Physiological Considerations in Applying Laboratory-Determined Buoyant Densities to Predictions of Bacterial and Protozoan Transport in Groundwater:  Results of In-Situ and Laboratory Tests

Abstract

Buoyant densities were determined for groundwater bacteria and microflagellates (protozoa) from a sandy aquifer (Cape Cod, 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 μm), unattached bacteria inhabiting a pristine zone just beneath the water table and a majority (∼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/cm3 before culturing. In the aquifer, sinking rates for the uncultured 2-μm size class of contaminant plume bacteria were comparable to that of the bromide tracer (1.9 × 10-3 M), also suggesting a low buoyant density. Culturing groundwater bacteria resulted in larger (0.8−1.3 μm), less neutrally-buoyant (1.043−1.081 g/cm3) cells with potential sedimentation rates up to 64-fold higher than 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 μm) 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/cm3 (using the DGC assay) and 1.017−1.039 g/cm3 (estimated from in-situ sedimentation rates), sug gesting good potential for subsurface transport under favorable conditions.

Simultaneous Transport of Two Bacterial Strains in Intact Cores from Oyster, Virginia: Biological Effects and Numerical Modeling

“…One vital requirement for successful implementation of bioaugmentation, the injection of bacteria with degradative capabilities into the subsurface to remediate a contaminated aquifer (30,36,43), is the delivery of selected bacteria to and through the contaminated zone of an aquifer. In most bacterial transport studies, bacterial attachment to mineral grains significantly impairs effective dispersion of the introduced bacteria throughout the aquifer (8,23,25). Bacterial attachment to mineral grain surfaces is influenced by biological factors associated with the bacterial cells and by physical and chemical factors associated with a granular aquifer (12,17,22,34).…”

Adaptations of Indiginous Bacteria to Fuel Contamination in Karst Aquifers in South-Central Kentucky

“…Buoyant density of bacteria collected in these three wells ranged from light (1.02 g cm 23 ) to dense (1.08 g cm 23 ). A similar difference in size and buoyant densities of groundwater bacteria observed in pristine versus contaminated zones of a sandy aquifer in Cape Cod, Massachusetts, resulted in an estimated 64-fold difference in their settling rates (Harvey et al, 1997). However, the relationship between bacteria shape, size, and sedimentation velocity is not fully understood, and other factors may affect sedimentation rate.…”

“…For example, karst conduits typically offer much smaller surface areas for biofilm development per volume of water relative to granular aquifers. It is therefore reasonable to assume that there are different adaptations and distributions in karst microbial communities as compared to those reported for sandy aquifers (Haack et al, 2012;Harvey et al, 1984;Harvey et al, 1997;Kö lbel-Boelke et al, 1988;Barton and Northup, 2007).…”

“…Buoyant-density determinations were performed using the method described by Harvey et al (1997). Approximately 2 L of uncontaminated groundwater or 1 L of contaminated groundwater was filtered through 47 mm (diameter), 0.2 mm (pore size) polycarbonate membrane filters at 20.3 atmosphere transmembrane pressure.…”

“…Groundwater bacteria may use directed locomotion in the presence of chemical gradients (chemotaxis) to actively pursue energy sources, thus facilitating biodegradation (Ford and Harvey, 2007). Many aquatic bacteria propel themselves using flagella (Harschey, 2003), glide by one of several methods (McBride, 2001), or modify their buoyant density and float with the groundwater (Harvey et al, 1997). The use of flagella for locomotion is energy intensive and may be limited in anaerobic environments, where rapid expenditure of energy is costly to the organism.…”