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 << 0.002) with total organic carbon. Near-surface concentrations of organic carbon rapidly decreased from approximately 3.6% to 2.0% at 13 mbsf, and thereafter remained at 1.5%–2.0% to the base of the core at 68.28 mbsf, indicating a high level of recalcitrance. The changes in the rate of bacterial population decrease with depth may be a response to increasingly recalcitrant organic carbon. High levels of methane (4100 µmol/L) were found at 9.0 mbsf, although the maximum concentration present in the sediment at this site may be in an unsampled horizon above this depth. At greater depth, methane concentrations were still high (>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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