Symbiosis between nitrogen-fixing bacteria and Medicago truncatula is not significantly affected by silver and silver sulfide nanomaterials

Reveal molecular mechanism on the effects of silver nanoparticles on nitrogen transformation and related functional microorganisms in an agricultural soil

Silver nanoparticles (AgNPs) are widely present in aquatic and soil environment, raising significant concerns about their impacts on creatures in ecosystem. While the toxicity of AgNPs on microorganisms has been reported, their effects on biogeochemical processes and specific functional microorganisms remain relatively unexplored. In this study, a 28-day microcosmic experiment was conducted to investigate the dose-dependent effects of AgNPs (10 mg and 100 mg Ag kg-1 soil) on nitrogen transformation and functional microorganisms in agricultural soils. The molecular mechanisms were uncovered by examining change in functional microorganisms and metabolic pathways. To enable comparison, the toxicity of positive control with an equivalent Ag+ dose from CH3COOAg was also included. The results indicated that both AgNPs and CH3COOAg enhanced nitrogen fixation and nitrification, corresponding to increased relative abundances of associated functional genes. However, they inhibited denitrification via downregulating nirS, nirK, and nosZ genes as well as reducing nitrate and nitrite reductase activities. In contrast to high dose of AgNPs, low levels increased bacterial diversity. AgNPs and CH3COOAg altered the activities of associated metabolic pathways, resulting in the enrichment of specific taxa that demonstrated tolerance to Ag. At genus level, AgNPs increased the relative abundances of nitrogen-fixing Microvirga and Bacillus by 0.02 %-629.39 % and 14.44 %-30.10 %, respectively, compared with control group (CK). The abundances of denitrifying bacteria, such as Rhodoplanes, Pseudomonas, and Micromonospora, decreased by 19.03 % to 32.55 %, 24.73 % to 50.05 %, and 15.66 % to 76.06 %, respectively, compared to CK. CH3COOAg reduced bacterial network complexity, diminished the symbiosis mode compared to AgNPs. The prediction of genes involved in metabolic pathways related to membrane transporter and cell motility showed sensitive to AgNPs exposure in the soil. Further studies involving metabolomics are necessary to reveal the essential effects of AgNPs and CH3COOAg on biogeochemical cycle of elements in agricultural soil.

Metal oxide nanoparticles inhibit nitrogen fixation and rhizosphere colonization by inducing ROS in associative nitrogen-fixing bacteria Pseudomonas stutzeri A1501

The potential effects of engineered metal oxide nanoparticles (MONPs) on bacterial nitrogen fixation are of great concern. Herein, the impact and mechanism of the increasing-used MONPs, including TiO₂, Al₂O₃, and ZnO nanoparticles (TiO₂NP, Al₂O₃NP, and ZnONP, respectively), on nitrogenase activity was studied at the concentrations ranging from 0 to 10 mg L^-1 using associative rhizosphere nitrogen-fixing bacteria Pseudomonas stutzeri A1501. Nitrogen fixation capacity was inhibited by MONPs in an increasing degree of TiO₂NP < Al₂O₃NP < ZnONP. Realtime qPCR analysis showed that the expressions of nitrogenase synthesis-related genes, including nifA and nifH, were inhibited significantly when MONPs were added. MONPs could cause the explosion of intracellular ROS, and ROS not only changed the permeability of the membrane but also inhibited the expression of nifA and biofilm formation on the root surface. The repressed nifA gene could inhibit transcriptional activation of nif-specific genes, and ROS reduced the biofilm formation on the root surface which had a negative effect on resisting environmental stress. This study demonstrated that MONPs, including TiO₂NP, Al₂O₃NP, and ZnONP, inhibited bacterial biofilm formation and nitrogen fixation in the rice rhizosphere, which might have a negative effect on the nitrogen cycle in bacteria-rice system.

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.

