Interactions between microbial predators and their prey can significantly influence the behavior of toxic trace metals. Metals associated with bacterial prey can be released into the dissolved phase following digestion by a predator, and/or metals can remain in the predator and potentially be transferred to the next level of the food chain. Toxic metal ions in the aqueous phase are also expected to modify the growth and predation rate of a microbial predator. A defined predator-prey system was developed to study metal behavior in simple microbial food chains using lead (Pb) as a representative metal. Desired features of this system were the ability to define the chemical speciation of dissolved metals as well as to distinguish between prey and predator-bound metals. Pseudomonas putida and the ciliate protozoan Tetrahymena thermophila were selected as representative bacterial prey and predator species, respectively. Batch reactors were used to measure microbial growth parameters, effects of prey density on predation and Pb phase distribution. A mathematical model was developed to describe predator-prey dynamics and their influence on the behavior and fate of Pb. Growth data were used to obtain model parameters, and model simulations for Pb fractionation were compared to experimental observations.
The methodological studies demonstrated successful predator-prey separation techniques with little metal loss. Results of batch reactor experiments demonstrated that some kinetic parameters related to prey consumption and growth of T. thermophila are altered by Pb. Upon addition of predator to prey cells in equilibrium with dissolved Pb, dissolved and prey-bound Pb became associated with the predator through ingestion and adsorption. Ingested Pb was later excreted as a bound metal associated with T. thermophila waste matter. Experimental observations that did not match model predictions prompted further mathematical modeling of this predator-prey system. These simulations also explored Pb behavior under other hypothetical experimental conditions such as a chemostat reactor and a pulsed Pb dosing regime. The generality of the model was demonstrated by matching the trends in experimental data reported by other investigators for a different trace metal (Cd) in a different predator-prey system.