Introduction to the Subsurface Biosphere

Only a submarine snow storm could so radically alter the human concept of a biosphere. During 2 decades of exploring sea floor hydrothermal vent systems, evidence has accrued that a microbial population is living within the crust of the planet, sustained by geothermal energy. The most dramatic demonstration of subsurface productivity has been observed directly via submersible: plumes of buoyant fluid rich in flocculant white organic material streams from fissures in the youngest oceanic crust -- like a snow storm on the sea floor -- apparently in response to volcanic activity (and coincident changes in subsurface environmental parameters).

Not only does the existence of such organisms warrant deepening of the concept of a (previously surficial, predominantly solar-energized) biosphere, but the biomass of the subsurface ecosystem may rival that of the surface, motivating scientific reassessment of diverse topics: from global biogeochemical budgets to origin of life hypotheses.

Other evidence of the crustal pervasiveness of life is strong. The recent eruption of MacDonald seamount resulted in a hydrothermal slick on the ocean's surface from which hyperthermophiles were cultured (Huber et al, 1990). A strong correlation between particulate DNA (deoxyribonucleic acid) concentrations and ridge crest vent fluid temperature discerned by Deming and Baross (1993) convinced many scientists that a productive community was indeed living within the crust, rather than only in the benthic vent ecosystem. Laboratory culture of microorganisms isolated from carefully sampled vent environments revealed that many of the organisms grew optimally at unprecedented temperatures and pressures (Pledger et al, 1994).

While Gold (1992) has speculated upon the nature and extent of a "deep, hot biosphere" within the Earth, specification of the how deep and how hot is only beginning. Definition of potential habitat for subsurface microorganisms -- beginning with the degree to which heat flux and permeablity constrain life within the oceanic crust -- represents a first step towards re-quantifying the Earth's biosphere.

To expound on the magnitude and significance of the subsurface biosphere is intellectually invigorating and best accompanied with fervent gesticulation. While indulging sporadically in such exertions, I will earnestly attempt to define the potential subsurface habitat through review and synthesis of lithospheric heat flux models, seismic velocity profiles for the upper oceanic crust, and pressure-temperature tolerance of lab culture hyperthermophiles (and (later! with you!!) fluid flow models, organic synthesis constraints,

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