Ocean 540 Project Proposal

Geologic Constraints on Subsurface Microorganisms

While Gold (1992) has speculated upon the nature and extent of a "deep, hot biosphere" within the Earth, and Deming and Baross (1993) have proffered upper temperature and pressure limits to the growth of hyperthermophilic microorganisms collected from deep-sea hydrothermal vent systems, the degree to which heat flux and geochemical processes constrain life within the oceanic crust has yet to be thoroughly examined.

I propose to characterize the habitat of hyperthermophilic organisms through both theory and observation. The pressure-temperature space conducive to hyperthermophile growth will be established by comparing the subsurface temperature field (established through critical synthesis of modelled and measured conductive and convective heat flux in the near-axis seafloor) with the most recent observations of hyperthermophile physiology (lab culture pressure and temperature limits). In particular, I intend to compare the heat flux models examined in class with others proposed in the literature, to further develop the implications for biology of Lister's cracking front hypothesis, and to consider what inferences may be drawn for the subsurface from our current understanding and observations of diffuse and focussed hydrothermal venting, event plume formation, and episodic volcanic activity.

Further insight into the potential habitat, life cycles, and geologic interactions of hyperthermophiles may be derived from a survey of known geochemical evolution in near-axis oceanic crust, seafloor observations of flocculant microorganisms and associated organic compounds, and speculations on the source of growth-limiting nutrients in the subsurface. I will glean the requisite information through personal communication (Baross, Lilley, Delaney, Butterfield, Kelley, McDuff) and further immersion in the published literature.

I expect to gather evidence supporting a hypothesis that biogeochemical processes regulate heat flux within the subsurface, and will examine the role of such processes in the context of global cycles. It is already apparent that there are confounding interactions in which biological, chemical, and geological processes compete for space, energy, and mass; the prospect of disentangling the roles of both mineral precipitation and biological colonization within the flow fields of the hydrothermal subsurface is simultaneously tantalizing and bewildering.

It is my intention to drive my increasingly-honed intellectual marlinspike into such interdisciplinary knots, ultimately working free another line in the conceptual rigging of the ridge crest research vessel. At the least, by juxtaposing geophysical estimates of the spatial extent of the global ridge crest system and biological estimates of hyperthermophile metabolism and morphology, I will enjoy inscribing the back of a re-used envelope with a new estimate of the oceanic lithosphere's contribution to global subsurface biomass. Furthermore, by adopting a paleoecological perspective on the coevolution of life and lithosphere, I will indulge by briefly discussing the implications of a subsurface biosphere for the Gaia hypothesis.