Eight samples for direct microscopic examination were obtained from the lower portion of Hole 1118A, the lowest of which represent the deepest samples obtained to date from deep-sea sediments for enumeration of in situ bacterial populations.
Bacteria were present in all samples examined from 505 to 842 mbsf (Table T12; Fig. F63). The size of the bacterial population at Site 1118 lies within the 95% prediction limits of Parkes' general model for bacterial depth distributions in deep-sea sediments (Parkes et al., 1994; Fig. F63). Bacterial numbers appear to decrease more rapidly than predicted in the indurated material below ~750 mbsf, although porosity remains fairly high (40%-50%). However, significant numbers of bacteria were present in the deepest sample analyzed (842 mbsf) at 3.2 × 105 cells/cm3. Dividing and divided cells follow a similar trend to total bacterial numbers and were present in all samples.
Microbial activity is responsible for many geochemical changes downhole. Depletion of IW sulfate by ~240 mbsf (Fig. F59A) and the subsequent rapid increase in methane concentrations (Fig. F61) are caused by the competitive advantage of sulfate-reducing bacteria over methanogenic bacteria in obtaining organic substrates. Conversely, below ~700 mbsf, the situation is reversed: an increase in pore-water sulfate concentration is accompanied by a rapid decrease in methane concentration (Fig. F61). Downhole ammonia profiles (Fig. F59A) also demonstrate that bacterial degradation of organic matter is continuing deep within the sediments. C1/C2 ratios remain high (>1000) throughout the hole, which is indicative of a biogenic source (Fig. F61).
Site 1118 data reinforce the findings from Site 1115 by demonstrating the presence of bacterial populations in indurated sedimentary strata at depths >800 mbsf. Below 842 mbsf, the degree of induration of the rocks is so great that the microbiological sampling methods (hammering a sterile plastic syringe into the core) were ineffective. Bacteria are present in all samples down to this point. Thus, although we have defined a limit for current sampling techniques, the limit of the biosphere in deep sediments remains undefined.