Abstract
The dynamics of a model of a microbial batch culture community consisting of Tetrahymena pyriformis and Escherichia coli populations are explored when the community is stressed by chemicals with a nonpolar narcosis mode of action. The framework for the community consists of two individualโbased population models, individual models of the organisms T. pyriformis and E. coli, and exposure and effect modules for each species. The exposure module is based on passive diffusion and describes both environmental and resource routes of exposure. The effect module utilizes quantitative structureโactivity relationships to determine resource uptake reduction, the effect chosen as the primary consequence of exposure to the chemical. The range of possible effects on the model ciliateโbacterial community includes results that do not arise from population studies; for example, chemically depressed growth rates of individuals can be stimulatory for a population in a community setting. Community structure and feedback mechanisms are demonstrated to be fundamental for effects determination in that structure evolves as a function of exposure concentration. In a closed system, the presence of nonviable organic material modifies bioavailability of the toxicant and allows for community persistence above concentration levels that cause mortality in a continuous culture. An indicator of stress, the community extinction threshold, is developed as a function of chemical diffusivity and hydrophobicity.