Impacts of silver nanoparticle coating on the nitrification potential of Nitrosomonas europaea

Antibacterial Potential and Biocompatibility of Chitosan/Polycaprolactone Nanofibrous Membranes Incorporated with Silver Nanoparticles

This study addresses the need for enhanced antimicrobial properties of electrospun membranes, either through surface modifications or the incorporation of antimicrobial agents, which are crucial for improved clinical outcomes. In this context, chitosan-a biopolymer lauded for its biocompatibility and extracellular matrix-mimicking properties-emerges as an excellent candidate for tissue regeneration. However, fabricating chitosan nanofibers via electrospinning often challenges the preservation of their structural integrity. This research innovatively develops a chitosan/polycaprolactone (CH/PCL) composite nanofibrous membrane by employing a layer-by-layer electrospinning technique, enhanced with silver nanoparticles (AgNPs) synthesized through a wet chemical process. The antibacterial efficacy, adhesive properties, and cytotoxicity of electrospun chitosan membranes were evaluated, while also analyzing their hydrophilicity and nanofibrous structure using SEM. The resulting CH/PCL-AgNPs composite membranes retain a porous framework, achieve balanced hydrophilicity, display commendable biocompatibility, and exert broad-spectrum antibacterial activity against both Gram-negative and Gram-positive bacteria, with their efficacy correlating to the AgNP concentration. Furthermore, our data suggest that the antimicrobial efficiency of these membranes is influenced by the timed release of silver ions during the incubation period. Membranes incorporated starting with AgNPs at a concentration of 50 µg/mL effectively suppressed the growth of both microorganisms during the early stages up to 8 h of incubation. These insights underscore the potential of the developed electrospun composite membranes, with their superior antibacterial qualities, to serve as innovative solutions in the field of tissue engineering.

Long-term exposure of polytetrafluoroethylene-nanoplastics on the nitrogen removal and extracellular polymeric substances in sequencing batch reactor

The impact of polytetrafluoroethylene-nanoplastics (PTFE-NPs) on biological sewage disposal was delved, containing nitrogen remotion, microbiological activity and composition of extracellular polymer (EPS). The addition of PTFE-NPs reduced the removal efficiencies of chemical oxygen demand (COD) and ammonia nitrogen (NH₄⁺-N) by 3.43 % and 2.35 %, respectively. In comparison with no PTFE-NPs, the specific oxygen uptake rate (SOUR), specific ammonia oxidation rate (SAOR), specific nitrite oxidation rate (SNOR) and specific nitrate reduction rate (SNRR) decreased by 65.26 %, 65.24 %, 41.77 % and 54.56 %, respectively. The PTFE-NPs inhibited the activities of nitrobacteria and denitrobacteria. It was worth noting that, nitrite oxidized bacterium was more resistant to adverse environments than ammonia oxidizing bacterium. Compared with no PTFE-NPs, the reactive oxygen species (ROS) content and lactate dehydrogenase (LDH) grew by 130 % and 50 % under PTFE-NPs pressure. The appearance of PTFE-NPs affected the normal function of microorganisms by inducing endocellular oxidative stress and destroying the completeness of the cytomembrane. The protein (PN) and polysaccharide (PS) levels in loosely bound EPS (LB-EPS) and tightly bound EPS (TB -EPS) increased by 4.96, 0.70, 3.07 and 0.71 mg g^-1 VSS, under PTFE-NPs. Meanwhile, the PN/PS ratios of LB-EPS and TB -EPS increased from 6.18 and 6.41-11.04 and 9.29, respectively. The LB-EPS might provide sufficient binding sites for PTFE-NPs adsorption due to its loose and porous structure. The defense mechanism of bacteria against PTFE-NPs was mainly the PN in loosely bound EPS. Moreover, the functional groups referred to the complexation of EPS with PTFE-NPs were mainly related to N-H, CO, and C-N in proteins and O-H in polysaccharides.

Effect of metal and metal oxide engineered nano particles on nitrogen bio-conversion and its mechanism: A review

Metal and metal oxide engineered nano particles (MMO-ENPs) are widely applied in various industries due to their unique properties. Thus, many researches focused on the influence on nitrogen transformation processes by MMO-ENPs. This review focuses on the effect of MMO-ENPs on nitrogen fixation, nitrification, denitrification and Anammox. Firstly, based on most of the researches, it can be concluded MMO-ENPs have negative effect on nitrogen fixation, nitrification and denitrification while the MMO-ENPs have no promotion effect on Anammox. Then, the influence factors are discussed in detail, including MMO-ENPs dosage, MMO-ENPs kind and exposure time. Both the microbial morphology and population structure were altered by MMO-ENPs. Also, the mechanisms of MMO-ENPs affecting the nitrogen transformation are reviewed. The inhibition of key enzymes and functional genes, the promotion of reactive oxygen species (ROS) production, MMO-ENPs themselves and the suppression of electron transfer all contribute to the negative effect. Finally, the key points for future investigation are proposed that more attention should be attached to the effect on Anammox and the further mechanism in the future studies.

Combined impact of TiO2 nanoparticles and antibiotics on the activity and bacterial community of partial nitrification system

The effects of TiO2 nanoparticles (nano-TiO2) together with antibiotics leaking into wastewater treatment plants (WWTPs), especially the partial nitrification (PN) process remain unclear. To evaluate the combined impact and mechanisms of nano-TiO2 and antibiotics on PN systems, batch experiments were carried out with six bench-scale sequencing batch reactors. Nano-TiO2 at a low level had minimal effects on the PN system. In combination with tetracycline and erythromycin, the acute impact of antibiotics was enhanced. Both steps of nitrification were retarded due to the decrease of bacterial activity and abundance, while nitrite-oxidizing bacteria were more sensitive to the inhibition than ammonia-oxidizing bacteria. Proteobacteria at the phylum level and Nitrosospira at the genus level remained predominant under single and combined impacts. The flow cytometry analysis showed that nano-TiO2 enhanced the toxicity of antibiotics through increasing cell permeability. Our results can help clarify the risks of nano-TiO2 combined with antibiotics to PN systems and explaining the behavior of nanoparticles in WWTPs.

Stability and Microbial Toxicity of Silver Nanoparticles under Denitrifying Conditions

While the antibacterial effect of silver nanoparticles (AgNPs) on environmentally beneficial microbes has drawn considerable attention, the stability and microbial toxicity of AgNPs in a system where nitrate reduction is the dominant terminal electron-accepting process remain understudied. Here, we explore the impact of citrate-coated AgNPs (cit-AgNPs) on the growth and metabolism of two metal-sensitive and one nonsensitive bacterial strains under denitrifying conditions. Dose-response analysis revealed that in contrast to the bacteriostatic effect exhibited at 1 ppm, 5 ppm cit-AgNPs were bactericidal to the metal-sensitive strains. It was observed that the growth of the cells initiated Ag(I) formation, and the supplement of chloride (2.7 mM) to the cultures substantially mitigated the bactericidal capacity of cit-AgNPs, indicating that AgNP dissolution to ionic Ag(I) played a key role in AgNP toxicity. Abiotic experiments confirmed that nitrite, not nitrate, had the capacity to oxidize cit-AgNPs. Transcriptomic analysis revealed that (i) the gene encoding for membrane stress was upregulated proportionally to cit-AgNP concentrations; (ii) cit-AgNPs and Ag(I) at higher levels upregulated genes involved in oxidative stress and iron-sulfur clusters, whereas expressions of the genes responsible for electron transport, ATP synthesis, and denitrification were substantially repressed; (iii) the addition of chloride significantly altered the level of transcriptional profiles of all of the genes. These results not only provide evidence of abiotic AgNP oxidation by metabolic intermediate nitrogen species but also suggest that AgNPs and Ag(I) may induce differential toxicity modes to prokaryotes. Our findings reinforce the importance of evaluating the potential ecological toxicity and risks associated with the transformation of nanomaterials.

