Lead or cadmium co-contamination alters benzene and toluene degrading bacterial communities

Abstract

Co-contamination of hydrocarbons with heavy metals in soils often complicates and hinders bioremediation. A comprehensive characterization of site-specific degraders at contaminated sites can help determine if in situ bioremediation processes are sufficient. This study aimed to identify differences in benzene and toluene degradation rates and the microbial communities enriched under aerobic conditions when different concentrations of Cd and Pb are introduced. Microcosms were used to study the degradation of 0.23 mM benzene or 0.19 mM toluene under various concentrations of Pb (up to 240 µM) and Cd (up to 440 µM). Soil collected from a stormwater retention basin receiving runoff from a large parking lot was utilized to seed the microcosms. The hydrocarbon degradation time and rates were measured. After further rounds of amendment and degradation of benzene and toluene, 16S rRNA gene amplicon sequencing and quantitative PCR were used to ascertain the microbial communities enriched under the various concentrations of the heavy metals. The initial degradation time for toluene and benzene was 7 to 9 days and 10 to 13 days, respectively. Degradation rates were similar for each hydrocarbon despite the concentration and presence of metal co-contaminant, however, the enriched microbial communities under each condition differed. Microcosms without metal co-contaminant contained a diversity of putative benzene and toluene degrading bacteria. Cd strongly reduced the richness of the microbial communities. With higher levels of heavy metals, genera such as Ralstonia, Cupriavidus, Azoarcus, and Rhodococcus became more dominant under various conditions. The study finds that highly efficient benzene- and toluene-degrading consortia can develop under variations of heavy metal co-contamination, but the consortia are dependent on the heavy metal type and concentrations.

Keywords: Cupriavidus; Heavy metal resistance; Hydrocarbon degradation; Microbial diversity; Ralstonia.

Fig. 1

The total number of OTUs found in each sample from 16S rRNA gene amplicon sequencing. Low concentration refers to 0.24 µM Pb or 0.44 µM Cd. High concentration refers to 240 µM Pb or 440 µM Cd. None indicates the samples, which do not have any heavy metals

Fig. 2

Results of the non-metric multidimensional scaling (NMDS) analysis of bacterial community composition as determined by 16S rRNA gene amplicon sequencing

Fig. 3

Partial results of the qPCR data analysis to demonstrate the dominant Bacteria in the microcosms. Shown is the % of 16S rRNA genes measured for the phylogenetic group as a total of 16S rRNA genes measured in all 39 assays for those microcosms. Full results of qPCR data are shown in the Supplementary Information

Fig. 4

Dendrogram and heatmap (correlation distance) analysis of the 39 Bacteria in an OTU level most abundant and unique for the co-contaminants’ type and concentration. The scale bar and heatmap indicates the log-unit increase or decrease for each OTU from 16S rRNA in the sample, derived from qPCR analysis, compared with its mean. Dark blue corresponds to low expression, dark red corresponds to high expression