Haining Huang

Distribution of tetracycline resistance genes in anaerobic treatment of waste sludge: The role of pH in regulating tetracycline resistant bacteria and horizontal gene transfer.

Although pH value has been widely regarded as an important factor that affects resource recovery of waste sludge, the potential influence of diverse pHs on the distribution of tetracycline resistance genes (TRGs) during sludge anaerobic treatment is largely unknown. Here we reported that in the range of pH 4-10, 0.58-1.18 log unit increase of target TRGs was observed at pH 4, compared with that at pH 7, while 0.70-1.31 log unit further removal were obtained at pH 10. Mechanism study revealed that varied pHs not only altered the community structures of tetracycline resistant bacteria (TRB), but also changed their relative abundances, benefitting the propagation (acidic pHs) or attenuation (alkaline pHs) of TRB. Further investigation indicated that the amount and gene-possessing abilities of key genetic vectors for horizontal TRGs transfer were greatly promoted at acidic pHs but restricted under alkaline conditions.

Carboxyl-modified single-walled carbon nanotubes negatively affect bacterial growth and denitrification activity

Single-walled carbon nanotubes (SWNTs) have been used in a wide range of fields, and the surface modification via carboxyl functionalization can further improve their physicochemical properties. However, whether carboxyl-modified SWNT poses potential risks to microbial denitrification after its release into the environment remains unknown. Here we present the possible effects of carboxyl-modified SWNT on the growth and denitrification activity of Paracoccus denitrificans (a model denitrifying bacterium). It was found that carboxyl-modified SWNT were present both outside and inside the bacteria, and thus induced bacterial growth inhibition at the concentrations of 10 and 50 mg/L. After 24 h of exposure, the final nitrate concentration in the presence of 50 mg/L carboxyl-modified SWNT was 21-fold higher than that in its absence, indicating that nitrate reduction was substantially suppressed by carboxyl-modified SWNT. The transcriptional profiling revealed that carboxyl-modified SWNT led to the transcriptional activation of the genes encoding ribonucleotide reductase in response to DNA damage and also decreased the gene expressions involved in glucose metabolism and energy production, which was an important reason for bacterial growth inhibition. Moreover, carboxyl-modified SWNT caused the significant down-regulation and lower activity of nitrate reductase, which was consistent with the decreased efficiency of nitrate reduction.

The effects of carbon nanotubes on nitrogen and phosphorus removal from real wastewater in the activated sludge system

The wide use of carbon nanotubes (CNTs), such as single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs), inevitably causes their release into the environment. Previous studies pointed out that the released CNTs would have negative effects on model animals, plants, or microorganisms. Nevertheless, to date, it is unclear whether the presence of CNTs in wastewater treatment plants (WWTPs) could affect biological nitrogen and phosphorus removal. In this paper, the potential effects of CNTs (SWNTs and MWNTs) on nitrogen and phosphorus removal from real wastewater in an activated sludge system were investigated. It was found that the presence of CNTs had no significant impacts on nitrogen and phosphorus removal even at the exposure concentration of 100 mg L−1. Mechanism studies indicated that the sludge membrane integrity and viability and the respiration of both heterotrophic and autotrophic microorganisms were not affected by CNTs. Further experiments revealed that the presence of CNTs also did not change the transformations of intracellular metabolites (mainly glycogen and polyhydroxyalkanoates) and activities of key enzymes (mainly ammonia monooxygenase, nitrite oxidoreductase, nitrate reductase, nitrite reductase, exopolyphosphatase, and polyphosphate kinase), which was consistent with no observed influences on nitrogen and phosphorus removal.

Alkaline fermentation of waste sludge causes a significant reduction of antibiotic resistance genes in anaerobic reactors

Alkaline fermentation has been reported to be an effective method to recover valuable products from waste sludge. However, to date, the potential effect of alkaline pH on the fate of antibiotic resistance genes (ARGs) during anaerobic fermentation of sludge has never been documented. In this study, the target ARGs in sludge was observed to be removed effectively and stably when sludge was anaerobically fermented at pH10. Compared with the control (without pH adjustment), the abundances of target ARGs at pH10 were reduced by 0.87 (sulI), 1.36 (sulII), 0.42 (tet(O)), 1.11 (tet(Q)), 0.79 (tet(C)) and 1.04 (tet(X)) log units. Further investigations revealed that alkaline fermentation shifted the community structures of potential ARGs hosts. Moreover, alkaline fermentation remarkably decreased the quantities and the ARGs-possessing ability of genetic vectors (plasmid DNA, extracellular DNA and phage DNA), which might limit the transfer of ARGs via conjugation, transformation and transduction. These results suggest that the shifted compositions of gene hosts and restricted gene transfer potential might be the critical reasons for the attenuation of ARGs at pH10.

Long-term effects of engineered nanoparticles on enzyme activity and functional bacteria in wastewater treatment plants

The pervasive use of engineered nanoparticles (NPs) in a wide range of fields raises concerns about their potential environmental impacts. Previous studies confirmed that some NPs had already entered wastewater treatment plants (WWTPs). Wastewater nutrient removal depends on the metabolisms of activated sludge bacteria and their related key enzymes. Therefore, this study compared the possible influences of Al2O3, SiO2, TiO2, and ZnO NPs on the key enzymes activities and microbial community structures involved in wastewater treatment facilities. It was found that long-term exposure to these NPs significantly affected the microbial communities and changed the relative abundances of key functional bacteria, such as ammonia-oxidizing bacteria. Also, the gene expressions and catalytic activities of essential enzymes, such as ammonia monooxygenase, nitrite oxidoreductase, nitrate reductase, and nitrite reductase, were decreased, which finally resulted in a lower efficiency of biological nitrogen removal.

