| Topic: |
Science > Philosophy |
| User: |
"Sir Frederick" |
| Date: |
16 Nov 2006 04:12:49 PM |
| Object: |
On the Ecosystem Deep Beneath the Seafloor |
ECOLOGY: ON THE ECOSYSTEM DEEP BENEATH THE SEAFLOOR
ScienceWeek http://scienceweek.com
The following points are made by B.B. Jørgensen and S. D'Hondt (Science 2006 314:932):
1) Over the past 20 years, scientific drilling into sediments and basaltic crust all over the world ocean has revealed the
omnipresence of microscopic life deep beneath the seafloor. Diverse communities of prokaryotic cells have been discovered in
sediments and rock reaching a subsurface depth of 1 km. Most of these microorganisms have no cultured or known relatives in the
surface world and are still only characterized by the genetic code of their DNA. Recent studies (1-4) have shed light on the ways in
which they differ from microorganisms in the surface world and on the energy sources that support life in this buried ecosystem.
2) About 20 years ago, R. John Parkes and Barry Cragg started to systematically enumerate microorganisms in deep cores (5). Much
later, rigorous contamination tests performed on the drill ship showed that the cells detected were indeed indigenous to the deep
subsurface. The cell counts were used for a bold extrapolation to the global ocean floor. The astonishing conclusion was that this
"unseen majority" of microorganisms accounts for 55 to 85% of Earth's prokaryotic biomass and about 30% of the total living biomass.
3) The first drilling expedition focused entirely on deep biosphere exploration was launched in 2002 by the Ocean Drilling Program
(ODP, Leg 201) (1). The target was the eastern tropical Pacific, with sites ranging from the continental shelf to ocean depths of
5000 m. By drilling through the seafloor down to the basaltic crust, sediments with ages up to 35 million years old could be
sampled. At all sites, prokaryotic cells (bacteria and archaea) were detected below the seafloor. Their numbers dropped from more
than 10^(8) cm^(-3) at the sediment surface to less than 10^(6) cm^(-3) just above the ocean crust, with an average density much
greater than in the ocean above. Occasional high cell numbers [up to 10^(10) cm^(-3)] coincided with sediment horizons in which more
energy was available from counterdiffusing methane and sulfate.
4) These large population sizes remain the greatest mystery of the deep biosphere. Although marine sediments harbor Earth's largest
reactive carbon pool, the organic matter becomes increasingly unreactive with depth and age and would seem to be practically
inaccessible for microorganisms several million years after its burial. How, then, can there be sufficient energy for all these
organisms to metabolize and grow? The metabolic activity of the subsurface populations can be calculated by transport-reaction
modeling of pore water solutes that are consumed or excreted by the microorganisms. Because microbial cells must metabolize a
certain minimum amount of substrate before they can double their cell size and divide into two daughter cells, their minimum
doubling time can also be calculated. On the basis of this calculation, the mean generation time of deep subseafloor microorganisms
is more than 1000 years. This extremely slow growth cannot be reconciled with our understanding of the minimum energy requirements
for life. All actively growing organisms must keep their enzymatic machinery going above a critical level to maintain vital cell
functions such as replacement of degraded enzymes, repair of DNA damaged by high-energy radiation from natural radionuclides, and,
presumably, the maintenance of an electrochemical gradient across the cell membrane. A possible explanation for the low apparent
rates of deep subsurface metabolism could be that most subseafloor cells are not active but dormant or even dead. However, when a
highly sensitive fluorescence technique (catalyzed reporter deposition-fluorescence in situ hybridization or CARDFISH) was used to
detect the presence of ribosome -- a component of all living and active cells that is rapidly degraded upon their death -- the
results showed that many of the subsurface cells were alive.
References (abridged):
1. S. D'Hondt et al., Science 306, [2216] (2004).
2. F. Inagaki et al., Proc. Natl. Acad. Sci. U.S.A. 103, 2815 (2006).
3. J. F. Biddle et al., Proc. Natl. Acad. Sci. U.S.A. 103, 3846 (2006).
4. J. P. Amend, A. Teske, Palaeogeogr. Palaeoclimatol. Palaeoecol. 219, 131 (2005).
5. R. J. Parkes, B. A. Cragg, P. Wellsbury, Hydrogeol. J.. 8, 11 (2000).
Science http://www.sciencemag.org
ScienceWeek http://scienceweek.com
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