This study performed mechanistic toxicity assessment of nanosilver (nAg) and nanotitanium dioxide anatase (nTiO2_a) via toxicogenomic approach, employing a whole-cell-array library consisting of 91 recombinated Escherichia coli K12 strains with transcriptional GFP-fusions covering most known stress response genes. The results, for the first time, revealed more detailed transcriptional information on the toxic mechanism of nAg and nTiO2_a, and led to a better understanding of the mode of action (MOA) of metal and metal oxide nanomaterials (NMs). The detailed pathways network established for the oxidative stress system and for the SOS (DNA damage) repair system based on the temporal gene expression profiling data revealed the relationships and sequences of key genes involved in these toxin response systems. Both NMs were found to cause oxidative stress as well as cell membrane and transportation damage. Genotoxicity and DNA damage were also observed, although nTiO2_a induced SOS response via previously identified pathway and nAg seemed to induce DNA repair via a pathway different from SOS. We observed that the NMs at lower concentration tend to induce more chemical-specific toxicity response, while at higher concentrations, more general global stress response dominates. The information-rich real-time gene expression data allowed for identification of potential biomarkers that can be employed for specific toxin detection and biosensor developments. The concentration-dependent gene expression response led to the determination of the No Observed Transcriptional Effect Level (NOTEL) values, which can be potentially applied in the regulatory and risk assessment framework as an alternative toxicity assessment end point.

Application of external carbon sources for denitrification becomes necessary for wastewater treatment plants that have to meet very stringent effluent nitrogen limits (e.g., 3 to 5 mgTN/L). In this study, we evaluated and compared three carbon sources—MicroCTM (Environmental Operating Solutions, Bourne, Massachusetts), methanol, and acetate—in terms of their denitrification rates and kinetics, effect on overall nitrogen removal performance, and microbial community structure of carbon‐specific denitrifying enrichments. Denitrification rates and kinetics were determined with both acclimated and non‐acclimated biomass, obtained from laboratory‐scale sequencing batch reactor systems or full‐scale plants. The results demonstrate the feasibility of the use of MicroCTM for denitrification processes, with maximum denitrification rates (kdmax) of 6.4 mgN/gVSS·h and an observed yield of 0.36 mgVSS/mgCOD. Comparable maximum nitrate uptake rates were found with methanol, while acetate showed a maximum denitrification rate nearly twice as high as the others. The maximum growth rates measured at 20°C for MicroCTM and methanol were 3.7 and 1.2 day−1, respectively. The implications resulting from the differences in the denitrification rates and kinetics of different carbon sources on the full‐scale nitrogen removal performance, under various configurations and operational conditions, were assessed using Biowin (EnviroSim Associates, Ltd., Flamborough, Ontario, Canada) simulations for both pre‐ and post‐denitrification systems. Examination of microbial population structures using Automated Ribosomal Intergenic Spacer Analysis (ARISA) throughout the study period showed dynamic temporal changes and distinct microbial community structures of different carbon‐specific denitrifying cultures. The ability of a specific carbon‐acclimated denitrifying population to instantly use other carbon source also was investigated, and the chemical‐structure‐associated behavior patterns observed suggested that the complex biochemical pathways/enzymes involved in the denitrification process depended on the carbon sources used.

The biotransformation of some micropollutants has previously been observed to be positively associated with ammonia oxidation activities and the transcript abundance of the archaeal ammonia monooxygenase gene (amoA) in nitrifying activated sludge. Given the increasing interest in and potential importance of ammonia-oxidizing archaea (AOA), we investigated the capabilities of an AOA pure culture, Nitrososphaera gargensis, to biotransform ten micropollutants belonging to three structurally similar groups (i.e., phenylureas, tertiary amides, and tertiary amines). N. gargensis was able to biotransform two of the tertiary amines, mianserin (MIA) and ranitidine (RAN), exhibiting similar compound specificity as two ammonia-oxidizing bacteria (AOB) strains that were tested for comparison. The same MIA and RAN biotransformation reactions were carried out by both the AOA and AOB strains. The major transformation product (TP) of MIA, α-oxo MIA was likely formed via a two-step oxidation reaction. The first hydroxylation step is typically catalyzed by monooxygenases. Three RAN TP candidates were identified from nontarget analysis. Their tentative structures and possible biotransformation pathways were proposed. The biotransformation of MIA and RAN only occurred when ammonia oxidation was active, suggesting cometabolic transformations. Consistently, a comparative proteomic analysis revealed no significant differential expression of any protein-encoding gene in N. gargensis grown on ammonium with MIA or RAN compared with standard cultivation on ammonium only. Taken together, this study provides first important insights regarding the roles played by AOA in micropollutant biotransformation.

Examining global effects of toxins on gene expression profiles is proving to be a powerful method for toxicity assessment and for investigating mechanisms of toxicity. This study demonstrated the application of prokaryotic real-time gene expression profiling in Escherichia coli for toxicity assessment of environmental pollutants in water samples, by use of a cell-array library of 93 E. coli K12 strains with transcriptional green fluorescent protein (GFP) fusions covering most known stress response genes. The high-temporal-resolution gene expression data, for the first time, revealed complex and time-dependent transcriptional activities of various stress-associated genes in response to mercury and mitomycin (MMC) exposure and allowed for gene clustering analysis based on temporal response patterns. Compound-specific and distinctive gene expression profiles were obtained for MMC and mercury at different concentrations. MMC (genotoxin) induced not only the SOS response, which regulates DNA damage and repair, but also many other stress genes associated with drug resistance/sensitivity and chemical detoxification. A number of genes belonging to the P-type ATPase family and the MerR family were identified to be related to mercury resistance, among which zntA was found to be up-regulated at an increasing level as the mercury concentration increased. A mechanism-based evaluation of toxins based on real-time gene expression profiles promises, to be an efficient and informative method for toxicity assessment in environmental samples.