Dispersal dynamics of groundwater bacteria.

Dispersal of bacteria in saturated, porous soils can be characterized by the partitioning of cells between the aqueous and solid phases, as a result of the physical and chemical nature of the soil and water and cell surface modifications. The purpose of this work is to understand variations in partitioning as a consequence of the nutrient conditions and to use this information in mathematical models to predict the dispersal rate of bacteria in aquifer material. Two different models were used to describe dispersal: an advective-dispersive-sorptive model with a first order kinetic sink term to account for irreversible cell reactions, such as death and sorption; and a two-site reaction model, in which the retardation was assumed to be determined by two types of sites, one characterized by instantaneous equilibrium sorption reactions and the other by kinetic nonequilibrium reactions. Water-saturated sand columns were used as continuous-flow groundwater microcosms to test the models under different nutrient regimes. Two strains of indigenous groundwater bacteria were isolated from aquifer material and labelled with(3)H-alanine,(14)C-pyruvic acid,(3)H-glucose, and(3)H-adenosine for different measurements of sorption and dispersal, which were estimated from breakthrough curves. Both experimental data and model variables showed that dispersal of bacteria was a dynamic nonequilibrium process, possibly shaped by two subpopulations, one strongly, even irreversibly, adsorbing to the solid particles, and one with very slow adsorption kinetics. The cell surfaces were modified in response to the growth conditions, which was demonstrated by hydrophobic and electrostatic interaction chromatography. Cell surface hydrophobicity was about eight times higher in groundwater than in eutrophic lake water. The partition coefficient varied between 12.6 in the groundwater and 6.4 in the lake water, indicating the prime importance of hydrophobic binding for attachment in low nutrient conditions. The partitioning was also sensitive to the hydrodynamics of the system and the oxygen supply, as demonstrated by comparison of sorption in agitated test tubes, gently shaken vials, and air-flushed bottles. Sorption kinetics were demonstrated in a continuous flow cell. About 45% of a population was associated with sand particles with a continuous flow of pure groundwater and as little as 20% in lake water. However, more than 50% of the bacteria in the aqueous phase were associated with suspended material of less than 60 μm in diameter. This association may enhance dispersal for example, by size exclusion of the colloidal material in the interstitial pores.