Behavior of lead in a model microbial predator-prey system.

Predation at the microbial level can affect the fate 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 be transferred potentially to the next level of the food chain. Toxic metal ions in the aqueous phase also are expected to modify the growth and predation rate of a microbial predator. A model predator-prey system was developed to test the effects of Pb on cells and to help elucidate the fate of Pb in this type of interaction. Established methods that have been shown to be suitable for distinguishing dissolved, prey-bound, predator-bound, and ingested Pb were used to establish the pathway of Pb over time. Growth parameters were measured using batch reactors for the protozoan predator Tetrahymena thermophila and the bacterial prey Pseudomonas putida without Pb and at several concentrations of Pb. The effect of prey density on predation and Pb phase distribution also was investigated. Results demonstrate 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 become associated with the predator through ingestion and adsorption. Ingested Pb is excreted later as a bound metal associated with T. thermophila waste matter. A preliminary 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 are compared to experimental observations.

Development of a model microbial predator-prey system suitable for studies of the behavior of toxic trace metals.

Interactions between microbial predators and their prey can significantly influence the behavior of toxic trace metals. Ingested bacterial prey-bound metals can either accumulate within a predator or be excreted and potentially reintroduced into the dissolved phase. A defined predator-prey system suitable for developing a more fundamental understanding of metal behavior in simple microbial food chains was designed and tested by using lead (Pb) as a representative cationic transition 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. In addition, the use of fluorescent microspheres was evaluated as an experimental surrogate for bacterial prey. Filtration techniques for size-selective separation were developed so that the distribution of Pb between cells of T. thermophila, cells of P. putida or microspheres, and the dissolved phase could be assessed. Filtration units were selected based on their ability to perform separations with minimal metal loss at circumneutral pH. Five-micron polycarbonate filter membranes successfully separated T. thermophila from P. putida with good cell retention and low metal loss. Centrifuge filters successfully separated dissolved and particle-bound metal (<5,000 nominal molecular wt limit). Exemplary experimental results are presented and show that predation on Pb-exposed cells of P. putida or microspheres increases uptake of Pb by T. thermophila.