C and N Isotope Fractionation Suggests Similar Mechanisms of Microbial Atrazine Transformation Despite Involvement of Different Enzymes (AtzA and TrzN)

Transformation of atrazine to hydroxyatrazine in the environment may be underestimated by current assessment schemes since immobilization and further transformation of the metabolite can render parent-to-daughter compound ratios unreliable. This study reports significant C and N isotope fractionation of atrazine in transformation to hydroxyatrazine by Chelatobacter heintzii, Pseudomonas sp. ADP, and Arthrobacter aurescens M highlighting an alternative approach to detecting this natural transformation pathway. Indistinguishable dual isotope slopes Delta (= delta N-15/delta C-13 approximate to epsilon(N)/epsilon(C)) for Chelatobacter heintzii (-0.65 +/- 0.08) and Arthrobacter aurescens TC1 (-0.61 +/- 0.02) suggest the same biochemical transformation mechanism despite different hydrolyzing enzymes (AtzA versus TrzN). With Pseudomonas sp. ADP (also AtzA) significantly smaller fractionation indicates masking effects by steps prior to enzyme catalysis, while a distinguishable Delta = -0.32 +/- 0.06 suggests that some of these steps showed slight isotope fractionation. Abiotic reference experiments reproduced the pattern of biotic transformation at pH 3 (enrichment of C-13, depletion of N-15 in atrazine), but showed enrichment of both C-13 and N-15 at pH 12. This indicates that the organisms activated atrazine by a similar Lewis acid complexation (e.g., with H+) prior to nucleophilic aromatic substitution, giving the first detailed mechanistic insight into this important enzymatic reaction.