Deep Subsurface Biosphere

By Sara Cox

Outline

• History• Deep Subsurface Biosphere• SLiMEs• Microbial Organisms• TEAPs• Anaerobic Degradation of Benzoate• Sample-taking and Contamination• Future• References

History

• 1960’s and 1970’s: discovery of microbes in geysers at temperatures of 160°F

• 1981: Dr Stetter discovers hyperthermophiles in Icelandic hot springs

• 1989: first routine use of the term deep subsurface biosphere

Deep Subsurface Biosphere

• Usually considered to begin 50m below surface of the Earth and extend to variable depth

• Depth determined by maximum temperature• Oceanic crust heats at a rate of 15°C per km, and

reaches 110°C at about 7 km depth• Continental crust heats at 25°C per km and

reaches 110 °C at 4 km• Deepest samples recovered at 75°C from a depth

of 2.8 km

SLiMEs

• Subsurface lithoautotrophic microbial ecosystems• fluid-filled pores, cracks and interstices of rock

and feed off heat and chemicals, main microbial habitat is in hot aquifers under continents and oceanic abyssesIf 1% of total pore space was occupied, the mass of microbes would be 200 trillion tons, enough to coat land surfaces 5 feet thick

Microbial organisms

• Hyperthermophilic methanogens at temperatures up to 110°C or 6 km deep

• Subsurface microbes may be able to withstand temperatures up to 230°F and possibly briefly to 700 °F, result of pressure

• Most terrestrial microbes die at the boiling point of water

• Bacteria, archaea and eukaryotic microorganisms are all well distributed, with the exceptions of algae and ciliates

• High clay layers have low microbial numbers but sandy layers have elevated numbers

TEAPs

• Terminal electron accepting processes• The most common TEAPs are O2, nitrate,

Mn (IV), Fe (III), sulfate and CO2

• Distribution of TEAPs in deep aquifers occur in this order: oxic, nitrate and Mn(IV) reducing, Fe(III) reducing, sulfate reducing and finally methanogenic

Anaerobic degradation of benzoate

• C6H5COO- + 7H20 3CH3COO- + HCO3- + 3H+ + CH2

• Not thermodynamically favorable unless linked with aceoclastic methanogenesis

• 4C6H5COO+ + 18H20 15 CH4 +13CO2

• Anaerobic degradation of phenol is also linked with acetoclastic methanogenesis

Sample-taking and Contanimation

• Debate: are subsurface microbes actually indigenous or are they merely surface contaminants?

• Lack of photosynthetic organisms in samples

• Specialized drilling and sample-collecting to try to prevent contamination

• Nitrogen or argon gases used in in drilling rather than fluids

• Sterilized drilling fluid or tracers

• Sterile and non-oxidizing containment of samples

• Argon-filled bags enclose all tools and samples kept in boxes of argon or nitrogen

• If non-oxidizing gases are not used in drilling, drilling fluids are often marked with tracers (fluorescent or organically labeled)

• When samples are taken either completely untagged samples are used or the contaminated layers are removed and the “clean” areas are inspected

Future

• Possible life on other planets? T. Gold predicts at least 10 possible deep biospheres in our solar system

• Drilling not feasible, collection of samples from deep layers that are now exposed, ex. Valley Marinara on Mars, once several km deep

• Untapped pool of genetic diversity

• Medical: investigation of microbes for anti-cancer and anti-AIDS drugs

• Bioaugmentation: pollution-eating bacteria for ground water cleanup

• Mary deFlaun (Envirogen) non-adhesive bacteria

• Storage of nuclear waste underground

References

Gold, Thomas. 1999. The Deep Hot Biosphere. Copernicus. New York. Jones, R, Beeman, R. & Suflita, J. 1989. Anaerobic Metabolic Processes

in the Deep Terrestrial Subsurface. Geomicrobiology Journal 7 pg. 117-130.

Fredrickson, J. & Onstott, TC. 1996. Microbes Deep Inside the Earth. Scientific American Oct. 1996.

Lovley, D. Chapelle, F. 1995. Deep Subsurface Microbial Processes. Reviews of Geophysics, 33,3.

Reysenbach, A. & Staley, J. ed. 2002. Biodiversity of Microbial Life. Wiley-Liss, Inc. New York.

Sinclair, J. & Ghiorse, W. Distribution of Aerobic Bacteria, Protozoa, Algae, and Fungi in Deep Subsurface Sediments. Geomicrobiology Journal 7. Pg. 15-31.