The effects of engineered nanoparticles on nitrification during biological wastewater treatment

Technological advancements in the past few decades have made it possible to manufacture nanomaterials at a large scale, and engineered nanoparticles (ENPs) are increasingly found in consumer products, such as cosmetics, sports products, and LED displays. A large amount of these ENPs end up in wastewater and potentially impact the performance of wastewater treatment plants (WWTPs). One important function of the WWTP is nitrification, which is carried out by the actions of two groups of bacteria, ammonia-oxidizing bacteria (AOB), and nitrite-oxidizing bacteria (NOB). Since most ENPs are found to have or are designed to have antimicrobial activities, it is a legitimate concern that ENPs entering WWTPs may have negative impacts on nitrification. In this paper, the effects of ENPs on nitrification are discussed, focusing mainly on autotrophic nitrification by AOBs and NOBs. This review also covers ENP effects on anaerobic ammonium oxidation (anammox). Generally, nitrifiers in pure and mixed cultures can be inhibited by a variety of ENPs, but stress response mechanisms may attenuate toxicity. Long-term studies demonstrated that a wide range of NPs could cause severe deterioration of AOBs and/or NOBs when the influent concentration exceeded an inhibition threshold. Proposed mechanisms include the generation of reactive oxygen species, dissolved metals, physical disruption of cell membranes, bacterial engulfment, and intracellular accumulation of ENPs. Future research needs are also discussed.

Interaction of Differently Coated Silver Nanoparticles With Skin and Oral Mucosal Cells

Silver nanoparticles (AgNP) can be found in different consumer products and various medical devices due to their excellent biocidal properties. Despite extensive scientific literature reporting biological effects of AgNP, there is still a lack of scientific evidence on how different surface functionalization affects AgNP interaction with the human skin and the oral epithelium. This study aimed to investigate biological consequences following the treatment of HaCaT and TR146 cells with AgNP stabilized with negatively charged sodium bis(2-ethylhexyl)-sulfosuccinate (AOT), neutral polyvinylpyrrolidone (PVP), and positively charged poly-l-lysine (PLL). All AgNP were characterized by means of size, shape and surface charge. Interactions with biological barriers were investigated in vitro by determining cell viability, particle uptake, oxidative stress response and DNA damages following AgNP treatment. Results showed a significant difference in cytotoxicity depending on the surface coating used for AgNP stabilization. All three types of AgNP induced apoptosis, oxidative stress response and DNA damages in cells, but AOT- and PVP-coated AgNP exhibited lower toxicity than positively charged PLL-AgNP. Considering the number of data gaps related to the safe use of nanomaterials in biomedicine, this study highlights the importance of particle surface functionalization that should be considered during design and development of future AgNP-based medical products.

Effects of silver nanoparticles on coupled nitrification-denitrification in suspended sediments

The effects of varying concentrations of Ag NPs on coupled nitrification and denitrification (CND) in two suspended sediments (SPSs) sizes were investigated using isotopic tracer method. In general, 0.5 and 5 mg/L Ag NPs had less effect on CND, while 2 and 10 mg/L Ag NPs exhibited the improvement and inhibition effect, respectively. The CND improvement by 2 mg/L NPs was mainly due to the enhanced nitrifying and denitrifying enzyme activity. However, 10 mg/L Ag NPs inhibited NH₄⁺ oxidation by directly reducing the AMO activity and AOB abundance. The inhibition on NAR and NIR activity and their encoding narG and nirK gene abundance further inhibited NO₃^- and NO₂^- reduction, leading to a dramatic decrease in the ¹⁵N-N₂ production. The above inhibition effects were attributed to the nano-effects of Ag NPs, which led to the excessive ROS amount and the decreased T-AOC level in microbial systems. But the connection between nitrification and denitrification was not broken after Ag NPs exposure. Moreover, the results indicated that N-cycling in clay and silt-type SPS systems could be more sensitive than sand-type SPS systems to NP exposure. The findings provide a basis for evaluating the environmental risks of Ag NPs in water-sediment systems.

Effects of PVP-coated silver nanoparticles on enzyme activity, bacterial and archaeal community structure and function in a yellow-brown loam soil

The undesirable effects of silver nanoparticles (AgNPs) on soil environment have caused much concern. The previous studies, however, focused on sandy soil, with little known on others. In present study, the effects of polyvinylpyrrolidone-coated AgNPs (0, 1, 10, and 100 mg kg^- 1 soil) on enzyme activities (urease and dehydrogenase), ammonia-oxidizing bacteria (AOB) and archaea (AOA), bacterial and archaeal communities, and microbial function profile in a yellow-brown loam soil were investigated. The significant dose-response inhibitions of AgNPs on enzyme activities were observed, with dehydrogenase more susceptible to AgNPs. Both of bacterial and archaeal amoA genes were reduced by AgNPs above 10 mg kg^- 1, with AOB more susceptible to AgNPs than AOA. AgNPs at 100 mg kg^- 1 caused reductions on the dominant Nitrosospira and Nitrosomonas, and even disappearance on Nitrosovibrio, while increase on Nitrososphaera significantly. AgNPs also changed bacterial and archaeal community structure. Exposure to AgNPs at 100 mg kg^- 1 caused significant increases by 186.79% and 44.89% for Bacteroidetes and Proteobacteria, while decreases by 47.82%, 44.09%, 43.67%, and 80.44% for Actinobacteria, Chloroflexi, Planctomycetes, and Verrucomicrobia, respectively. Moreover, three dominant archaeal phyla (Thaumarchaeota, Euryarchaeota, and Parvarchaeota) were also reduced in the presence of AgNPs, especially Thaumarchaeota with the significant reduction of 13.71%. PICRUSt prediction revealed that AgNPs indeed had the potential to change soil microbial community's functional contributions. It must be cautious on the interference of AgNPs to soil ecological functions in the future.

Underlying Promotion Mechanism of High Concentration of Silver Nanoparticles on Anammox Process

Silver nanoparticles (AgNPs) are largely discharged into sewers and mostly accumulated in the sediments and sludge. The toxicity of AgNPs to environmental microorganisms has attracted great attention. However, the effect of AgNPs on anaerobic ammonium-oxidizing (anammox) granules remains unknown. Here we present the underlying promotion mechanism of AgNPs on anammox granules from a morphological and molecular biology perspective. Our results demonstrate a positive effect of AgNPs on the proliferation of anammox bacteria. AgNPs resulted in a change in the three-dimensional structure of anammox granules and led to larger pore size and higher porosity. In addition, the diffusion capacity of the substrate and metal ions was enhanced. Furthermore, the expression of anammox-related enzymes, such as nitrite oxidoreductase (NirS), hydrazine dehydrogenase (Hdh), and hydrazine synthase (HZS), was upregulated. Therefore, the growth rate and the nitrogen removal performance of the anammox granules were improved. Our findings clarify the underlying mechanism of AgNPs on anammox granules and provide a promising method for the treatment of AgNPs-rich wastewater.

