Physiological considerations in applying laboratory-determined buoyant densities to predictions of bacterial and protozoan transport in groundwater: Results of in-situ and laboratory pests

Buoyant densities were determined for groundwater bacteria and microflagellates (protozoa) from a sandy aquifer (Cape God, MA) using two methods: (1) density-gradient centrifugation (DGC) and (2) Stoke's law approximations using sedimentation rates observed during natural-gradient injection and recovery tests. The dwarf (average cell size, 0.3 mu m), unattached bacteria inhabiting a pristine zone just beneath the water table and a majority (similar to 80%) of the morphologically diverse community of free-living bacteria inhabiting a 5-km-long plume of organically-contaminated groundwater had DGC-determined buoyant densities <1.019 g/cm(3) before culturing. In the aquifer, sinking rates for the uncultured 2-mu m size class of contaminant plume bacteria were comparable to that of the bromide tracer (1.9 x 10(-3) M), also suggesting a low buoyant density. Culturing groundwater bacteria resulted in larger (0.8-1.3 mu m), less neutrally-buoyant (1.043-1.081 g/cm(3)) cells with potential sedimentation rates up to 64-fold higher th an those predicted for the uncultured populations. Although sedimentation generally could be neglected in predicting subsurface transport for the community of free-living groundwater bacteria, it appeared to be important for the cultured isolates, at least until they readapt to aquifer conditions. Culturing-induced alterations in size of the contaminant-plume microflagellates (2-3 mu m) were ameliorated by using a lower nutrient, acidic (pH 5) porous growth medium. Buoyant densities of the cultured microflagellates were low, i.e., 1.024-1.034 g/cm(3) (using the DGC assay) and 1.017-1.039 g/cm(3) (estimated from in-situ sedimentation rates), suggesting good potential for subsurface transport under favorable conditions.