Rate-LimitingMass Transfer in Micropollutant DegradationRevealed by Isotope Fractionation in Chemostat

Biodegradation of persistent micropollutants like pesticides often slows down at low concentrations (μg/L) in the environment. Mass transfer limitations or physiological adaptation are debated to be responsible. Although promising, evidence from compound-specific isotope fractionation analysis (CSIA) remains unexplored for bacteria adapted to this low concentration regime. We accomplished CSIA for degradation of a persistent pesticide, atrazine, during cultivation of Arthrobacter aurescens TC1 in chemostat under four different dilution rates leading to 82, 62, 45, and 32 μg/L residual atrazine concentrations. Isotope analysis of atrazine in chemostat experiments with whole cells revealed a drastic decrease in isotope fractionation with declining residual substrate concentration from e(C) = −5.36 ± 0.20‰ at 82 μg/L to e(C) = −2.32 ± 0.28‰ at 32 μg/L. At 82 μg/L e(C) represented the full isotope effect of the enzyme reaction. At lower residual concentrations smaller e(C) indicated that this isotope effect w...