An Assessment of the Effect of Green Synthesized Silver Nanoparticles Using Sage Leaves (Salvia officinalis L.) on Germinated Plants of Maize (Zea mays L.)

AgNPs have attracted considerable attention in many applications including industrial use, and their antibacterial properties have been widely investigated. Due to the green synthesis process employed, the nanoparticle surface can be coated with molecules with biologically important characteristics. It has been reported that increased use of nanoparticles elevates the risk of their release into the environment. However, little is known about the behaviour of AgNPs in the eco-environment. In this study, the effect of green synthesized AgNPs on germinated plants of maize was examined. The effects on germination, basic growth and physiological parameters of the plants were monitored. Moreover, the effect of AgNPs was compared with that of Ag(I) ions in the form of AgNO₃ solution. It was found that the growth inhibition of the above-ground parts of plants was about 40%, and AgNPs exhibited a significant effect on photosynthetic pigments. Significant differences in the following parameters were observed: weights of the caryopses and fresh weight (FW) of primary roots after 96 h of exposure to Ag(I) ions and AgNPs compared to the control and between Ag compounds. In addition, the coefficient of velocity of germination (CVG) between the control and the AgNPs varied and that between the Ag(I) ions and AgNPs was also different. Phytotoxicity was proved in the following sequence: control < AgNPs < Ag(I) ions.

Responses of flocculated activated sludge to bimetallic Ag-Fe nanoparticles toxicity: Performance, activity enzymatic, and bacterial community shift

Ever-increasing production and use of nanoparticles (NPs) have aroused overarching concerns for their toxic effects on the environment and human. In the present study, the toxic effects of Silver (Ag) and Iron (Fe) NPs on the performance of activated sludge were investigated under continuous aerobic/anoxic/anaerobic conditions in laboratory-scale sequencing batch reactors (SBRs).Activated sludge was exposed to various concentrations (5-100 mg/L) of Ag-Fe NPs for 60 days and its response was assessed through the enzymatic activity, COD, nitrogen (TN) and phosphorus (TP) removal, toxicity tests, as well as variations in bacterial community. Compared with the pristine control sample, the exposure to NPs suppressed TN and TP removal efficiencies. Indeed, the respiration rate and biomass concentration were significantly affected by the NPs. Although the simultaneous exposure to Ag-Fe NPs did affect the integrity of cell membrane (LDH) and key enzymes activities, the higher concentration induced an increased generation of reactive oxygen species (ROS). The metagenome analysis revealed a marked shift in the microbial community structure suggesting that both heterotrophic and autotrophic communities were affected by the presence of Ag-Fe NPs. Our results provide some evidence for compounded effects of NPs in their simultaneous presence, and generate new leads for future research efforts.

Size-dependent cytotoxicity of silver nanoparticles to Azotobacter vinelandii: Growth inhibition, cell injury, oxidative stress and internalization

The influence of nanomaterials on the ecological environment is becoming an increasingly hot research field, and many researchers are exploring the mechanisms of nanomaterial toxicity on microorganisms. Herein, we studied the effect of two different sizes of nanosilver (10 nm and 50 nm) on the soil nitrogen fixation by the model bacteria Azotobacter vinelandii. Smaller size AgNPs correlated with higher toxicity, which was evident from reduced cell numbers. Flow cytometry analysis further confirmed this finding, which was carried out with the same concentration of 10 mg/L for 12 h, the apoptotic rates were20.23% and 3.14% for 10 nm and 50 nm AgNPs, respectively. Structural damage to cells were obvious under scanning electron microscopy. Nitrogenase activity and gene expression assays revealed that AgNPs could inhibit the nitrogen fixation of A. vinelandii. The presence of AgNPs caused intracellular reactive oxygen species (ROS) production and electron spin resonance further demonstrated that AgNPs generated hydroxyl radicals, and that AgNPs could cause oxidative damage to bacteria. A combination of Ag content distribution assays and transmission electron microscopy indicated that AgNPs were internalized in A. vinelandii cells. Overall, this study suggested that the toxicity of AgNPs was size and concentration dependent, and the mechanism of antibacterial effects was determined to involve damage to cell membranes and production of reactive oxygen species leading to enzyme inactivation, gene down-regulation and death by apoptosis.

Potential Clinical Risk of Inflammation and Toxicity from Rare-Earth Nanoparticles in Mice

Background:

Nanotechnology is emerging as a promising tool to perform noninvasive therapy and optical imaging. However, nanomedicine may pose a potential risk of toxicity during in vivo applications. In this study, we aimed to investigate the potential toxicity of rare-earth nanoparticles (RENPs) using mice as models.

Methods:

We synthesized RENPs through a typical co-precipitation method. Institute of Cancer Research (ICR) mice were randomly divided into seven groups including a control group and six experimental groups (10 mice per group). ICR mice were intravenously injected with bare RENPs at a daily dose of 0, 0.5, 1.0, and 1.5 mg/kg for 7 days. To evaluate the toxicity of these nanoparticles in mice, magnetic resonance imaging (MRI) was performed to assess their uptake in mice. In addition, hematological and biochemical analyses were conducted to evaluate any impairment in the organ functions of ICR mice. The analysis of variance (ANOVA) followed by a one-way ANOVA test was used in this study. A repeated measures' analysis was used to determine any significant differences in white blood cell (WBC), alanine aminotransferase (ALT), and creatinine (CREA) levels at different evaluation times in each group.

Results:

We demonstrated the successful synthesis of two different sizes (10 nm and 100 nm) of RENPs. Their physical properties were characterized by transmission electron microscopy and a 980 nm laser diode. Results of MRI study revealed the distribution and circulation of the RENPs in the liver. In addition, the hematological analysis found an increase of WBCs to (8.69 ± 0.85) × 10⁹/L at the 28^th day, which is indicative of inflammation in the mouse treated with 1.5 mg/kg NaYbF₄:Er nanoparticles. Furthermore, the biochemical analysis indicated increased levels of ALT ([64.20 ± 15.50] U/L) and CREA ([27.80 ± 3.56] μmol/L) at the 28^th day, particularly those injected with 1.5 mg/kg NaYbF₄:Er nanoparticles. These results suggested the physiological and pathological damage caused by these nanoparticles to the organs and tissues of mice, especially to liver and kidney.

Conclusion:

The use of bare RENPs may cause possible hepatotoxicity and nephritictoxicity in mice.

Effects of short- and long-term exposure of silver nanoparticles and silver ions to Nitrosomonas europaea biofilms and planktonic cells

The increasing use of silver nanoparticles (AgNPs) in consumer products, and their resulting influx into wastewater, may pose a threat to biological nutrient removal in wastewater treatment plants. Planktonic ammonia-oxidizing bacteria (AOB), which convert ammonia to nitrite in the first step of nitrification, are highly sensitive to AgNPs and their released silver ions (Ag⁺), but the sensitivity of AOB biofilms to AgNPs and Ag⁺ is less clear. This study demonstrated that biofilms of Nitrosomonas europaea, a model AOB, were more resistant to both short-term and long-term exposure to AgNP and Ag⁺ than planktonic cells. The increased resistance of N. europaea biofilms was attributed primarily to the increased biomass and slower growth rates present in the biofilm. Similar inhibition mechanisms were observed for AgNPs and Ag⁺ in both planktonic cells and biofilms with enzymatic inhibition observed at lower concentrations and cell lysis observed at higher concentrations. Long-term continuous exposure to AgNPs lowered the inhibitory concentration by 1-2 orders of magnitude below that required by short-term exposures. Although the total AgNP load was similar between the short and long-term exposure scenarios, the long-term exposure resulted in an order of magnitude more silver being associated in the biofilms and is the primary reason for the increased sensitivity observed. This suggests that short-term batch toxicity assays may greatly underestimate the sensitivity of biofilm treatment systems to long-term exposures of low concentrations of AgNPs and Ag⁺.

