MICROBIOLOGICAL COMPARISONS WITHIN AND ACROSS CONTIGUOUS LACUSTRINE, PALEOSOL, AND FLUVIAL SUBSURFACE SEDIMENTS

Twenty-six subsurface samples were collected from a borehole at depths of 173.3 to 196.8 m in the saturated zone at the Hanford Site in sout h central Washington State. The sampling was performed throughout stra ta that included fine-grained lacustrine (lake) sediments, a paleosol (buried soil) sequence, and coarse-grained fluvial (river) sediments. A subcoring method and tracers were used to minimize and quantify cont amination to obtain samples that were representative of subsurface str ata. Sediment samples were tested for total organic carbon, inorganic carbon, total microorganisms by direct microscopic counts, culturable aerobic heterotrophs by plate counts, culturable anaerobes by most-pro bable-number enumeration, basal respiration rates, and mineralization of C-14-labeled glucose and acetate. Total direct microscopic counts o f microorganisms were low, ranging from below detection to 1.9 x 10(5) cells g (dry weight)(-1). Culturable aerobes and anaerobes were below minimum levels of detection in most samples. Direct microscopic count s, basal respiration rates, and C-14-glucose mineralization were all p ositively correlated with total organic carbon and were highest in the lacustrine sediments. In contrast to previous subsurface studies, the se saturated-zone samples did not have higher microbial abundance and activities than unsaturated sediments sampled from the same borehole, the fine-textured lacustrine sediment had higher microbial numbers and activities than the coarse-textured fluvial sands, and the paleosol s amples did not have higher biomass and activities relative to the othe r sediments. The results of this study expand the subsurface microbiol ogy database to include information from an environment very different from those previously studied.