Samples for microbiological analysis were obtained from Holes 1115A (APC), 1115B (APC/XCB), and 1115C (RCB). Forty-four samples were prepared for direct microscopic examination aboard ship. Twelve whole-round samples were taken for further shore-based analysis to measure potential bacterial activities and culturable microorganisms. Samples obtained from the bottom of Hole 1115C represent the deepest samples obtained to date from deep-sea sediments for both microscopy (Section 180-1115C-54R-7, 801 mbsf) and potential activity/culturability experiments (Section 180-1115C-54R-4, 798 mbsf). (Note that deeper samples were subsequently recovered at Site 1118.)
Bacteria were present in all samples examined (Table T10; Fig. F48); their numbers decreased rapidly with increasing depth. Total bacterial numbers were 2.83 × 108 cells/cm3 in the mudline sample (Sample 180-1115A-1H-1, 0-1 cm). Total organic carbon in the uppermost sample obtained from Hole 1115A (2.82 mbsf) was 0.05% (see Table T9). Thus the numbers of bacteria present in surficial sediment at Site 1115 are nearly the same as those at other sites with similar overlying water depths and near-surface organic carbon concentrations (see Table T10 in the "Site 1108" chapter). Dividing and divided cells follow a similar trend to total bacterial numbers, although no cell involved in division was observed in the deepest sample, 801 mbsf.
Total bacterial numbers in sediments from Site 1115 conform to the general model for bacterial depth distributions in deep-sea sediments (Parkes et al., 1994). Bacteria were present in the deepest sample analyzed, 801 mbsf, at 3.51 × 105 cells/cm3. Although this value is three orders of magnitude lower than near-surface bacterial populations in deep-sea sediments, it still represents a substantial bacterial community at considerable depth. In spite of the absence of dividing or recently divided cells, the continued presence of a bacterial community over 800 mbsf, into indurated sandstones, is of fundamental significance. The persistence of microbial life in indurated sedimentary rock (with a biostratigraphic age of ~15 Ma; see "Biostratigraphy") adds to a steadily growing body of evidence for an extensive biosphere in deep-sea sediments (e.g., Parkes et al., 1994) and even into basalts (Furnes et al., 1996; Giovannoni et al., 1996). The current limit of the deep subseafloor biosphere remains unknown.
The activity of the deep subsurface microbial populations is also evident in geochemical data from Site 1115. Pore-water sulfate is fully depleted by ~200 mbsf (see "Inorganic Geochemistry"). In contrast, headspace methane concentrations remain low in the uppermost ~200 mbsf (in the range 2-4 ppmv between 0 and 200 mbsf), increase rapidly to ~1200 ppmv by 246 mbsf, and remain high downcore, although there was a distinct minimum at ~515 mbsf, which may indicate two separate production zones. The C1/C2 ratios remain high (>1000) throughout the hole, indicative of a biogenic source (see Fig. F46). A similar profile is also evident in pore-water ammonia concentrations, with a distinct minimum at ~520-580 mbsf, which again reinforces the notion of continuing action of bacterial processes downhole. In situ temperature increases with depth, which may result in greater organic matter bioavailability. This may in turn be reflected in increases in the concentration of important metabolic intermediates such as acetate (Wellsbury et al., 1997). The temperature gradient at Site 1115 is ~28°C·km-1 below a seafloor at ~4°C (see "In Situ Temperature Measurements"), which suggests a temperature of ~22°-25°C around 650-750 mbsf. Methane concentrations are high at this depth (e.g., ~62,000 ppmv at 716 mbsf). Whether deep methane production at Site 1115 can be attributed to acetoclastic methanogenesis will be investigated further.