Effects of metal oxide nanoparticles on nitrification in wastewater treatment systems: A systematic review

While the variety of engineered nanoparticles used in consumer products continues to grow, the use of metal oxide nanoparticles in electronics, textiles, cosmetics and food packaging industry has grown exponentially in recent years, which will inevitably result in their release into wastewater streams in turn impacting the important biological processes in wastewater treatment plants. Among these processes, nitrification play a critical role in nitrogen removal during wastewater treatment, however, it is sensitive to a wide range of inhibitory substances including metal oxide nanoparticles. Therefore, it is essential to systematically asses the effects of metal oxide nanoparticles on nitrification in biological wastewater treatment systems. In this review we discuss the present scenario of metal oxide nanoparticles and their impact on biological wastewater treatment processes, specifically nitrogen removal through nitrification. We also summarize the various methods used to measure nitrification inhibition by metal oxide nanoparticles and highlight corresponding results obtained using those methods. Finally, the key research gaps that need to be addressed in future are discussed.

Shifts of system performance and microbial community structure in a constructed wetland after exposing silver nanoparticles

The increasing utilization of silver nanoparticles (Ag NPs) in industry and commerce inevitably raises its release into wastewater. In this work, effects of Ag NPs on system performance and microbial community along the way of a vertical flow constructed wetland (VFCW) were investigated, along with the removal and fate of Ag NPs within the system. Results showed that the performance of control wetland kept stable during the experimental period, and the top substrate layer (soil layer) of wetland could remove most of pollutants in the influent. The study also suggested that addition of Ag NPs did not significantly affect organic matters removal. However, adverse effects were observed on the nitrogen and phosphorus removal. Removal efficiencies of TN, NH₄⁺-N and TP approximately obviously reduced by approximately 10.10%, 8.42% and 28.35% respectively in contrast to before dosing after exposing 100 μg/L Ag NPs for 94 d, while the no dosing wetland with the stable performance. It was found that Ag NPs accumulated in the upper soil layer more than in the lower soil layer, and Ag NPs could enter into the plant tissues. After continuous input of Ag NPs, removal efficiency of Ag NPs was measured as 95.72%, which showed that the CW could effectively remove Ag NPs from the wastewater. The high-throughput sequencing results revealed that Ag NPs caused the shifts in microbial community structures and changed the relative abundances of key functional bacteria, which finally resulted in a lower efficiency of biological nitrogen and phosphorus removal.

Metagenomic analysis of microbial communities yields insight into impacts of nanoparticle design

Next-generation DNA sequencing and metagenomic analysis provide powerful tools for the environmentally friendly design of nanoparticles. Herein we demonstrate this approach using a model community of environmental microbes (that is, wastewater-activated sludge) dosed with gold nanoparticles of varying surface coatings and morphologies. Metagenomic analysis was highly sensitive in detecting the microbial community response to gold nanospheres and nanorods with either cetyltrimethylammonium bromide or polyacrylic acid surface coatings. We observed that the gold-nanoparticle morphology imposes a stronger force in shaping the microbial community structure than does the surface coating. Trends were consistent in terms of the compositions of both taxonomic and functional genes, which include antibiotic resistance genes, metal resistance genes and gene-transfer elements associated with cell stress that are relevant to public health. Given that nanoparticle morphology remained constant, the potential influence of gold dissolution was minimal. Surface coating governed the nanoparticle partitioning between the bioparticulate and aqueous phases.

Microbial community response to silver nanoparticles and Ag+ in nitrifying activated sludge revealed by ion semiconductor sequencing

Silver nanoparticles (AgNPs), which are known to act as biocides, are incorporated into medical and consumer products including athletic clothing, stuffed animals, liquid dietary supplements, and more. The increasing use of AgNPs in these products is likely to lead to their entry into both natural and engineered systems, which has the potential to disrupt bacterial processes including those involved in nutrient cycling in wastewater treatment. In the present study, sequencing batch reactors (SBR) mimicking secondary wastewater treatment were operated to determine the effects of AgNPs on the microbial communities contained within activated sludge of wastewater treatment plants (WWTP). SBRs were treated with 0.2 and 2ppm of either gum Arabic (GA)-coated AgNPs, citrate (Ca)-coated AgNPs, or Ag⁺ as AgNO₃. Cell samples were collected and DNA isolated periodically throughout SBR operation. DNA was used for Ion Torrent Next Gen Sequencing of the V3 region of the 16S rDNA gene. Subsequent analyses revealed that the microbial community both shifted and recovered quickly in response to Ag⁺. Both AgNP treatments resulted in slower initial community shifts than that observed with the Ag⁺ treatment. GA-AgNPs elicited the longest lasting effect. Additional examination of nitrogen removal bacteria suggested the possibility of an increase in sludge bulking species with increased concentrations of AgNPs in WWTPs. This study supports the hypothesis that Ag⁺ release from AgNPs is largely coating-dependent and thus a key driver in dictating AgNP toxicity.

Examining relationships between total silver concentration and Sil silver resistance genes in domestic wastewater treatment plants

AIMS

The overarching aims of this study were to determine if microbes found in wastewater treatment plants (WWTPs) carry silver resistance genes and ascertain whether a relationship exists between silver loading and sil silver resistance gene copy numbers.

METHODS AND RESULTS

Activated sludge samples were collected from 17 WWTPs across the United States and characterized for chemical oxygen demand (COD) and total silver concentration. Gene copy numbers were quantified using qPCR for four common silver resistance genes (silC, silP, silR and silS). Spearman correlation analyses were performed to examine associations between three WWTP characteristics (i.e. size, COD and total silver concentration) and sil gene copy numbers. sil genes were found in all activated sludge samples, however the quantity of gene copy numbers was inconsistent. Of the sil genes screened, silS was the most abundant, followed by silR. Gene copy numbers for silC and silP were generally similar and overall lower than silS and silR.

CONCLUSIONS

Neither individual or combined sil gene copy numbers correlated significantly with total silver concentration, suggesting either the presence of alternate mechanisms of silver resistance or that a critical silver concentration threshold within WWTPs was not met that would require specific resistance against silver.

SIGNIFICANCE AND IMPACT OF THIS STUDY

To the authors' knowledge, this is one of the first studies to measure the prevalence of sil genes in engineered systems. Results from this study suggest that at least four silver resistance genes are commonly found within WWTP-activated sludge, however it is unclear if these genes are being expressed under current total silver loadings.

Silver nanoparticles stimulate the proliferation of sulfate reducing bacterium Desulfovibrio vulgaris

The intensive use of silver nanoparticles (AgNPs) in cosmetics and textiles causes their release into sewer networks of urban water systems. Although a few studies have investigated antimicrobial activities of nanoparticles against environmental bacteria, little is known about potential impacts of the released AgNPs on sulfate reducing bacteria in sewers. Here, we investigated the effect of AgNPs on Desulfovibrio vulgaris Hidenborough (D. vulgaris), a typical sulfate-reducing bacterium (SRB) in sewer systems. We found AgNPs stimulated the proliferation of D. vulgaris, rather than exerting inhibitory or biocidal effects. Based on flow cytometer detections, both the cell growth rate and the viable cell ratio of D. vulgaris increased during exposure to AgNPs at concentrations of up to 100 mg/L. The growth stimulation was dependent on the AgNP concentration. These results imply that the presence of AgNPs in sewage may affect SRB abundance in sewer networks. Our findings also shed new lights on the interactions of nanoparticles and bacteria.

