Previous research on deep marine sediments obtained from the Ocean Drilling Program (ODP) has conclusively demonstrated the presence of a deep bacterial biosphere to at least 750 meters below seafloor (mbsf) (Whelan et al., 1985; Tarafa et al., 1987; Parkes et al., 1990, 1994; Cragg, 1994; Cragg and Parkes, 1994; Cragg and Kemp, 1995; Cragg et al., 1990, 1992, 1995a, 1995b, 1996, 1997, 1998; Wellsbury et al., 1997). Additionally, indirect chemical evidence (e.g., chemical changes in pore water and isotopic evidence) corroborates the evidence that microbial activity is continuous to considerable sediment depths. In the majority of sediments, high temperatures do not impose a limit on bacterial activity because the thermal gradient of the Earth's crust is ~10°-40°C/km and bacteria can grow at temperatures of 113°C (Huber et al., 1989; Pledger and Baross, 1991; Stetter et al., 1990; Blochl et al., 1997). Nevertheless, in areas of high geothermal activity such as mid-ocean ridges and subduction zones, where there may be diffuse or focused hydrothermal fluid flow, temperatures in excess of 350°C can occur. This exceeds the growth temperature of even the most hyperthermophilic bacteria ("hyperthermophile" is defined as bacteria able to grow above 80°C) and hence limits bacterial distributions (Karl, 1985; Karl et al., 1988; Stetter et al., 1990).
Venting chimneys, vent fluids, the surrounding water, and adjacent shallow sediments of hydrothermal systems have been microbiologically investigated and a number of extreme thermophiles and hyperthermophiles isolated (Baross and Deming, 1985; Jannasch and Mottl, 1985; Deming and Baross, 1986; Fiala and Stetter, 1986; Jannasch et al., 1988; Karl et al., 1988; Huber et al., 1989, 1990; Stetter et al., 1990; Straube et al., 1990), including samples from the Juan de Fuca Ridge (Pledger and Baross, 1989, 1991; Reysenbach and Deming, 1991). However, deeper sediment layers have been little studied. One investigation at the Juan de Fuca Ridge (Cragg and Parkes, 1994) showed, with two exceptions, no bacterial cells below ~16 mbsf, or at >76°C, in four holes at Site 858. The two exceptions were in two "hot" holes (Holes 858B and 858D), where a single occurrence of significant bacterial populations (>5 × 106 cells/cm3) was measured in each hole at ~17 mbsf at an estimated in situ temperature of 165°-170°C, well above the current documented upper survival temperature for bacteria. Contamination during drilling was not a likely explanation for the presence of these bacteria, and Leg 169 represented an opportunity to revisit Site 858 and re-examine bacterial distributions in these deep, high-temperature hydrothermal sediments.