Eighteen samples for microbiological analysis were obtained from Hole 1176A for direct microscopic enumeration aboard ship. Eleven whole-round cores were taken for shipboard enrichment cultures, cell viability, and shore-based microbiological analysis to measure potential bacterial activities, culture microorganisms, characterize nucleic acids, and investigate fatty acid biomarkers.
Bacteria are present in all 18 samples (Table T15; Fig. F19). The surface sample (190-1176A-1H-1, 0-1 cm) contains 6.67 × 108 cells/cm3, which follows a general trend observed at other Ocean Drilling Program (ODP) sites where near-surface bacterial populations decrease as overlying water depths increase (Table T16).
The deepest sample is at 363.49 mbsf (Sample 190-1176A-40X-1, 49-50 cm) with 1.71 × 106 cells/cm3, representing 0.25% of the surface population. Bacterial populations decline rapidly from the surface, consistent with the decrease in sulfate concentrations to near zero at 14.6 mbsf (see "Inorganic Geochemistry"). Below 14.6 mbsf, bacterial populations increase with increases in methane concentrations (see "Organic Geochemistry").
The depth distribution of total bacterial numbers in sediments from Site 1176 conforms to the general model for bacterial populations in deep-sea sediments (Parkes et al., 1994) from the surface to 363 mbsf (Fig. F19). Although deviations from the expected profile are greater than those observed at Site 1175 (see "Microbiology" in the "Site 1175" chapter), they are nonetheless small, consistently remaining within the given 95% prediction limits, and there is no apparent correlation with any geochemical component of the sediment. The single datum with the greatest deviation is at 51 mbsf (Sample 190-1176A-6H-4, 114-115 cm), and interestingly, this deviation is coincident with the lowest organic carbon concentration measured at this site (0.05 wt%) (see "Organic Geochemistry").