Analyzing (Initial) Biotransformation Reactions as an Organizing Principle for Unraveling the Extent of Trace Organic Chemical Biotransformation in Biofiltration Systems

Due to the low environmental concentrations and substance-specific behavior of trace organic chemicals (TOrCs), general mechanistic insights can hardly be gained from biotransformation studies with individual substances. In this study, we aimed to identify prevalent enzymatic reactions under different redox and substrate conditions in a well-defined laboratory-scale column system. Biotransformation of chemicals in the first column, which was characterized by a relatively high biomass and rapid depletion of dissolved oxygen, was initiated by frequent and fast reactions such as the oxidation of alcohols or amide hydrolysis. Slower reactions, e.g., the initial dealkylation of primary and secondary amines, were inefficient in the first column and benefited from oxic and carbon-limited conditions prevalent in the second column system after reaeration. Moreover, several compound-specific reactions such as the cleavage of C−Cl and C−O bonds through substitution with glutathione occurred only under carbon-limited conditions. The link between system-specific TOrC removal and suggested initial reactions from the literature can serve as a starting point for a suspect screening of relevant enzymes for the biotransformation of TOrCs based on metagenomic or metatranscriptomic data. Complementary batch experiments with media from the columns confirmed the observed removal under carbon-rich conditions but revealed a limited reproducibility of microbial degradation under oligotrophic conditions.