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Bacterial depth profiles were obtained from sediment samples taken from a deep-water (3761 m) site in the eastern equatorial Pacific to a depth of 317.4 m below seafloor (mbsf) using the Acridine Orange direct count (AODC) technique. Bacterial cells were observed at all depths and dividing/divided cells were observed in 47 of 58 samples. Numbers were consistent with a low organic carbon environment. Total counts decreased from near-surface levels of 2.08 × I0 to 4.88 × I0 cells/cm at 12 mbsf before increasing to a local maximum of 9.33 × I0 cells/cm at 91 mbsf. The maximum number of dividing/divided cells (4.85 × I0 cells/cm) were detected at the near-surface. They decreased to "not detectable" by 4.5 mbsf and did not display a sustained increase in numbers until approximately 30 mbsf, with a peak at 2.62 × I0 cells/cm at 91 mbsf. The increase in counts below 30 mbsf was attributed to a change in the depositional history resulting in elevated buried organic carbon concentrations between 30 and 290 mbsf. Dividing/divided cell counts may provide an index of the bioavailability of this organic carbon. Inorganic and organic chemical profiles correlated significantly with bacterial counts and indicated a zone of limited bacterial activity (sulfate reduction and methanogenesis) between 30 and 290 mbsf. The reasons for the existence of a band of low total counts and almost negligible dividing/divided cell counts between 1.5 and 25-30 mbsf is not understood, as the chemical profiles suggest that bacteria should be both present and active. When compared with equivalent bacterial counts taken on sediment samples from the Peru Margin and Japan Sea (areas of higher oceanic productivity) and Lau Basin (lower oceanic productivity), near-surface total counts were lower than those in the Lau Basin. This is attributed to considerably lower concentrations of organic carbon (0.06% compared with 0.27%) in the near-surface sediments at this site owing to greater water depth. The rate decrease in both the total and dividing/divided cell counts over the top 80 mbsf was less than that observed in the Lau Basin (but greater than in the Peru Margin or Japan Sea); however, this observation is probably related to the depositional history of the site, rather than the overlying oceanic productivity.

Bacterial Profiles in Deep Sediment Layers from the Eastern Equatorial Pacific Ocean, Site 851