Status Quo in Data Availability and Predictive Models of Nano-Mixture Toxicity

Co-exposure of nanomaterials and chemicals can cause mixture toxicity effects to living organisms. Predictive models might help to reduce the intensive laboratory experiments required for determining the toxicity of the mixtures. Previously, concentration addition (CA), independent action (IA), and quantitative structure–activity relationship (QSAR)-based models were successfully applied to mixtures of organic chemicals. However, there were few studies concerning predictive models for toxicity of nano-mixtures before June 2020. Previous reviews provided comprehensive knowledge of computational models and mechanisms for chemical mixture toxicity. There is a gap in the reviewing of datasets and predictive models, which might cause obstacles in the toxicity assessment of nano-mixtures by using in silico approach. In this review, we collected 183 studies of nano-mixture toxicity and curated data to investigate the current data and model availability and gap and to derive research challenges to facilitate further experimental studies for data gap filling and the development of predictive models.

Silver Sulfide Nanoparticles Reduce Nitrous Oxide Emissions by Inhibiting Denitrification in the Earthworm Gut

The accumulation of Ag₂S in agricultural soil via application of Ag-containing sludge potentially affects the functioning of soil microorganisms and earthworms (EWs) due to the strong antimicrobial properties of Ag. This study examined the effects of Ag₂S nanoparticles (Ag₂S-NPs) on the EW-mediated (Eisenia fetida and Pontoscolex corethrurus) soil N cycle. We used 16S rRNA gene-based sequencing and quantitative polymerase chain reaction to examine the bacterial community and nitrification/denitrification-related gene abundance. The presence of either EWs or Ag significantly increased denitrification and N₂O emissions. However, the addition of Ag₂S to EW-inhabited soil reduced N₂O emissions by 14-33%. Furthermore, Ag₂S caused a low-dose stimulation but a high-dose inhibition to N₂O flux from the EW gut itself. Accordingly, an increase in Ag in the EW gut caused a decrease in the relative abundance of denitrifiers in both the soil and the gut, especially for the dominant genus Bacillus. Ag₂S also decreased the copy numbers of nitrification gene (nxrB) and denitrification genes (napA, nirS, and nosZ) in EW gut, leading to the observed decrease in N₂O emissions. Collectively, applying Ag₂S-containing sludge disturbs the denitrification function of the EW gut microbiota and the cycling of N in soil-based systems.

The effects of silver nanoparticles on the microbial nitrogen cycle: a review of the known risks

The nitrogen cycle is an integral biogeochemical function for maintaining healthy environments. Nitrogen is a key nutrient that must be continuously replenished through recycling mechanisms to sustain ecosystems, disruption to which can result in compromised ecosystem functioning. Certain stages in the microbial conversion of nitrogen compounds are performed by a limited range of micro-organisms making these key functional species in ecosystems. The growing industrial use of silver nanoparticles (AgNPs) potentially poses significant risks for microbial nitrogen cycling species. AgNPs possess potent antimicrobial properties and are expected to reach a range of natural environments through several routes of exposure. Certain functional nitrogen cycling microbes have been shown to be highly susceptible to AgNP toxicity. The current literature indicates that AgNPs can negatively affect certain nitrogen fixing, nitrifying and denitrifying microbes in vitro. In vivo studies investigating the effect of AgNPs on nitrogen cycling microbial communities and nitrogen transformation rates in soil, sediment and sludge environments have also indicated disruption of these functional processes. This review provides a comprehensive description of the current state of knowledge regarding the toxicity of AgNPs to nitrogen cycling communities. The aim of the review is to highlight the most susceptible stages in the nitrogen cycle and the implications for the affected ecosystems.

