INTRODUCTION
The Ocean Drilling Program (ODP) is uniquely positioned to sample one of the least known and potentially strangest ecosystems on Earth—the microbial biosphere of deep marine sediments and the oceanic crust. The growing international interest in the study of this subsurface biosphere is driven by a variety of factors, including the role of subsurface microbial activity in Earth's biogeochemical cycles, the possibility of life on other planets, and sheer fascination with the nature of life on the margin of existence.Nearly twenty years ago, Deep Sea Drilling Project (DSDP) experiments with methane concentrations and radiotracer uptake first demonstrated active microbial processes in cores recovered from deeply buried marine sediments (Oremland et al., 1982; Whelan et al., 1986; Tarafa et al., 1987). Over the last fifteen years, studies of ODP cores have extended our understanding of those processes (e.g., Cragg et al., 1992; Getliff et al., 1992) and consistently identified abundant microbes in deeply buried oceanic sediments (e.g., Cragg et al., 1990, 1992; Thierstein and Störrlein, 1991; Parkes et al., 1994, 2000). Microbes have been recovered from burial depths as great as 800 meters below the seafloor (mbsf) (Shipboard Scientific Party, 1999). Recent contamination tracer experiments suggest that most of the microbes reported from ODP cores are inherent to the drilled sediments (Smith et al., 2000).
The number and mass of prokaryotes in the subsurface biosphere of oceanic sediments has been estimated by extrapolation from direct counts of sedimentary microbes at a small number of ODP sites. Based on that extrapolation, the marine subsurface biosphere may compose as much as one tenth (Parkes et al., 2000) or even one-third (Whitman et al., 1998) of the world's living biomass. In situ metabolic activity by at least a portion of this biosphere is spectacularly demonstrated by hydrates of methane produced by microbes in deep-sea sediments. On a global scale, these hydrates contain 7,500 to 15,000 Gigatons (Gt) of carbon—four to eight times as much carbon as in living surface biosphere and soils combined (Kvenvolden, 1993). Pore water chemical studies (Borowski et al., 1996) and recent microbiological discoveries (Hinrichs et al., 1999; Boetius et al., 2000) suggest that, on an ongoing basis, the CH4 produced in deep-sea sediments is primarily destroyed by the sulfate-reducing activity of microbes in overlying sediments.
Despite these recent advances, very little is known about the nature and activity of life in deep marine sediments. In particular, we know almost nothing about (1) the continuity of subsurface life from one oceanographic region to another; (2) the specialized metabolic strategies, if any, that are required to survive in deeply buried marine sediments; or (3) the conditions under which subsurface microbes are active or inactive and living or dead.