Complex Dynamics of Adaptation in a Nonaxenic Microcystis Culture: 2. Computer Simulation of Dinitrophenol Effects

Abstract

A hypothesis was modeled to account for complex 20-day dynamics in a culture of blue-green algae Microcystis and heterotrophic bacteria exposed to 2,4-dinitrophenol (DNP). In trials with little or no added DNP, a limiting factor (light or CO2) may cause algal density to fluctuate after 14 days of increase. Such factors may be unimportant at levels of DNP that restrict photosynthesis. Bacterial growth may be limited by organic substrate, and bacteria may be more resistant to DNP than blue-green algae. Hence, at intermediate levels of DNP, substrate provided by increased algal death stimulates bacterial growth more than DNP retards it, causing a bacterial peak. Sorption of DNP to cells may cause the DNP decline. Greater growth and slower DNP decline in experiments with preexposed organisms indicate lower DNP sorption affinity in preexposed cells. Bacterial assimilation of DNP-containing substrate may cause the reappearance of DNP. The model reproduced the fluctuation in algal density after growth was limited and better growth and lower DNP decline with preexposed organisms. Reappearance of DNP occurred, but was not obvious. Bacterial dynamics were least well reproduced. Changes in bacterial constants most affected output. Despite model inadequacies, probable aspects of toxicant action in nature have been revealed. Ecological relationships among populations of different species and genetic differences among individuals may have led to lower than expected toxicity, adaptation, and even growth stimulation. Responses of single species tested in isolation may be inadequate to predict toxicant impact.