Microbial degradation of organic micropollutants in hyporheic zone sediments

Abstract

The hyporheic zone is the water-saturated streambed sediment layer characterized by the simultaneous occurrence of multiple physical, biological, and chemical processes. The hyporheic zone contributes to the self-purification capacity of streams by removing point and nonpoint source pollutants mainly via microbial activity. However, such potentials, associated microbial communities, and impacts on microbial community structure are largely unknown for specific widely distributed compounds of environmental concern. Thus, pollutant removal potentials in hyporheic zone sediments were investigated using the non-steroidal anti-inflammatory drug ibuprofen. Ibuprofen biodegradation in oxic sediment microcosms amended with ibuprofen, or ibuprofen and acetate was determined. Unsupplemented and heat-sterilized sediments served as controls. Ibuprofen biotransformation occurred via 1-, 2-, 3-hydroxy and carboxy-ibuprofen as transient primary transformation products. Quantitative PCR analysis revealed a significantly higher 16S rRNA abundance in ibuprofen-amended relative to un-amended incubations. Time-resolved amplicon Illumina MiSeq sequencing targeting 16S rRNA genes and 16S rRNA revealed a clear effect of ibuprofen on the microbial community structure and many new ibuprofen responsive taxa affiliating with Acidobacteria, Actinobacteria, Bacteroidetes, Gemmatimonadetes, Latescibacteria and Proteobacteria. This was confirmed by specific 16S rRNA gene expression analysis of representative taxa. Two strains utilizing ibuprofen as sole carbon and energy source of the genera Novosphingobium and Pseudomonas were isolated and will serve as model organisms for elucidating ibuprofen degradation pathways. Redox gradients along the streambed depth profile impact pollutant removal. Microbial removal of the beta-blocker metoprolol, in the redox-delineated hyporheic zones was investigated using oxic and anoxic sediment microcosms. Biotransformation of metoprolol occurred under both incubation conditions. In the oxic microcosms, metoprolol was transformed mainly to metoprolol acid, while under anoxic conditions, metoprolol acid and hydroxy metoprolol were formed, indicating dissimilar metabolic pathways for metoprolol degradation under the contrasting incubation conditions. The transformation products were subsequently completely degraded under both conditions. The responsive taxa associated with metoprolol degradation were affiliated with the Proteobacteria and Bacteroidetes under oxic conditions, and Proteobacteria, Acidobacteria, Chloroflexi, Firmicutes and Gemmatimonadetes under anoxic conditions. The organic matter content influences multiple biogeochemical reactions. The effect of organic carbon on the fate of 13 trace organic compounds (TrOCs) was investigated using sediments differing in the total organic carbon content (TOC). Oxic sediment microcosms with low and high TOC content differed in the biotransformation and sorption removal efficiencies of the TrOCs. Significantly higher biotic removal efficiencies of compounds…