The presence of a deep bacterial biosphere in marine sediments has been established by extensive research on sediments primarily obtained from the Ocean Drilling Program (ODP) (Parkes et al., 2000). In addition, bacteria have been shown to be present in basaltic basement rocks (Furnes et al., 1996; Giovannoni et al., 1996; McKinley and Stevens, 2000) and Cretaceous shales and sandstones (Krumholz et al., 1997). Bacteria appear to be ubiquitous in marine sediments, and depth profiles of bacteria are remarkably consistent across different oceans. Population sizes range from ~109 cells/cm3 at the near surface, decreasing exponentially to ~106 cells/cm3 at the mean ocean sediment depth of 500 meters below seafloor (mbsf). Near the sediment surface, bacteria act as a filter on the burial of sedimented organic material and in the subsurface they continue degradation, recycling, and selective preservation of organic matter, albeit at much lower rates. These subseafloor bacterial populations have been shown to be stimulated by variations in their in situ environment (i.e., influxes of electron acceptors—a brine intrusion replenishing sulfate at the Peru margin [Cragg et al., 1992b]; the presence of high concentrations of organic carbon—sapropels containing up to 30 wt% organic carbon in the Mediterranean [Cragg et al., 1998]; the presence of free methane gas around hydrates—below the bottom-simulating reflector at Cascadia margin [Cragg et al., 1996] and the Blake Ridge [Wellsbury et al., 2000]; and the presence of thermogenic methane in Japan Sea sediments [Cragg et al., 1992a]). Conversely, where organic carbon concentrations are low or electron acceptors are depleted, bacterial populations tend to be considerably reduced (e.g., the Lau Basin [Cragg, 1994]; the eastern equatorial Pacific [Cragg and Kemp, 1995]; the sedimented flanks of the Juan de Fuca Ridge [Mather and Parkes, 2000], and in the relatively deepwater low-organic carbon sediments of the Woodlark Basin [Wellsbury et al., 2002]).
ODP Site 1149 in the western equatorial Pacific is a deepwater area (5818 m). The bulk of the material deposited as sediments onto the seafloor has consisted of carbonate-free clays with mixtures of volcanic ash accounting for 35%-50% of the sediment to 118 mbsf. Below this, dark brown pelagic clay, devoid of calcareous and siliceous microfossils, dominates to 179.1 mbsf (Shipboard Scientific Party, 2000). This site represents the greatest water depth sampled to date for microbiological studies. At these depths, only very low amounts of bioavailable organic carbon may reach the sediments, which could limit the depth distribution of sediment bacterial populations. Core samples were obtained from this site in order to determine the presence, concentration, and depth distribution of sediment bacteria in such an extreme and challenging environment.