Microbial degradation kinetics of toxic organic chemicals over a wide range of concentrations in natural aquatic systems

Abstract

AbstractThe degradation kinetics for several toxic organic compounds (p‐cresol, phenol, p‐ chlorophenol, acetone and methanol) exposed to natural aquatic microbial assemblages were examined over a wide range of concentrations. Compounds were added in radiolabeled form and incubated at 25°C for periods ranging from several hours to one day. The results revealed that degradation kinetics were multiphasic for each of the compounds during at least one sampling period, i.e., the kinetic parameters Kt and Vmax increased with increasing substrate concentration. The observed multiphasicity suggests the presence within the natural microbial assemblages of multiple uptake/degradative systems, each with distinguishable kinetic parameters and each dominating degradation at a different substrate concentration range. If the kinetic diversity is ignored and rates of degradation are calculated based on the assumption that kinetics follow simple hyperbolic (Michaelis‐Menten) relationships, then errors as large as 40,000‐fold can result. Thus, our results suggest that models for predicting the persistence of such compounds in natural waters should consider changes in kinetic constants with substrate concentration.