Metagenomic insights into Cr(VI) effect on microbial communities and functional genes of an expanded granular sludge bed reactor treating high-nitrate wastewater.

In this study, a lab-scale expanded granular sludge bed reactor was continuously operated to treat high-nitrate wastewater containing different concentrations of hexavalent chromium (Cr(VI)). Nearly complete nitrate removal was achieved even at 120 mg/L influent Cr(VI). Pyrosequencing of 16S rRNA gene showed that Cr(VI) decreased the biodiversity of the bacterial community and potential denitrifiers. Proteobacteria dominated in the bioreactor, and Betaproteobacteria had increased abundance after Cr(VI) feeding. Thauera and Halomonas were the two predominant genera in the bioreactor fed with Cr(VI), demonstrating opposite responses to the Cr(VI) stress. Metagenomic analysis indicated that Cr(VI) feeding posed no obvious effect on the overall function of the bacterial community, but altered the abundance of specific denitrifying genes, which was evidenced by quantitative real time PCR. This study revealed that Halomonas mainly contributed to the denitrification under no or low Cr(VI) stress, while Thauera played a more important role under high Cr(VI) stress.