Transformation, CO₂ formation and uptake of four organic micropollutants by carrier-attached microorganisms

A tiered process was developed to assess the transformation, CO₂ formation and uptake of four organic micropollutants by carrier-attached microorganisms from two municipal wastewater treatment plants. At the first tier, primary transformation of ibuprofen, naproxen, diclofenac, and mecoprop by carrier-attached microorganisms was shown by the dissipation of the target compounds and the formation of five transformation products using LC-tandem MS. At the second tier, the microbial cleavage of the four organic micropollutants was confirmed with ¹⁴C-labeled micropollutants through liquid scintillation counting of the ¹⁴CO₂ formed. At the third tier, microautoradiography coupled with fluorescence in situ hybridization (MAR-FISH) was used to screen carrier-attached microorganisms for uptake of the four radiolabeled micropollutants. Results from the MAR-FISH screening indicated that only a small fraction of the microbial community (≤1‰) was involved in the uptake of the radiolabeled micropollutants and that the responsible microorganisms differed between the compounds. At the fourth tier, the microbial community structure of the carrier-attached biofilms was analyzed by 16S rRNA gene amplicon sequencing. The sequencing results showed that the MAR-FISH screening targeted ∼80% of the microbial community and that several taxonomic families within the FISH-probed populations with MAR-positive signals (i.e. Firmicutes, Gammaproteobacteria, and Deltaproteobacteria) were present in both biofilms. From the broader perspective of organic micropollutant removal in biological wastewater treatment, the MAR-FISH results of this study indicate a high degree of microbial substrate specialization that could explain differences in transformation rates and patterns between micropollutants and microbial communities.