The response of bacterial populations to sapropels in deep sediments of the Eastern Medierranean (Site 969)

Sediment samples were obtained from 36 depths between the near-surface to almost 100 mbsf (meters below seafloor) at Site 969, south of Crete. Bacterial populations were determined by the acrinine orange direct count technique. The bacterial profile agreed with a general relationship between sediment depth and bacterial concentration previously obtained from a range of different marine sites. Bacterial populations decreased from a near-surface value of 7.76 ´ 108 cells/cm3 to 1.0 ´ 106 cells/ cm3 by 97.79 mbsf. Dividing and divided cell numbers roughly paralleled total bacterial numbers representing ~9.9% of the total population. At two depths, sapropels were specifically sampled in duplicate and enumeration suggested that bacterial populations within sapropels were homogeneous. Two additional sapropels were also encountered by chance. The data strongly indicated an active and probably growing bacterial population within the sapropels because: 1. Bacterial populations were considerably, and significantly, greater than those in adjacent nonsapropel sediment layers. 2. There were locally high TOC concentrations and abundant electron acceptors (sulfate). 3. Evidence of postburial bacterial sulfate reduction activity was supported by independent sulfur isotope data. 4. Dividing bacterial cells were present. These results demonstrate the surprising ability of 4.7 Ma organic matter to continue to provide energy for bacterial populations during burial, and this supports the global presence of a deep bacterial biosphere in marine sediments