacterial species, and many different ones are found in SLiMEs.
In fact, finding a new bacterial species is not very noteworthy in itself. As near as microbiologists can figure, we only know about one-tenth of one percent of all bacterial species that are found in nature. One can find a new bacterial species in virtually any handful of soil. Microbial ecology is one of the frontiers of science, with much yet to be discovered. We expect that as the CRB communities are examined more closely, new species will be described.
Good sources of information about microorganisms include the American
Society for Microbiology
In SLiMEs, bacteria appear to live on chemical energy contained in basalt rocks. This material erupted from the Earth�s mantle, which is chemically more reduced than material at the Earth�s surface. We believe that it is this difference in reduction potential that provides potential energy that microorganisms can exploit. We now know that under certain environmental conditions, hydrogen gas is produced in a chemical reaction between water and ferrous silicate minerals found in the basalt.
Hydrogen has been observed in gases exsolved from groundwater at a few
localities around the world. All of these occurences are associated
with buried bodies of igneous rock that contain an abundance of ferrous
silicates. This is consistent with our laboratory observations that ferrous
silicates can react with water to produce hydrogen, under the right conditions.
These other studies did not look for microorganisms, but we think that
it is likely that they were present. The widespread distribution
of igneous rocks containing ferrous silicates may indicate also a broader
occurrence of SLiMEs.
Many anaerobic microorganisms perform �reductive dehalogenation,� of
chlorinated solvents, while using hydrogen as an energy source. In
this process, chlorine atoms are removed from chemicals like carbon tetrachloride
and trichloroethylene. Some microorganisms actually gain energy from
reductive dechlorination, coupled to hydrogen oxidation. Other organisms
can immobilize heavy metals (such as uranium) by a similar reductive reaction,
from which they gain energy. The metals are converted from a soluble
form, which travels with groundwater, to an insoluble form, which becomes
trapped in the rock, thus purifying the water.
The reference to life on Mars is based on the fact that the basic components that appear to drive SLiMEs in the Columbia River Basalts are also thought to exist underground on Mars. These include abundant basalt, liquid water, and inorganic carbon. Conceivably, SLiMEs could live in the subsurface of Mars, if any were there. No other living things on Earth, as far as we know, could survive on Mars today. This may be a timely observation because the next wave of Martian exploration is now being planned. The possibility of subsurface life on Mars was discussed by P.J. Boston, M.V. Ivanov, and C.P. McKay in Icarus (1992) 95:300-308.
A recent, highly controvorsial report by D.S. McKay, et al., suggests that one of the martian meteorites, ALH84001, contains evidence for ancient life on Mars (Science (1996) 273:924-930.) Although this hypothesis remains highly speculative, the physical setting of the ALH84001 sample is thought, by some, to be very much like the microbial habitat in the CRB; a subsurface fracture in basalt, through which water once flowed.
Many scientists think that a layer of liquid water, or even a subsurface ocean, could exist below the surface ice layers of Europa. Europa is thought to have a rocky core, containing ferrous minerals much like those in basalt. If sufficient carbon is available, a SLiME like ecosystem could be conceivable in the subsurface of Europa.
Additional information can be found through the "Astrobiology
Web"
