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

摘要

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 ε(C) = −5.36 ± 0.20‰ at 82 μg/L to ε(C) = −2.32 ± 0.28‰ at 32 μg/L. At 82 μg/L ε(C) represented the full isotope effect of the enzyme reaction. At lower residualconcentrations smaller ε(C) indicated that this isotope effectwas masked indicating that mass transfer across the cell membranebecame rate-limiting. This onset of mass transfer limitation appearedin a narrow concentration range corresponding to about 0.7 μMassimilable carbon. Concomitant changes in cell morphology highlightthe opportunity to study the role of this onset of mass transfer limitationon the physiological level in cells adapted to low concentrations.