Nanoscaled gold and silver: Simultaneous removal and transformation to functional materials

Based on an "acid-assisted cool welding" technology which was realized by virtue of the freezing process in the presence of acid, nanoscaled gold (Au) and silver (Ag) from wastewater could be removed very facilely and efficiently. The technology was independence of the freezing temperature, size as well as shape of those nanoscaled units. Besides, some functional materials like porous nanostructures with highly and stably catalytic activity could be also obtained during the removal. Our research not only provided a new method to remove nanoscaled Ag or Au from wastewater, but also built up a unique route to transform those nano-units into functional materials simultaneously.

Contradictory effects of silver nanoparticles on activated sludge wastewater treatment

Increased amount of nano-silver will be released into domestic and industrial waste streams due to its extensive application. However, great controversy still exists on the effects of silver nanoparticle (Ag-NP) on biological wastewater treatment processes and a toxicology model has not been built yet. Four sequencing batch reactors with activated sludge has been run for over three months with different silver species at a concentration of 1mg Ag/L in influent. Both freshly prepared Ag-NPs and aged Ag-NPs were tested with released silver ion as control. Results in this study showed that Ag-NPs, especially freshly prepared Ag-NPs, can help to maintain or even increase the diversity of microbial community in activated sludge and the biomass concentration even under long-term treatment. It indicates that the hormesis model need to be considered for the toxicology of Ag-NPs.

Bioaccumulation-based silver nanoparticle toxicity in Daphnia magna and maternal impacts

In the present study, we tested whether bioaccumulation in specific tissues of Daphnia magna could explain silver nanoparticle (AgNP) toxicity. Daphnids were exposed to different concentrations of well-suspended AgNPs and AgNO₃ . The accumulations of Ag in the whole body, gut, and nongut tissues, as well as the mortality of daphnids, were recorded over a period of 7 d. Regression analysis showed a higher degree of positive correlation between the concentration of Ag in the nongut tissues than gut tissues and the mortality of daphnids. The results strongly suggested that the toxicity of AgNPs could be better explained in terms of bioaccumulation of AgNPs in the nongut tissues. We further tested the maternal transfer of AgNPs in daphnids into the next generation using radioactive tracers, which were able to detect as low as 1.0 to 3.2% of total accumulated Ag transferred to the neonates. The AgNPs significantly affected the reproduction process during the first 2 broods after exposure, whereas AgNO₃ only had significant effects on the first brood. It is possible that AgNPs have longer adverse effects than AgNO₃ on the reproduction of Daphnia. The present study identified the sensitive site of AgNP toxic action in daphnids and documented the extent of maternal transfer and the significant influence of AgNPs on the reproduction of daphnids. Environ Toxicol Chem 2017;36:3359-3366. © 2017 SETAC.

Nitrogen rich core-shell magnetic mesoporous silica as an effective adsorbent for removal of silver nanoparticles from water

The production and increasing use of silver nanoparticles (AgNPs) obviously results in their release into the environment, leading to a risk to the environment due to their toxic effects. Thus, the removal of AgNPs from water is highly needed. Here, we demonstrate that nitrogen rich (∼10% nitrogen content) core-shell magnetic mesoporous silica is a promising adsorbent for the removal of AgNPs. For this, the poly(ethylenimine) functionalized core-shell magnetic mesoporous silica composites (Fe₃O₄@SiO₂-PEI) were prepared, and characterized by TEM, FT-IR, XRD, TG and N₂ adsorption-desorption. The removal of AgNPs by Fe₃O₄@SiO₂-PEI as a function of contact time, concentration of AgNPs, solution pH and ionic strength were studied. The adsorption kinetic data could be described by the pseudo-second-order rate model. Both Langmuir and Freundlich models fitted the adsorption data well. The adsorption capacity for AgNPs is 909.1mg/g, which is 5-181 times higher than that of the previously reported adsorbents for AgNPs. Interestingly, the silver adsorbed onto Fe₃O₄@SiO₂-PEI exhibits highly catalytic activity for 4-nitropheol (4-NP) reduction with a rate constant of 0.072min^-1, which is much higher than those by other AgNPs reported before. The silver-loaded Fe₃O₄@SiO₂-PEI promises good recyclability for at least five cycles, showing great potential in practical applications.

Effects of graphite nanoparticles on nitrification in an activated sludge system

Graphite nanoparticles (GNPs) might result in unexpected effects during their transportation and transformation in wastewater treatment systems, including strong thermo-catalytic and catalytic effects and microbial cytotoxicity. In particular, the effects of GNPs on the nitrification process in activated sludge systems should be addressed. This study aimed to estimate the influence of GNPs on the nitrification process in a short-term nitrification reactor with exposure to different light sources. The results indicated that GNPs could only improve the efficiency of photothermal transformation slightly in the activated sludge system because of its photothermal effects under the standard illuminant (imitating 1 × sun). However, even with better photothermal effects, the nitrification efficiency still decreased significantly with GNP dosing under the standard illuminant, which might result from stronger cytotoxic effects of GNPs on the nitrifying bacteria. The disappearance of extracellular polymeric substances (EPS) around bacterial cells was observed, and the total quantity of viable bacteria decreased significantly after GNP exposuring. Variation in bacterial groups primarily occurred in nitrifying microbial communities, including Nitrosomonas sp., Nitrosospira sp., Comamonas sp. and Bradyrhizobiace sp. Nitrifiers significantly decreased, while the phyla Gammaproteobacteria, Deinocccus, and Bacteroidetes exhibited greater stability during GNP treatment.

Aerobic condition enhances bacteriostatic effects of silver nanoparticles in aquatic environment: an antimicrobial study on Pseudomonas aeruginosa

The intensive applications of silver nanoparticles (AgNPs) inevitably cause continuous release of such materials into environments, as a consequence posing potential risks to microbial communities in engineered or natural ecosystems. However, the magnitude of antibacterial capacity of nanoparticles is still inconclusive, owing to influential factors such as the size of nanoparticle, microbial species, or environmental conditions. To reveal whether the presence of air would alter AgNPs ecotoxicity, Pseudomonas aeruginosa PAO1, a facultative denitrifying bacterium and an opportunity pathogen, was used to study antibacterial assays under both anaerobic and aerobic conditions. The results indicate that the respiration status of P. aeruginosa affect the ecotoxicity of AgNPs. P. aeruginosa cultured under aerobic condition were more susceptible to AgNPs than that under anaerobic condition. Aerobic condition greatly enhanced bacteriostatic effects of AgNPs but not their bactericidal effects, as the ratio of viable but nonculturable (VBNC) bacteria remained above 90% when 5 mg L⁻¹ AgNPs applied. Our findings offer further understanding for the degree of toxicity of nanoparticles on microbial ecosystems and underscore the importance of exposure condition (e.g. oxygen) in the mode of action of AgNPs.

