Biodegradation of Trace-level Organic Contaminants by Rhizosphere Microbial Communities

Abstract

Wastewater treatment plants (WWTPs) remove simple organic compounds from wastewater but cannot fully degrade the thousands of trace-level organic contaminants (TOrCs) added to the waste stream each year. These TOrCs are released into the environment in WWTP effluent and can negatively impact aquatic ecosystems. One proposed method for treating TOrCs is rhizotreatment, which breaks down contaminants using the metabolic capability of the rhizosphere. In rhizotreatment, effluent is discharged onto a field of trees. As the water moves through the soil, it encounters rhizosphere microorganisms that degrade the TOrCs. Rhizotreatment is used at several WWTPs to treat nitrogen and phosphorus compounds, but little is known about the fate of TOrCs in the system, or about the changes to the soil microbial community induced by long-term rhizotreatment. This study examined the impact of rhizotreatment on the degradation of five TOrCs, as well as changes to the rhizosphere community. Rhizosphere soil samples were collected from three WWTP effluent discharge fields planted with trees and exposed to effluent for at least ten years, and from nearby control plots that received no effluent. Short-term batch testing was conducted to compare the degradation of bisphenol A (BPA), carbamazepine, gemfibrozil, ibuprofen, and naproxen between effluent-exposed and control soils. Microbial community composition was also examined using 16S rRNA Illumina sequencing, and bacterial and archaeal biomass, along with the presence of a putative BPA degradation gene, were quantified via droplet digital PCR. Soils from all three sites exhibited intrinsic degradation of BPA and ibuprofen. Additionally, BPA degradation was enhanced in all treated soils, two of which also showed increased abundance of the BPA degradation gene. The other three TOrCs exhibited minimal degradation over the timeframe of the experiment. Principal component analysis of the sequencing results revealed distinct clustering of the treated and control communities at two of three sites where BPA degradation was enhanced, suggesting differences in microbial community composition between treated and control soils. In a correlation analysis, seven bacterial taxa were found to correlate with BPA degradation rate, and several additional taxa correlated with soil moisture. Together, these results suggest that long-term TOrC exposure may enhance BPA degradation and, in combination with increased soil moisture due to irrigation, alter rhizosphere microbial community composition. Further research is needed to examine the relationships between long-term TOrC exposure, enhanced TOrC degradation, and microbial community composition.