title: Nitrification inhibition by hexavalent chromium Cr(VI)--Microbial ecology, gene expression and off-gas emissions.
abstract: The goal of this study was to investigate the responses in the physiology, microbial ecology and gene expression of nitrifying bacteria to imposition of and recovery from Cr(VI) loading in a lab-scale nitrification bioreactor. Exposure to Cr(VI) in the reactor strongly inhibited nitrification performance resulting in a parallel decrease in nitrate production and ammonia consumption. Cr(VI) exposure also led to an overall decrease in total bacterial concentrations in the reactor. However, the fraction of ammonia oxidizing bacteria (AOB) decreased to a greater extent than the fraction of nitrite oxidizing bacteria (NOB). In terms of functional gene expression, a rapid decrease in the transcript concentrations of amoA gene coding for ammonia oxidation in AOB was observed in response to the Cr(VI) shock. In contrast, transcript concentrations of the nxrA gene coding for nitrite oxidation in NOB were relatively unchanged compared to Cr(VI) pre-exposure levels. Therefore, Cr(VI) exposure selectively and directly inhibited activity of AOB, which indirectly resulted in substrate (nitrite) limitation to NOB. Significantly, trends in amoA expression preceded performance trends both during imposition of and recovery from inhibition. During recovery from the Cr(VI) shock, the high ammonia concentrations in the bioreactor resulted in an irreversible shift towards AOB populations, which are expected to be more competitive in high ammonia environments. An inadvertent impact during recovery was increased emission of nitrous oxide (N2O) and nitric oxide (NO), consistent with recent findings linking AOB activity and the production of these gases. Therefore, Cr(VI) exposure elicited multiple responses on the microbial ecology, gene expression and both aqueous and gaseous nitrogenous conversion in a nitrification process. A complementary interrogation of these multiple responses facilitated an understanding of both direct and indirect inhibitory impacts on nitrification.
authors.affiliation: Department of Earth and Environmental Engineering, Columbia University, 500 West 120th Street, New York, NY 10027, USA.
authors.forename: Young Mo
authors.initials: YM
authors.lastname: Kim
authors.affiliation: Department of Earth and Environmental Engineering, Columbia University, 500 West 120th Street, New York, NY 10027, USA.
authors.forename: Hongkeun
authors.initials: H
authors.lastname: Park
authors.affiliation: Department of Earth and Environmental Engineering, Columbia University, 500 West 120th Street, New York, NY 10027, USA. Electronic address: kc2288@columbia.edu.
authors.forename: Kartik
authors.initials: K
authors.lastname: Chandran
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D001419:Bacteria / Q000187:drug effects / Q000235:genetics; D019149:Bioreactors / Q000382:microbiology; D002857:Chromium / Q000494:pharmacology*; D058645:Denaturing Gradient Gel Electrophoresis; D017753:Ecosystem; D015964:Gene Expression Regulation, Bacterial / Q000187:drug effects*; D009569:Nitric Oxide / Q000032:analysis; D058465:Nitrification / Q000187:drug effects / Q000235:genetics*; D009609:Nitrous Oxide / Q000032:analysis; D010802:Phylogeny; D017422:Sequence Analysis, DNA
chemical_list: D000393:Air Pollutants; D002857:Chromium; C074702:chromium hexavalent ion; D009569:Nitric Oxide; D009609:Nitrous Oxide
keywords: Gene expression; Hexavalent chromium (Cr(VI)); Inhibition; Microbial ecology; Nitrification; Nitrous oxide