Effects of silver nanoparticles on nitrification and associated nitrous oxide production in aquatic environments

Silver nanoparticles (AgNPs) are the most common materials in nanotechnology-based consumer products globally. Because of the wide application of AgNPs, their potential environmental impact is currently a highly topical focus of concern. Nitrification is one of the processes in the nitrogen cycle most susceptible to AgNPs but the specific effects of AgNPs on nitrification in aquatic environments are not well understood. We report the influence of AgNPs on nitrification and associated nitrous oxide (N₂O) production in estuarine sediments. AgNPs inhibited nitrification rates, which decreased exponentially with increasing AgNP concentrations. The response of nitrifier N₂O production to AgNPs exhibited low-dose stimulation (<534, 1476, and 2473 μg liter^-1 for 10-, 30-, and 100-nm AgNPs, respectively) and high-dose inhibition (hormesis effect). Compared with controls, N₂O production could be enhanced by >100% at low doses of AgNPs. This result was confirmed by metatranscriptome studies showing up-regulation of nitric oxide reductase (norQ) gene expression in the low-dose treatment. Isotopomer analysis revealed that hydroxylamine oxidation was the main N₂O production pathway, and its contribution to N₂O emission was enhanced when exposed to low-dose AgNPs. This study highlights the molecular underpinnings of the effects of AgNPs on nitrification activity and demonstrates that the release of AgNPs into the environment should be controlled because they interfere with nitrifying communities and stimulate N₂O emission.

An Efficient and Benign Antimicrobial Depot Based on Silver-Infused MoS2

Silver nanoparticles (AgNPs) have been used as a broad-spectrum antimicrobial agent, whose toxicity originates from the localized release of Ag⁺ ions. However, the residual AgNPs core could generate potential risk to humans and waste of noble metals. Herein, we infused the cysteine-modified molybdenum disulfide with minimum Ag⁺ ions and coated with a layer of cationic polyelectrolyte to construct an efficient and benign antimicrobial depot. The system exhibited much enhanced broad-spectrum antibacterial activity compared with an equivalent amount of silver nitrate, owing to its increasing accessibility of released Ag⁺ to the cell walls of microorganisms. More importantly, the antibacterial system could be successfully applied to treat wound infection, while retaining high antibacterial activities, exhibiting negligible biotoxicity and avoiding the waste of Ag.

Inhibition of an enriched culture of ammonia oxidizing bacteria by two different nanoparticles: Silver and magnetite

Metal and metal oxide nanoparticles are getting attention over the past years. They can be used to several purposes, especially in commercial and medical applications. Undoubtedly, this lead to higher production and, consequently, increasing the risks of exposition, once they can be released into environment without a proper control. However, their impact over the bacteria present in wastewater treatment plants (WWTP), manly over nitrifying bacteria, which are the most susceptible to toxic compounds, are still not very well established. Herein it was investigated the impact of silver nanoparticles (AgNP) and magnetite nanoparticles (FeNP), separately, over an ammonia oxidizing bacteria (AOB), during short-term exposure tests and it was also verified their impact on bacterial surface. The concentrations assessed were from 0 to 30mgAgNPL^-1 and from 0 to 1000mgFeNPL^-1. Results showed that AOB specific nitrite production rate reduced 90% when exposed to 30mgAgNPL^-1, and in almost 71% in the presence of 1000mgFeNPL^-1. The concentration necessary to reduce 50% of AOB activity was 10.75mgAgNPL^-1 and 483.01mgFeNPL^-1 highlighting that AgNP can be 45 times more toxic to AOB than FeNP. Both nanoparticles attached to bacterial surface, even in the lower concentration tested, hindering AOB activity due to changes in the membrane permeability. Once nanoparticles remain attached in the biological sludge, which is used as fertilizer to soil, they can affect not only WWTP performance but also hindering soil quality and the ecosystem balance.

Potential impacts of silver nanoparticles on bacteria in the aquatic environment

It is inevitable that nano-silver will be released into the environment. Therefore, there is an urgent need to better understand the effects of silver nanoparticles (Ag-NPs) on microbes in natural and engineered environments. The most remarkable gap in our knowledge on this lies on the low Ag-NPs dose side. This review summarized studies on the effects of Ag-NPs on bacteria from simple to complicated aquatic systems. A hormetic model with a narrow stimulatory zone has been proposed based on both experimental phenomenon and the potential mechanisms of the observed effects. Spectrum of the stimulating zone depends on Ag-NP properties, bacterial types and environmental conditions tested. This may become a concern in terms of Ag-NP disposal, and further research is required to build a sophisticated toxicity model for Ag-NPs.

Influence of silver nanoparticles and liberated silver ions on nitrifying sludge: ammonia oxidation inhibitory kinetics and mechanism

Silver nanoparticles (AgNPs) are widely used in commercial products because of their excellent antimicrobial activity. Entrance of AgNPs and its released Ag ions (Ag⁺) into wastewater treatment plants could harm ammonia oxidation (AO) process resulting in environmental problems. This study investigated inhibitory kinetics and mechanism of AO from nitrifying sludge influenced by AgNPs and Ag⁺. The findings demonstrated that AgNPs and Ag⁺ adversely influenced on AO. Silver ions were more toxic to AO than AgNPs, which was indicated by the lower inhibitory constant (K _i ) of 0.29 mg/L compared to that of AgNPs (K _i of 73.5 mg/L). Over the experimental period of 60 h, AgNPs at 1, 10, and 100 mg/L released Ag⁺ in the average concentrations of 0.059, 0.171, and 0.503 mg/L, respectively. Silver nanoparticles of 1-100 mg/L inhibited AO by 45-74%, whereas Ag⁺ of 0.05-0.50 mg/L inhibited AO by 53-94%. This suggested that the AgNP toxicity mainly derived from the liberated Ag⁺. Scanning electron microscopy results revealed that AgNPs attached on microbial cell surfaces, and both AgNPs and Ag⁺ induced cell morphological change from rod shape to shorter rod shape. Transmission electron microscopy showed that AgNPs and Ag⁺ diminished the thickness of the outer layer and reduced the density of internal parts of the exposed microbial cells, which could be the reasons for the morphology change. Live/dead results also confirmed that AgNPs and Ag⁺ damaged membrane integrity of cells in the nitrifying sludge. This study suggested that the primary mechanism for toxicity of AgNPs was the liberation of Ag⁺ and then both of silver species caused cell death.

The effect of chronic silver nanoparticles on aquatic system in microcosms

Silver nanoparticles (AgNPs) inevitably discharge into aquatic environments due to their abundant use in antibacterial products. It was reported that in laboratory conditions, AgNPs display dose-dependent toxicity to aquatic organisms, such as bacteria, algae, macrophytes, snails and fishes. However, AgNPs could behave differently in natural complex environments. In the present study, a series of microcosms were established to investigate the distribution and toxicity of AgNPs at approximately 500 μg L^-1 in aquatic systems. As a comparison, the distribution and toxicity of the same concentration of AgNO₃ were also determined. The results showed that the surface layer of sediment was the main sink of Ag element for both AgNPs and AgNO₃. Both aquatic plant (Hydrilla verticillata) and animals (Gambusia affinis and Radix spp) significantly accumulated Ag. With short-term treatment, phytoplankton biomass was affected by AgNO₃ but not by AgNPs. Chlorophyll content of H. verticillata increased with both AgNPs and AgNO₃ short-term exposure. However, the biomass of phytoplankton, aquatic plant and animals was not significantly different between control and samples treated with AgNPs or AgNO₃ for 90 d. The communities, diversity and richness of microbes were not significantly affected by AgNPs and AgNO₃; in contrast, the nitrification rate and its related microbe (Nitrospira) abundance significantly decreased. AgNPs and AgNO₃ may affect the nitrogen cycle and affect the environment and, since they might be also transferred to food web, they represent a risk for health.

