MICROBIAL COMMUNITY STRUCTURE AND BIOGEOCHEMISTRY OF MIOCENE SUBSURFACE SEDIMENTS - IMPLICATIONS FOR LONG-TERM MICROBIAL SURVIVAL

Thirty closely spaced cores were obtained from Miocene-aged fluvial, lacustrine and palaeosol subsurface sediments ranging in depth from 173 to 197 m at a site in southcentral Washington to investigate the size and composition of the microbial community in relation to sediment geochemical and geophysical properties. Total phospholipid fatty acid (PLFA) analysis indicated that the greatest concentrations of microbial biomass were in low-permeability lacustrine sediments that also contained high concentrations of organic carbon. Community structure, based on lipid analyses and on in situ hybridization of bacterial cells with 16S RNA-directed DNA probes, also revealed the presence of metabolically active bacteria that respire sulphate and/or Fe(III) in the lacustrine sediments. Concentrations of pore water sulphate were low (4-8 mg/L) and HCl-extractable Fe was predominantly Fe(II) in the same samples where total biomass and organic carbon were highest. The low hydraulic conductivity (10(-6) to < 10(-9) cm/s) of these sediments has likely contributed to the long term maintenance of both bacteria and organic carbon by limiting the supply of soluble electron accepters for microbial respiration. These results suggest that the current subsurface microbial population was derived from organisms that were present during lake sedimentation approximate to 6-8 million years ago.