Samples for microbiological analysis were obtained from Holes 1109B (APC), 1109C (APC/XCB), and 1109D (RCB). Forty-five samples were obtained for direct microscopic examination aboard ship. Fourteen whole-round samples were taken for shore-based microbiological analysis to measure potential bacterial activities and micro-organisms that can be cultured.
Bacteria were present in all samples examined (Table T14; Fig. F74); their numbers decreased rapidly with increasing depth. In the mudline sample (180-1109B-1H-1, 0-1 cm), total bacterial numbers were 3.28 × 108 cells/cm3. The TOC at 0.13 mbsf was 0.44% (see Table T13). Bacterial populations in surficial sediment at Site 1109 are similar to those at other sites with similar overlying water depths and near-surface organic carbon concentrations (see Table T10 in the "Site 1108" chapter). Total bacterial populations in the deepest sample analyzed, 746 mbsf, were 1.24 × 106 cells/cm3. Although this represents only 0.4% of the near-surface population, it still represents a substantial bacterial community at considerable depth. The population at this depth is similar to that in the deepest sample yet analyzed for the presence of bacteria, 748 mbsf at Site 997 during Leg 164, which contained 1.8 × 106 cells/cm3 (Wellsbury et al., in press).
The depth distribution of total bacterial numbers in sediments from Site 1109 conforms to the general model for bacterial populations in deep-sea sediments (Parkes et al., 1994). This hole is only the second time sediments from deeper than 650 mbsf have been microscopically analyzed for the presence of bacteria, and it confirms that bacteria are ubiquitous in deep-marine sediments.
Evidence for continued microbial activity at depth in Site 1109 sediments is provided by geochemical data. Sulfate concentrations in interstitial water decrease rapidly with increasing depth in the uppermost 100 m of sediment, with complete depletion at 106.9 mbsf (see Fig. F70). In contrast, headspace methane concentrations remain low in the uppermost 100 mbsf (in the range 2-13 ppmv between 10 and 100 mbsf), increasing rapidly to 570 ppmv at 104 mbsf and reaching 4800 ppmv by 116 mbsf (see Table T12).
In addition, subsurface changes in alkalinity and ammonia reflect the continuing action of bacterial processes downhole. The uppermost peak in pore-water ammonia concentrations is associated with a peak in bacterial numbers between 106 and 145 mbsf (Fig. F74) and is clear evidence of organic matter degradation. Deeper in the sediment, between ~430 and 480 mbsf, a secondary alkalinity maximum occurred, reflecting increased CO2 production; concomitant with this was a second peak in pore-water ammonia between ~450 and 580 mbsf. Once again, this is reflected in a broad zone where bacterial numbers are higher than the general model (Fig. F74). Deeper in the sediment column, the in situ temperature increases; previous research has demonstrated that the bioavailability of organic matter may be affected by such temperature increases, resulting in increases in the concentration of important metabolic intermediates such as acetate (Wellsbury et al., 1997). At Site 1109, the temperature gradient was 31ºC·km-1 (see "Temperature Measurements"), which suggests a temperature of 17.5ºC around 480 mbsf. Methane concentrations are high at this depth (4748 ppmv at 479 mbsf). Whether this methane production is derived from acetate (which would account for the increase in CO2) will be investigated further onshore.
It is significant to note that at Site 1109 there is no evidence for lateral fluid flow (see "Temperature Measurements"), and the presence of the dolerite at 773 mbsf physically prevents any fluid flux upward. Furthermore, ratios of C1/C2 indicate that no thermogenic gas production occurs in the sediments at Site 1109 (Table T12; also see "Organic Geochemistry"). This further substantiates the role of bacteria in controlling geochemical parameters in Site 1109 sediments.
As Holes 1109C (XCB) and 1109D (RCB) both drilled through an overlapping depth interval, they provided an opportunity to compare the effects of RCB and XCB coring on microscopic determinations of bacterial populations. Total bacterial numbers in Samples 180-1109C-38X-5, 0-1 cm (352.6 mbsf); 180-1109D-1R-2, 0-1 cm (354.3 mbsf); 180-1109C-40X-4, 0-1 cm (366.7 mbsf); and 180-1109D-3R-2, 0-1 cm (370.2 mbsf), were not significantly different (see Fig. F74).