Isotope Fractionation Reveals Limitations and Microbial Regulation of Pollutant Biodegradation at Low Concentrations

Compound-specific isotope fractionation analysis (CSIA) of chemical trace contaminants (“micropollutants”) revealed bottlenecks of degradation at low, relevant (mg/L) concentrations. Isotope fractionation was pronounced at high concentrations, but masked at trace levels providing evidence that mass transfer into and out of the cell became limiting for biodegradation specifically at low concentrations. An onset of masking was observed for atrazine when degraded by Arthrobacter aurescens TC1 at concentrations below 60 mg/L in chemostat with complete rate control at 10 mg/L in retentostat . Proteomics revealed that such mass transfer limitation served as trigger for physiological adaptation, where catabolic enzymes remained highly expressed, whereas other cellular functions were downregulated.

CSIA also demonstrated mass transfer limitations in a quasi-two dimensional sediment tank system mimicking realistic conditions of natural aquifers. High, unmasked isotope fractionation in the center of the plume indicated that 2,6-dichlorobenzamide degradation by Aminobacter sp. MSH1 was not limited by substrate availability, but rather by oxygen supply. In contrast, masked isotope fractionation pinpointed rate-limiting mass transfer during cellular uptake towards the lower end of the concentration profile. For Isotope Biogeochemistry these findings imply that, based on isotopic evidence, turnover of substances at low concentrations may be widely underestimated.