Physiological considerations in applying laboratory-determined buoyant densities to predictions of bacterial and protozoan transport in ground water: results of in-situ and laboratory tests

Buoyant densities of ground water bacteria and microflagellates from a sandy aquifer were determined by density-gradient centrifugation or by Stoke's law approximations from sedimentation rates observed during natural-gradient injection and recovery tests. Buoyant densities were less than 1.019 g per cm3 for the dwarf unattached bacteria, average cell size was 0.3 um, in a pristine zone just below the water table, and for the diverse free-living bacteria, average cell size was 0.6 um, in the organically contaminated 5-km long plume. In the aquifer, uncultured bacteria of 2 um-size showed sinking rates comparable with those of the bromide tracer. Cultured ground water bacteria had larger cells with buoyant densities of 1.043-1.081 g per cm3 and a 64-fold greater sedimentation rate potential than that predicted for the uncultured population. Culturing-induced alterations in the size of contaminant-plume microflagellates were minimized with a low nutrient porous growth medium at pH 5. This kept buoyant densities low and maintained good potential for sub-surface transport. There are 37 references.