Kennett, J.P., Baldauf, J.G., and Lyle, M. (Eds.), 1995 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 146 (Pt. 2) 10. BACTERIAL PROFILES IN DEEP SEDIMENTS OF THE SANTA BARBARA BASIN, SITE 893 1 B.A. Cragg, 2 R.J. Parkes, 2 J.C. Fry, 3 A.J. Weightman, 3 J.R. Maxwell, 4 M. Kastner, 5 M. Hovland, 6 M.J. Whiticar, 7 J.C. Sample, 8 and R. Stein 9 ABSTRACT 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 × 10 9 cells/ cm 3 . Numbers of bacteria decreased rapidly to 4.77 × 10 6 cells/cm 3 at 12 mbsf and then more slowly to 2.51 × 10 6 cells/cm 3 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 rap- idly 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 (>IOOO µmol/L), and although this area has many seeps of oil and gas, the C|/C 2+ ratios indicate a biogenic rather than a fhermogenic source.This work represents the first detailed microbiological analysis of deep sediment layers from the Santa Barbara Basin. INTRODUCTION Bacteria play a central role in the degradation and selective pres- ervation of organic matter in marine sediments and are thus intimate- ly involved in biogeochemical cycling of elements (J0rgensen, 1983). Their presence and activity in surface sediments has long been accepted (Novitsky and Karl, 1986; J0rgensen et al., 1990). At the in- terface between the water column and the sediment surface, detrital microaggregates will accumulate in an oxic, or suboxic, environ- ment, producing a favorable habitat for bacterial growth (Lochte and Turley, 1988; Turley and Lochte, 1990). Geochemical evidence (chemical changes in pore water, kerogen production, concretion for- mation, etc.) has suggested that bacterial populations remain active at considerable depths (Krumbein, 1983); some early evidence of the existence of such populations, based only on enrichment of viable bacteria exists (Rittenberg, 1940; ZoBell, 1958; Davis, 1967). More recently, with improved techniques, there have been a number of re- ports on the cultivation of bacteria from 200 meters below seafloor (mbsf) (Oremland et al., 1982; Belyaev and Ivanov, 1983; Bianchi, 1986), and work on deep aquifers has shown the presence of bacterial populations in sediments over 1000 m deep (White et al., 1983; Balk- will, 1989; Erlich and Ghiorse, 1989; Phelps et al., 1989; Chapelle and Lovley, 1990; Fliermans et al., 1993). 'Kennett, J.P., Baldauf, J.G., and Lyle, M. (Eds.), 1995. Proc. ODP, Sci. Results, 146 (Pt. 2): College Station, TX (Ocean Drilling Program). department of Geology, University of Bristol, Bristol BS8 1RJ, United Kingdom. 'School of Pure and Applied Biology, University of Wales College of Cardiff, P.O. Box 915, Cardiff CF1 3TL, United Kingdom. -• Departmen t of Chemistry, University of Bristol, Bristol BS8 1RJ, United King- dom. 'Scripps Institute of Oceanography, Geological Research Division A-012, La Jolla, CA 92093, U.S.A. 6 Statoil, P.O. Box 300, N-4001 Stavanger, Norway. 7 SEOS, University of Victoria, P.O. Box 1700, Victoria, B.C. V8N 1YU, Canada. "Department of Geological Sciences, California State University, 1250 Bellflower Blvd., Long Beach, CA 90840, U.S.A. l) Alfred-Wegener Institut für Polar und Meeresforschung, Columbusstrasse, D-2850 Bremerhaven, Federal Republic of Germany. In the marine environment, evidence for low levels of anaerobic bacterial methanogenesis and sulfate reduction has been reported in sediments to 167 mbsf in the Gulf of Mexico and the North Atlantic (Whelan et al., 1986; Tarafa et al., 1987). In a comprehensive study of high-organic-load sediments from the Peru Margin, Cragg et al. (1990) and Parkes et al. (1990) have described significant levels of bacterial sulfate reduction and methanogenic activity to 80 mbsf, and significant bacterial populations associated with low levels of activi- ty have been reported to 500 mbsf in the Japan Sea (Cragg et al., 1992; Getliff et al., 1992; Parkes et al., 1994). Most recently, exami- nation of sediments from sites of considerably lower oceanic produc- tivity and, consequently, much lower sediment organic carbon concentrations has demonstrated the continued presence of signifi- cant numbers of bacteria, associated with low levels of bacterial ac- tivity, in sediments to 100 mbsf in the Lau Basin (Cragg, 1994) and to 310 mbsf in the Eastern Equatorial Pacific (Cragg and Kemp, in press). The Santa Barbara Basin (SBB) sediments have long been recog- nized as providing a detailed index of recent environmental climatic history (Soutar and Grill, 1977). The surficial sedimentation rate is approximately 4 mm/yr (Schimmelmann et al., 1990), with sediment containing total organic carbon concentrations of 2-8 wt% laid down and preserved in the form of millimeter-scale laminae thought to be related to an annual cycle of oxygen replenishment and depletion in bottom waters (Reimers et al., 1990; Schimmelmann and Tegner, 1991). Bacterial degradation of organic matter settling out from the highly productive overlying surface waters reduces the oxygen con- tent of the deepest waters, resulting in dysaerobic to anaerobic condi- tions with less than 0.1 mL/L dissolved oxygen (Kennedy and Brassell, 1992). This lack of oxygen inhibits the development of benthic animal populations and bioturbation of the surface sediments is prevented (Soutar and Crill, 1977; Savrda et al., 1984; Kennedy and Brassell, 1992). Disturbance is further reduced by the growth of bacterial mats covering some 20% of the sediment surface, coinci- dent with the highest levels of total organic carbon (Grant, 1991), consisting predominantly of the filamentous sulfur-oxidizing Beg- giatoa spp. with some iron-oxidizing bacteria (Soutar and Crill, 139