CuO Nanoparticles Alter the Rhizospheric Bacterial Community and Local Nitrogen Cycling for Wheat Grown in a Calcareous Soil

The application of nanoparticles (NPs) to soils, as either fertilizers or fungicides (e.g., CuO NPs), has been proposed to improve the sustainability of agriculture. The observed effects could result directly from the NP-plant interactions or indirectly through effects on the soil microbiome. The objective of this study was to assess the effects of CuO NPs on the changes in the bacterial community structure and nitrogen-cycling-associated functions in a high pH soil and to correlate these changes with nitrate accumulation, soil parameter changes, and plant growth over 28 days. Triticum aestivum seedlings were exposed to 50 mg/kg CuO NPs, 50 mg/kg CuSO₄, or 0.5 mg/kg CuSO₄ in a standard soil (Lufa 2.1 soil, pH adjusted to 7.6). While Cu treatments reduced nitrate accumulation in the bulk soil, the effects were opposite in the rhizosphere (the soil influenced by root exudates). While nitrate accumulation in bulk soil negatively correlated with total Cu concentration, part of the nitrate concentration in the rhizosphere was explained by root uptake during plant growth, the rest being modulated by Cu treatments. The abundance of genes involved in the nitrogen cycle in the rhizosphere soil correlated with the ionic copper concentration. The increased nitrate concentration in the rhizosphere correlated with an increase of the gene abundance related to the nitrogen fixation and a decrease of denitrification gene abundance. Microbial diversity in bulk or rhizosphere soil under the different treatments alone could not explain these variations, while differences in the assemblages of bacteria associated with these functional gene abundances gave good insights. This study highlights the complexity of microbial N-related function in the rhizosphere and the need to characterize the rhizosphere soil, plant growth and root activity, NP (bio)transformations, along with microbial networks, to understand the impact of agrochemicals (here CuO NPs) on soil fertility.

Physical Properties of Carbon Nanomaterials and Nanoceria Affect Pathways Important to the Nodulation Competitiveness of the Symbiotic N2 -Fixing Bacterium Bradyrhizobium diazoefficiens

The pathogenicity and antimicrobial properties of engineered nanomaterials (ENMs) are relatively well studied. However, less is known regarding the interactions of ENMs and agriculturally beneficial microorganisms that affect food security. Nanoceria (CeO2 nanoparticles (NPs)), multiwall carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), and carbon black (CB) have been previously shown to inhibit symbiotic N2 fixation in soybeans, but direct rhizobial susceptibility is uncertain. Here, Bradyrhizobium diazoefficiens associated with symbiotic N2 fixation in soybeans is assessed, evaluating the role of soybean root exudates (RE) on ENM-bacterial interactions and the effects of CeO2 NPs, MWCNTs, GNPs, and CB on bacterial growth and gene expression. Although bacterial growth is inhibited by 50 mg L-1 CeO2 NPs, MWCNTs, and CB, all ENMs at 0.1 and 10 mg L-1 cause a global transcriptomic response that is mitigated by RE. ENMs may interfere with plant-bacterial signaling, as evidenced by suppressed upregulation of genes induced by RE, and downregulation of genes encoding transport RNA, which facilitates nodulation signaling. MWCNTs and CeO2 NPs inhibit the expression of genes conferring B. diazoefficiens nodulation competitiveness. Surprisingly, the transcriptomic effects on B. diazoefficiens are similar for these two ENMs, indicating that physical, not chemical, ENM properties explain the observed effects.

Prospect of phytoremediation combined with other approaches for remediation of heavy metal-polluted soils

Accumulation of heavy metals in agricultural soils due to human production activities-mining, fossil fuel combustion, and application of chemical fertilizers/pesticides-results in severe environmental pollution. As the transmission of heavy metals through the food chain and their accumulation pose a serious risk to human health and safety, there has been increasing attention in the investigation of heavy metal pollution and search for effective soil remediation technologies. Here, we summarized and discussed the basic principles, strengths and weaknesses, and limitations of common standalone approaches such as those based on physics, chemistry, and biology, emphasizing their incompatibility with large-scale applications. Moreover, we explained the effects, advantages, and disadvantages of the combinations of common single repair approaches. We highlighted the latest research advances and prospects in phytoremediation-chemical, phytoremediation-microbe, and phytoremediation-genetic engineering combined with remediation approaches by changing metal availability, improving plant tolerance, promoting plant growth, improving phytoextraction and phytostabilization, etc. We then explained the improved safety and applicability of phytoremediation combined with other repair approaches compared to common standalone approaches. Finally, we established a prospective research direction of phytoremediation combined with multi-technology repair strategy.

