Bacterial depth profiles were obtained from high-organic-load sediments (water depth 538 m) to a depth of 68.28 m below seafloor. Using the Acridine Orange Direct Count (AODC) technique, near-surface bacterial populations were 1.27 × 109 cells/cm3. Numbers of bacteria decreased rapidly to 4.77 × 106 cells/cm3 at 12 mbsf and then more slowly to 2.51 × 106 cells/cm3 at 68.28 mbsf (505-fold decrease). Dividing cells represented approximately 10% of the total count but decreased at a greater rate (850-fold). There was an abrupt change in the rate of total bacterial population decrease at approximately 13 mbsf. Bacterial numbers were strongly correlated (P 1000 µmol/L), and although this area has many seeps of oil and gas, the C1/C2+ ratios indicate a biogenic rather than a thermogenic source.This work represents the first detailed microbiological analysis of deep sediment layers from the Santa Barbara Basin.
Date of initial receipt: 23 August 1994
Date of acceptance: 11 January 1995
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