Toxic Response Of Microorganisms After Joint Exposure Of Repeated BDE209 And Pb In Soil

Decabromodiphenyl (BDE209) and lead (Pb) were two kinds of typical pollutants in soil around electric waste (e-waste) recycling sites (EWRSs). Recently, the study was conducted in the laboratory with one-off exposure treatments to determine the ecological effects of the two contaminants. However, in the real environment, pollutants usually gradually accumulated into soil, and it was essential to carry out the research to reveal the biotoxicity of repeated pollution events. In our study, the experiments were conducted to explore the toxic response of microorganisms after joint exposure of repeated BDE209 and Pb in soil as well as the transformation of target contaminants. Such observations would provide useful information for the ecological risk assessment and bioremediation of BDE209 and Pb at EWRSs. The results were as follows:(1) Pb alone or joint exposure with BDE209 both facilitated dehydrogenase and sucrase activity, but showed inhibited and then turned to stimulated effects on urease. The dose-effect relationship between soil contaminants and enzymes was not obvious. Compared to one-off exposure treatments, the enzymes were lower in repeated exposure groups. For dehydrogenase and sucrase, the interaction between BDE209 and Pb were antagonistic and synergistic, while for urease, synergistic role was dominant.(2) During the entire incubation, no obvious changes of BDE209 concentrations were observed in two exposure treatments, suggesting that it was hard to be degraded. Pb mainly existed in the form of CAR-Pb and FeMnOX-Pb, and with the incubation time extended, Pb gradually transformed to RES-Pb. In the 250 mg kg-1 Pb repeated treatments, the percentage of different fractions followed the order:FeMnOX-Pb> CAR-Pb> RES-Pb> OMB-Pb> EXCH-Pb. But in the 500 mg kg-1 Pb repeated treatments, the order was:CAR-Pb> FeMnOX-Pb> RES-Pb> OMB-Pb> EXCH-Pb. Compared to the repeated pollution events, the distribution of Pb speciation in one-off exposure groups was a little different. Moreover, the addition of BDE209 influenced the distribution of Pb speciation in soil. The parameter of Ir (Reduced Partition Index) and Uts (Whole-soil Redistribution Index) increased at the end of incubation period, and different exposure modes had an influence on Ir and Uts values. Besides, significant correlations between Ir and the soil enzymes were observed (p< 0.05).(3) Different Pb exposure modes and exogenous BDE209 input could influence soil microbial diversity and community composition. Sequence analysis showed that repetitive BDE209-Pb exposure caused significant lower microbial diversity indices (Shannon) compared to one-off treatments (p< 0.05).100 mg kg-1 BDE209 input elicited a significant reduction of microbial diversity and abundance in repeated exposure groups (p< 0.05). The results also illustrated that Proteobacteria, Acidobacteria and Bacteroidetes were predominant phyla. And the predominant genus were Lysobacter and Thermomonas (under Gammaproteobacteria level).