NADH-limited enzyme assays were used to determine the turnover of nitroaromatic substrates to organic and inorganic reaction products, O 2 disappearance, and H 2 O 2 formation, as well as changes in C/ 12 C ratios of the organic substrate and for experimental determination of the initial concentration of the substrates. From selected enzyme-substrate combinations, we also quantified H 2 O 2 formation using a horse radish peroxidase (HRP) assay to catalyze the reduction of H 2 O 2 by oxidizing 4-methoxyaniline (Section S2.4). 900 µL aliquots were withdrawn from each NADH-limited enzyme assay and mixed with 100 µL of an HRP assay in MES buffer to final concentrations of 10 mg L -1 HRP and 500 µM 4-methoxyaniline. Kinetics of O 2 consumption and substrates oxygenation were performed in modified assays as described before and in Sections S2.2 and S2.3.