Removal of silver nanoparticles by mussel-inspired Fe3O4@ polydopamine core-shell microspheres and its use as efficient catalyst for methylene blue reduction

The removal of silver nanoparticles (AgNPs) from water is highly needed because of their increasing use and potential risk to the environment due to their toxic effects. Catalysis over AgNPs has received significant attention because of their highly catalytic performance. However, their use in practical applications is limited due to high cost and limited resources. Here, we present for the first time that the mussel-inspired Fe3O4@polydopamine (Fe3O4@PDA) nanocomposite can be used for efficient removal and recovery of AgNPs. Adsorption of AgNPs over Fe3O4@PDA was confirmed by TEM, FT-IR, XRD, TGA and magnetic property. The adsorption efficiency of AgNPs by Fe3O4@PDA was investigated as a function of pH, contact time, ionic strength and concentration of AgNPs. The kinetic data were well fitted to a pseudo-second order kinetic model. The isotherm data were well described by Langmuir model with a maximum adsorption capacity of 169.5 mg/g, which was higher than those by other adsorbents. Notably, the obtained AgNPs-Fe3O4@PDA exhibited highly catalytic activity for methylene blue reduction by NaBH4 with a rate constant of 1.44 × 10-3/s, which was much higher than those by other AgNPs catalysts. The AgNPs-Fe3O4@PDA promised good recyclability for at least 8 cycles and acid resistant with good stability.

Governing factors affecting the impacts of silver nanoparticles on wastewater treatment

Silver nanoparticles (nanosilver or AgNPs) enter municipal wastewater from various sources, raising concerns about their potential adverse effects on wastewater treatment processes. We argue that the biological effects of silver nanoparticles at environmentally realistic concentrations (μgL^-1 or lower) on the performance of a full-scale municipal water resource recovery facility (WRRF) are minimal. Reactor configuration is a critical factor that reduces or even mutes the toxicity of silver nanoparticles towards wastewater microbes in a full-scale WRRF. Municipal sewage collection networks transform silver nanoparticles into silver(I)-complexes/precipitates with low ecotoxicity, and preliminary/primary treatment processes in front of biological treatment utilities partially remove silver nanoparticles to sludge. Microbial functional redundancy and microbial adaptability to silver nanoparticles also greatly alleviate the adverse effects of silver nanoparticles on the performance of a full-scale WRRF. Silver nanoparticles in a lab-scale bioreactor without a sewage collection system and/or a preliminary/primary treatment process, in contrast to being in a full scale system, may deteriorate the reactor performance at relatively high concentrations (e.g., mgL^-1 levels or higher). However, in many cases, silver nanoparticles have minimal impacts on lab-scale bioreactors, such as sequencing batch bioreactors (SBRs), especially when at relatively low concentrations (e.g., less than 1mgL^-1). The susceptibility of wastewater microbes to silver nanoparticles is species-specific. In general, silver nanoparticles have higher toxicity towards nitrifying bacteria than heterotrophic bacteria.

Influence of Silver nanoparticles on nutrient removal and microbial communities in SBR process after long-term exposure

The widespread utilization of silver nanoparticles (AgNPs) in industrial and commercial products inevitably raises the release into wastewater that might cause potential negative impacts on sewage treatment system. In this paper, long-term exposure experiments at four levels were conducted to determine whether AgNPs caused adverse impacts on nutrient removals in sequencing batch reactors (SBRs) and changes of microbial community structure. Compared with the control reactor (without AgNPs), carbon, nitrogen and phosphorus removal in presence of 0.1mg/L AgNPs was no difference. However, presence of 1.0 and 10mg/L AgNPs decreased the average removal efficiencies of COD from 95.4% to 85.2% and 68.3%, ammonia nitrogen from 98.8% to 71.2% and 49%, SOP from 97.6% to 75.5% and 54.1%, respectively. It was found that AgNPs could accumulate in sludge with the distribution coefficients of 39.2-114L/g, inhibit the protein and polysaccharide production in EPS, reduce the SOUR of sludge, and greatly increase LDH release from microbial cells. The illumina high-throughput sequencing results indicated that AgNPs concentration changed the structures of bacterial communities, associating with the effects of AgNPs on reactor performance. Sequence analyses showed that Proteobacteria, Bacteroidetes and Acidobacteria were the dominant phyla. It was notable that AgNPs addition reduced the contents of several nitrifying bacteria at genera level in sludge, leading to the lower removal of nitrogen.

Attenuation of Microbial Stress Due to Nano-Ag and Nano-TiO2 Interactions under Dark Conditions

Engineered nanomaterials (ENMs) are incorporated into thousands of commercial products, and their release into environmental systems creates complex mixtures with unknown toxicological outcomes. To explore this scenario, we probe the chemical and toxicological interactions of nanosilver (n-Ag) and nanotitania (n-TiO₂) in Lake Michigan water, a natural aqueous medium, under dark conditions. We find that the presence of n-Ag induces a stress response in Escherichia coli, as indicated by a decrease in ATP production observed at low concentrations (in the μg L^-1 range), with levels that are environmentally relevant. However, when n-Ag and n-TiO₂ are present together in a mixture, n-TiO₂ attenuates the toxicity of n-Ag at and below 20 μg L^-1 by adsorbing Ag⁺_(aq). We observe, however, that toxic stress cannot be explained by dissolved silver concentrations alone and, therefore, must also depend on silver associated with the nanoscale fraction. Although the attenuating effect of n-TiO₂ on n-Ag's toxicity is limited, this study emphasizes the importance of probing the toxicity of ENM mixtures under environmental conditions to assess how chemical interactions between nanoparticles change the toxicological effects of single ENMs in unexpected ways.

Quantifying the Sensitivity of Soil Microbial Communities to Silver Sulfide Nanoparticles Using Metagenome Sequencing

Soils are a sink for sulfidised-silver nanoparticles (Ag₂S-NPs), yet there are limited ecotoxicity data for their effects on microbial communities. Conventional toxicity tests typically target a single test species or function, which does not reflect the broader community response. Using a combination of quantitative PCR, 16S rRNA amplicon sequencing and species sensitivity distribution (SSD) methods, we have developed a new approach to calculate silver-based NP toxicity thresholds (HCx, hazardous concentrations) that are protective of specific members (operational taxonomic units, OTUs) of the soil microbial community. At the HC20 (80% of species protected), soil OTUs were significantly less sensitive to Ag₂S-NPs compared to AgNPs and Ag⁺ (5.9, 1.4 and 1.4 mg Ag kg^-1, respectively). However at more conservative HC values, there were no significant differences. These trends in OTU responses matched with those seen in a specific microbial function (rate of nitrification) and amoA-bacteria gene abundance. This study provides a novel molecular-based framework for quantifying the effect of a toxicant on whole soil microbial communities while still determining sensitive genera/species. Methods and results described here provide a benchmark for microbial community ecotoxicological studies and we recommend that future revisions of Soil Quality Guidelines for AgNPs and other such toxicants consider this approach.

