Candidate Stress Genes ofNitrosomonas europaeafor Monitoring Inhibition of Nitrification by Heavy Metals

Abstract

Heavy metals have been shown to be strong inhibitors of nitrification in wastewater treatment plants. In this research, the effects of cadmium, copper, and mercury on Nitrosomonas europaea were studied in quasi-steady-state batch reactors. When cells were exposed to 1 μM CdCl₂, 6 μM HgCl₂, or 8 μM CuCl₂, ammonia oxidation rates were decreased by about 90%. Whole-genome transcriptional and proteomic responses of N. europaea to cadmium were used to identify heavy metal stress response genes. When cells were exposed to 1 μM CdCl₂ for 1 h, 66 genes (of the total of 2,460 genes) were upregulated, and 50 genes were downregulated more than twofold. Of these, the mercury resistance genes (merTPCADE) averaged 277-fold upregulation under 1 μM CdCl₂, with merA (mercuric reductase) showing 297-fold upregulation. In N. europaea cells exposed to 6 μM HgCl₂ or to 8 μM CuCl₂, merA showed 250-fold and 1.7-fold upregulation, respectively. Cells showed the ability to recover quickly from Hg²⁺-related toxic effects, apparently associated with upregulation of the mercury resistance genes and amoA, but no such recovery was evident in Cd²⁺-exposed cells even though merTPCADE were highly upregulated. We suggest that the upregulation of merA in response to CdCl₂ and HgCl₂ exposure may provide a means to develop an early-warning indicator for inhibition of nitrification by these metals.