Nanomaterials in the environment: Behavior, fate, bioavailability, and effects-An updated review

The present review covers developments in studies of nanomaterials (NMs) in the environment since our much cited review in 2008. We discuss novel insights into fate and behavior, metrology, transformations, bioavailability, toxicity mechanisms, and environmental impacts, with a focus on terrestrial and aquatic systems. Overall, the findings were that: 1) despite substantial developments, critical gaps remain, in large part due to the lack of analytical, modeling, and field capabilities, and also due to the breadth and complexity of the area; 2) a key knowledge gap is the lack of data on environmental concentrations and dosimetry generally; 3) substantial evidence shows that there are nanospecific effects (different from the effects of both ions and larger particles) on the environment in terms of fate, bioavailability, and toxicity, but this is not consistent for all NMs, species, and relevant processes; 4) a paradigm is emerging that NMs are less toxic than equivalent dissolved materials but more toxic than the corresponding bulk materials; and 5) translation of incompletely understood science into regulation and policy continues to be challenging. There is a developing consensus that NMs may pose a relatively low environmental risk, but because of uncertainty and lack of data in many areas, definitive conclusions cannot be drawn. In addition, this emerging consensus will likely change rapidly with qualitative changes in the technology and increased future discharges. Environ Toxicol Chem 2018;37:2029-2063. © 2018 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Uptake of Engineered Nanoparticles by Food Crops: Characterization, Mechanisms, and Implications

With the rapidly increasing demand for and use of engineered nanoparticles (NPs) in agriculture and related sectors, concerns over the risks to agricultural systems and to crop safety have been the focus of a number of investigations. Significant evidence exists for NP accumulation in soils, including potential particle transformation in the rhizosphere and within terrestrial plants, resulting in subsequent uptake by plants that can yield physiological deficits and molecular alterations that directly undermine crop quality and food safety. In this review, we document in vitro and in vivo characterization of NPs in both growth media and biological matrices; discuss NP uptake patterns, biotransformation, and the underlying mechanisms of nanotoxicity; and summarize the environmental implications of the presence of NPs in agricultural ecosystems. A clear understanding of nano-impacts, including the advantages and disadvantages, on crop plants will help to optimize the safe and sustainable application of nanotechnology in agriculture for the purposes of enhanced yield production, disease suppression, and food quality.

The toxicity of silver to soil organisms exposed to silver nanoparticles and silver nitrate in biosolids-amended field soil

The use of engineered silver nanoparticles (AgNPs) is widespread, with expected release to the terrestrial environment through the application of biosolids onto agricultural lands. The toxicity of AgNPs and silver nitrate (AgNO₃ ; as ionic Ag⁺ ) to plant (Elymus lanceolatus and Trifolium pratense) and soil invertebrate (Eisenia andrei and Folsomia candida) species was assessed using Ag-amended biosolids applied to a natural sandy loam soil. Bioavailable Ag⁺ in soil samples was estimated using an ion-exchange technique applied to KNO₃ soil extracts, whereas exposure to dispersible AgNPs was verified by single-particle inductively coupled plasma-mass spectrometry and transmission electron microscopy-energy dispersive X-ray spectroscopy analysis. Greater toxicity to plant growth and earthworm reproduction was observed in AgNP exposures relative to those of AgNO₃ , whereas no difference in toxicity was observed for F. candida reproduction. Transformation products in the AgNP-biosolids exposures resulted in larger pools of extractable Ag⁺ than those from AgNO₃ -biosolids exposures, at similar total Ag soil concentrations. The results of the present study reveal intrinsic differences in the behavior and bioavailability of the 2 different forms of Ag within the biosolids-soils pathway. The present study demonstrates how analytical methods that target biologically relevant fractions can be used to advance the understanding of AgNP behavior and toxicity in terrestrial environments. Environ Toxicol Chem 2017;36:2756-2765. © 2017 Crown in the Right of Canada. Published Wiley Periodicals Inc., on behalf of SETAC.