Different responses of soil microbial metabolic activity to silver and iron oxide nanoparticles

The knowledge regarding the effects of metal or metal oxide nanoparticles on soil microbial metabolic activity and key ecological functions is limited, relative to the information about their species diversity. For this reason, the responses of soil microbial metabolic activity to silver (AgNPs) and iron oxide (FeONPs) nanoparticles, along concentration gradients of each, were evaluated by microcalorimetry and soil nitrification potential. The changes in abundances of bacteria, eukaryotes and ammonia-oxidizing bacteria were measured by real time quantitative PCR. It was found that AgNP (at 0.1, 1 and 10 mg kg(-1) soil) amendments decreased soil microbial metabolic activity, nitrification potential and the abundances of bacteria and ammonia-oxidizing bacteria; on the contrary, FeONPs had the positive effects on soil microbial metabolic activity (at 1 and 10 mg kg(-1) soil) and soil nitrification potential (at 0.1 and 1 mg kg(-1) soil). Specific microbial metabolic activity and specific nitrification potential further revealed that metal or metal oxide nanoparticles could change the C and N cycles of the agricultural soil through influencing soil microbial metabolism. These findings could deepen the understanding of the influence of NPs on soil microorganisms and their driven soil ecology process.

Physiological and transcriptional responses of nitrifying bacteria exposed to copper in activated sludge

Cu inhibition of gene transcription in ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were rarely studied simultaneously in activated sludge. In this study, the transcription of amoA (for AOB) and nxrB (for NOB), nitrification efficiencies, AOB and NOB respiratory rates, and Cu distribution were simultaneously investigated. Modeling the relationships among the aforementioned parameters revealed that in complex activated sludge systems, nitrification efficiency was an insensitive parameter for showing Cu inhibition. Respiration activities and gene transcription were sensitive to Cu and positively correlated with each other. The transcription of amoA and nxrB genes indicated that the Cu had different inhibitory effects on AOB and NOB. AOB were more susceptible to Cu toxicity than NOB. Moreover, the degree of Cu inhibition on ammonia oxidation was greater than on nitrite oxidation. The analysis and related modeling results indicate that the inhibitory actions of Cu on nitrifying bacteria could mainly be attributed to intracellular Cu. The findings from this study provide insight into the mechanism of Cu inhibition on nitrification in complex activated sludge systems.

Silver nanoparticles in aquatic environments: Physiochemical behavior and antimicrobial mechanisms

Nanosilver (silver nanoparticles or AgNPs) has unique physiochemical properties and strong antimicrobial activities. This paper provides a comprehensive review of the physicochemical behavior (e.g., dissolution and aggregation) and antimicrobial mechanisms of nanosilver in aquatic environments. The inconsistency in calculating the Gibbs free energy of formation of nanosilver [ΔGf(AgNPs)] in aquatic environments highlights the research needed to carefully determine the thermodynamic stability of nanosilver. The dissolutive release of silver ion (Ag(+)) in the literature is often described using a pseudo-first-order kinetics, but the fit is generally poor. This paper proposes a two-stage model that could better predict silver ion release kinetics. The theoretical analysis suggests that nanosilver dissolution could occur under anoxic conditions and that nanosilver may be sulfidized to form silver sulfide (Ag2S) under strict anaerobic conditions, but more investigation with carefully-designed experiments is required to confirm the analysis. Although silver ion release is likely the main antimicrobial mechanism of nanosilver, the contributions of (ion-free) AgNPs and reactive oxygen species (ROS) generation to the overall toxicity of nanosilver must not be neglected. Several research directions are proposed to better understand the dissolution kinetics of nanosilver and its antimicrobial mechanisms under various aquatic environmental conditions.

The effects of silver nanoparticles on intact wastewater biofilms

Silver nanoparticles (Ag-NPs) have strong antibacterial properties, which may adversely affect biological wastewater treatment processes. To determine the overall effect, intact biofilm samples were collected from the rotating biological contactor at the local wastewater treatment plant and treated with 200 mg Ag/L Ag-NPs for 24 h. The biofilm uptake of Ag-NPs was monitored with transmission electron microscopy. Forty-five minutes after Ag-NP application, Ag-NPs were seen in the biofilm extracellular polymeric substances (EPS). After 24 h, Ag-NPs had entered certain microbial cells, while other cells contained no observable Ag-NPs. Some cells were dying after the uptake of Ag-NPs. However, there was no significant reduction in cultivable bacteria in the biofilms, based on heterotrophic plate counts (HPC). While this may indicate that wastewater biofilms are highly resistant to Ag-NPs, the HPC represents only a small portion of the total microbial population. To further investigate the effects of Ag-NPs, a GeoChip microarray was used to directly detect changes in the functional gene structure of the microbial community in the biofilm. A clear decrease (34.6% decreases in gene number) in gene diversity was evident in the GeoChip analysis. However, the complete loss of any specific gene was rare. Most gene families present in both treated and untreated biofilms. However, this doesn’t necessarily mean that there was no change in these families. Signal intensity decreased in certain variants in each family while other variants increased to compensate the effects of Ag-NPs. The results indicate that Ag-NP treatment decreased microbial community diversity but did not significantly affect the microbial community function. This provides direct evidence for the functional redundancy of microbial community in engineered ecosystems such as wastewater biofilms.

Phosphorus Availability Alters the Effects of Silver Nanoparticles on Periphyton Growth and Stoichiometry

Exposure to silver nanoparticles (AgNPs) may alter the structure and function of freshwater ecosystems. However, there remains a paucity of studies investigating the effects of AgNP exposure on freshwater communities in the natural environment where interactions with the ambient environment may modify AgNP toxicity. We used nutrient diffusing substrates to determine the interactive effects of AgNP exposure and phosphorus (P) enrichment on natural assemblages of periphyton in three Canadian Shield lakes. The lakes were all phosphorus poor and spanned a gradient of dissolved organic carbon availability. Ag slowly accumulated in the exposed periphyton, which decreased periphyton carbon and chlorophyll a content and increased periphyton C:P and N:P in the carbon rich lakes. We found significant interactions between AgNP and P treatments on periphyton carbon, autotroph standing crop and periphyton stoichiometry in the carbon poor lake such that P enhanced the negative effects of AgNPs on chlorophyll a and lessened the impact of AgNP exposure on periphyton stoichiometry. Our results contrast with those of other studies demonstrating that P addition decreases metal toxicity for phytoplankton, suggesting that benthic and pelagic primary producers may react differently to AgNP exposure and highlighting the importance of in situ assays when assessing potential effects of AgNPs in fresh waters.

Transport, fate, and stimulating impact of silver nanoparticles on the removal of Cd(II) by Phanerochaete chrysosporium in aqueous solutions

Despite the knowledge about increasing discharge of silver nanoparticles (AgNPs) into wastewater and its potential toxicity to microorganisms, the interaction of AgNPs with heavy metals in the biological removal process remains poorly understood. This study focused on the effect of AgNPs (hydrodynamic diameter about 24.3±0.37 nm) on the removal of cadmium (Cd(II)) by using a model white rot fungus species, Phanerochaete chrysosporium. Results showed that the biological removal capacity of Cd(II) increased with the concentration of AgNPs increasing from 0.1 mg/L to 1 mg/L. The maximum removal capacity (4.67 mg/g) was located at 1 mg/L AgNPs, and then decreased with further increasing AgNPs concentration, suggesting that an appropriate concentration of AgNPs has a stimulating effect on the removal of Cd(II) by P. chrysosporium instead of an inhibitory effect. Results of Ag(+) and total Ag concentrations in the solutions together with those of SEM and XRD demonstrated that added AgNPs had undergone oxidative dissolution and transported from the solution to the surface of fungal mycelia (up to 94%). FTIR spectra confirmed that amino, carboxyl, hydroxyl, and other reducing functional groups were involved in Cd(II) removal, AgNPs transportation, and the reduction of Ag(+) to AgNPs.