Carbon nanotubes affect the toxicity of CuO nanoparticles to denitrification in marine sediments by altering cellular internalization of nanoparticle

Denitrification is an important pathway for nitrate transformation in marine sediments, and this process has been observed to be negatively affected by engineered nanomaterials. However, previous studies only focused on the potential effect of a certain type of nanomaterial on microbial denitrification. Here we show that the toxicity of CuO nanoparticles (NPs) to denitrification in marine sediments is highly affected by the presence of carbon nanotubes (CNTs). It was found that the removal efficiency of total NOX−-N (NO3−-N and NO2−-N) in the presence of CuO NPs was only 62.3%, but it increased to 81.1% when CNTs appeared in this circumstance. Our data revealed that CuO NPs were more easily attached to CNTs rather than cell surface because of the lower energy barrier (3.5 versus 36.2 kT). Further studies confirmed that the presence of CNTs caused the formation of large, incompact, non-uniform dispersed, and more negatively charged CuO-CNTs heteroaggregates, and thus reduced the nanoparticle internalization by cells, leading to less toxicity to metabolism of carbon source, generation of reduction equivalent, and activities of nitrate reductase and nitrite reductase. These results indicate that assessing nanomaterial-induced risks in real circumstances needs to consider the “mixed” effects of nanomaterials.

Effects of titanium dioxide and zinc oxide nanoparticles on methane production from anaerobic co-digestion of primary and excess sludge.

Anaerobic co-digestion of primary and excess sludge is regarded as an efficient way to reuse sludge organic matter to produce methane. In this study, short-term and long-term exposure experiments were conducted to investigate the possible effects of titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles (NPs) on methane production from anaerobic co-digestion of primary and excess sludge. The data showed that TiO2 NPs had no measurable impact on methane production, even at a high concentration (150 mg/g total suspended solids (TSS)). However, short-term (8 days) exposure to 30 or 150 mg/g-TSS of ZnO NPs significantly decreased methane production. More importantly, these negative effects of ZnO NPs on anaerobic sludge co-digestion were not alleviated by increasing the adaptation time to 105 days. Further studies indicated that the presence of ZnO NPs substantially decreased the abundance of methanogenic archaea, which reduced methane production. Meanwhile, the activities of some key enzymes involved in methane production, such as protease, acetate kinase, and coenzyme F420, were remarkably inhibited by the presence of ZnO NPs, which was also an important reason for the decreased methane production. These results provide a better understanding of the potential risks of TiO2 and ZnO NPs to methane production from anaerobic sludge co-digestion.

Effect of CO 2 on NADH production of denitrifying microbes via inhibiting carbon source transport and its metabolism

The potential effect of CO2 on environmental microbes has drawn much attention recently. As an important section of the nitrogen cycle, biological denitrification requires electron donor to reduce nitrogen oxide. Nicotinamide adenine dinucleotide (NADH), which is formed during carbon source metabolism, is a widely reported electron donor for denitrification. Here we studied the effect of CO2 on NADH production and carbon source utilization in the denitrifying microbe Paracoccus denitrificans. We observed that NADH level was decreased by 45.5% with the increase of CO2 concentration from 0 to 30,000ppm, which was attributed to the significantly decreased utilization of carbon source (i.e., acetate). Further study showed that CO2 inhibited carbon source utilization because of multiple negative influences: (1) suppressing the growth and viability of denitrifier cells, (2) weakening the driving force for carbon source transport by decreasing bacterial membrane potential, and (3) downregulating the expression of genes encoding key enzymes involved in intracellular carbon metabolism, such as citrate synthase, aconitate hydratase, isocitrate dehydrogenase, succinate dehydrogenase, and fumarate reductase. This study suggests that the inhibitory effect of CO2 on NADH production in denitrifiers might deteriorate the denitrification performance in an elevated CO2 climate scenario.

Global transcriptional responses of denitrifying bacteria to functionalized single-walled carbon nanotubes revealed by weighted gene-coexpression network analysis

Functionalized single-walled carbon nanotubes (f-SWNTs) are widely used in many fields due to the unique structure and the excellent properties. Although these nanomaterials have been reported to enable to cause negative effects on denitrifying bacteria once they enter the environment, the toxic behaviors and regulatory mechanisms of f-SWNTs to denitrification remain unclear. In this study, the denitrification performance of a model denitrifier exposed to pristine and functionalized SWNTs was investigated, and the global transcriptional responses were comprehensively explored by RNA-seq and weighted gene-coexpression network analysis (WGCNA). Although both hydroxyl SWNTs (SWNTs-OH) and carboxyl SWNTs (SWNTs-COOH) showed inhibitory effects on bacterial denitrification, the former more severely inhibited denitrification than the latter. Transcriptional profiles showed that compared with SWNTs-COOH, SWNTs-OH much more strongly influenced the expressions of the key genes related to signal transduction, substance transport, electron transfer and transcriptional regulation. Functional analysis further indicated that the genes associated with substrate transport, carbon source metabolism and electron transfer underwent dramatic down-regulation. Using WGCNA, 12 gene modules were established corresponding to various types of carbon nanotubes, and eigengene adjacency analysis revealed the key gene modules related to denitrification performance under different conditions. Hub gene network analysis revealed the key regulatory factors of bacterial denitrification induced by f-SWNTs. The results suggested that f-SWNTs modulated the key genes responsible for the glycerolipid/free fatty acid (GL/FFA) cycle, and thus disturb processes associated with denitrification, including signaling process, energy homeostasis, intracellular redox balance and transportation.