COMPARISON BETWEEN GEOCHEMICAL AND BIOLOGICAL ESTIMATES OF SUBSURFACEMICROBIAL ACTIVITIES

Geochemical and biological estimates of in situ microbial activities w ere compared from the aerobic and microaerophilic sediments of the Atl antic Coastal Plain. Radioisotope time-course experiments suggested ox idation rates greater than millimolar quantities per year for acetate and glucose. Geochemical analyses assessing oxygen consumption, solubl e organic carbon utilization, sulfate reduction, and carbon dioxide pr oduction suggested organic oxidation rates of nano- to micromolar quan tities per year. Radiotracer timecourse experiments appeared to overes timate rates of organic carbon oxidation, sulfate reduction, and bioma ss production by a factor of 10(3)-10(6) greater than estimates calcul ated from groundwater analyses. Based on the geochemical evidence, in situ microbial metabolism was estimated to be in the nano- to micromol ar range per year, and the average doubling time for the microbial com munity was estimated to be centuries.