Adsorption of extracellular polymeric substances from two microbes by TiO2 nanoparticles

Combined control of plant diseases by Bacillus subtilis SL44 and Enterobacter hormaechei Wu15

Co-incubation of plant growth promoting rhizobacteria (PGPRs) have been proposed as a potential alternative to pesticides for controlling fungal pathogens in crops, but their synergism mechanisms are not yet fully understood. In this study, combined use of Bacillus subtilis SL44 and Enterobacter hormaechei Wu15 could decrease the density of Colletotrichum gloeosporioides and Rhizoctonia solani and enhance the growth of beneficial bacteria on the mycelial surface, thereby mitigating disease severity. Meanwhile, PGPR application led to a reorganization of the rhizosphere microbial community through modulating its metabolites, such as extracellular polymeric substances and chitinase. These metabolites demonstrated positive effects on attracting and enhancing conventional periphery bacteria, inhibiting fungal pathogens and promoting soil health effectively. The improvement in the microbial community structure altered the trophic mode of soil fungal communities, effectively decreasing the proportion of saprotrophic soil and reducing fungal plant diseases. Certain combinations of PGPR have the potential to serve as precise instruments for managing plant pathogens.

Elucidating the effect of TiO2 nanoparticles on mung bean rhizobia via in vitro assay: Influence on growth, morphology, and plant growth promoting traits

Titanium dioxide nanoparticles (TiO2 NPs) are among the most commonly used nanomaterials and are most likely to end up in soil. Therefore, it is pertinent to study the interaction of TiO2 NPs with soil microorganisms. The present in vitro broth study evaluates the impacts of low-dose treatments (0, 1.0, 5.0, 10.0, 20.0, and 40.0 mg L-1 ) of TiO2 NPs on cell viability, morphology, and plant growth promoting (PGP) traits of rhizobia isolated from mung bean root nodule. Two types of TiO2 NPs, that is, mixture of anatase and rutile, and anatase alone were used in the study. These TiO2 NPs were supplemented in broth along with a multifunctional isolate (Bradyrhizobium sp.) and two reference cultures. The exposure of TiO2 (anatase+rutile) NPs at low concentrations (less than 20.0 mg L-1 ) enhanced the cell growth, and total soluble protein content, besides improving the phosphate solubilization, Indole-3-acetic acid (IAA) production, siderophore, and gibberellic acid production. The TiO2 (anatase) NPs enhanced exopolysaccharide (EPS) production by the test rhizobial cultures. The radical scavenging assay was performed to reveal the mode of action of the nano-TiO2 particles. The study revealed higher reactive oxygen species (ROS) generation by the TiO2 (anatase) NPs as compared with TiO2 (anatase+rutile) NPs. Exposure to TiO2 NPs also altered the morphology of rhizobial cells. The findings suggest that TiO2 NPs could act as promoters of PGP traits of PGP bacteria when applied at appropriate lower doses.

Formation of extracellular polymeric substances corona on TiO2 nanoparticles: Roles of crystalline phase and exposed facets

Nanoparticles (NPs) in the environment can interact with macromolecules in the surrounding environment to form eco-corona on their surfaces, which in turn affects the environmental fate and toxicity of nanoparticles. Wastewater treatment plants containing large amounts of microbial extracellular polymeric substances (EPS) are an important source of NPs into the environment, where the formation of EPS coronas on NPs is critical. However, it remains unclear how the crystalline phase and exposed facets, which are intrinsic properties of NPs, affect the formation of EPS coronas on NPs. This study investigated the formation of EPS corona on three TiO2 NPs (representing the most widely used engineered NPs) with different crystalline phases and exposed facets. The protein type and abundance in EPS coronas on TiO2 NPs varied depending on the crystalline phase and exposed facets. Anatase with {101} facets and {001} facets preferred to adsorb proteins with lower molecular weights and higher H-bonding relevant amino acids, respectively, while EPS corona on rutile with {110} facets had proteins with higher hydrophobicity. In addition, the selective adsorption of proteins was primarily determined by steric hindrance, hydrogen bonding, and hydrophobic interaction between TiO2 NPs and proteins, which were affected by changes in aggregation state, surface hydroxyl density, and hydrophobicity of TiO2 NPs induced by crystalline phase and exposed facets. Moreover, crystalline phase and exposed facets-induced EPS corona changes altered the aggregation state and oxidation potential of TiO2-EPS corona complexes. These findings emphasize the important role of crystalline phase and exposed facets in the environmental behavior of nanoparticles and may provide insights into the safe design of nanoparticles.

Ecotoxicity of a mixture of nanoparticles on algal species Scendesmus obliquus in OECD growth media, wastewater, and pond water

The possible impact of ZnO and CuO nanoparticles (NPs) (individually and in binary mixture) was investigated using the freshwater microalgae, Scenedesmus obliquus. The present study shows the effect of nanoparticles on algae in OECD growth media, wastewater, and pond water during a 96-h toxicity test. At 0.1 mg/L concentration of the mixture of NPs, the reduction in the chlorophyll a content was 13.61 ± 1.34% (OECD media), 28.83 ± 1.85% (wastewater), and 31.81 ± 2.23% (pond water). Values of reduction in biomass were observed to be 42.13 ± 1.38, 39.96 ± 1.03, and 33.10 ± 1.29% for OECD media, wastewater, and pond water, respectively. The highest increase in lipid values was observed in the case of pond water (6.3 ± 1.31%). A significant increase in the value of EPS-generated protein was observed in the wastewater sample. EPS-generated carbohydrate values were increased in OECD media but decreased in the wastewater matrix. The transmission electron microscope images showed structural damage to algae cells due to the exposure to a mixture of nanoparticles at higher concentrations. Fourier transform infrared analysis showed an addition of bonds and differences in the peak and its intensity during exposure to high concentrations of NPs. Overall, this study gives fundamental insights into the interaction and toxicity of a mixture of NPs to algal species in different water matrices.

Eco-Corona Dictates Mobility of Nanoplastics in Saturated Porous Media: The Critical Role of Preferential Binding of Macromolecules

Nanoplastics are an increasing environmental concern. In aquatic environments, nanoplastics will acquire an eco-corona by interacting with macromolecules (e.g., humic substances and extracellular polymeric substances (EPS)). Here, we show that the properties of the eco-corona and, consequently, its ability to enhance the transport of nanoplastics vary significantly with the surface functionality of nanoplastics and sources of macromolecules. The eco-corona derived from the EPS of Gram-negative Escherichia coli MG1655 enhances the transport of polystyrene (PS) nanospheres in saturated porous media to a much greater extent than the eco-corona derived from soil humic acid and fulvic acid. In comparison, the eco-corona from all three sources significantly enhance the transport of carboxylated PS (HOOC-PS). We show that the eco-corona inhibits the deposition of the two types of nanoplastics to the porous media mainly via steric repulsion. Accordingly, an eco-corona consisting of a higher mass of larger-sized macromolecules is generally more effective in enhancing transport. Notably, HOOC-PS tends to acquire macromolecules of lower hydrophobicity than PS. The more disordered and flexible structures of such macromolecules may result in greater elastic repulsion between the nanoplastics and sand grains and, consequently, greater transport enhancement. The findings of this study highlight the critical role of eco-corona formation in regulating the mobility of nanoplastics, as well as the complexity of this process.

Aquatic organisms modulate the bioreactivity of engineered nanoparticles: focus on biomolecular corona

The increased use of nanoparticle (NP)-enabled materials in everyday-life products have raised concerns about their environmental implications and safety. This motivated the extensive research in nanoecotoxicology showing the possibility that NPs could cause harm to the aquatic organisms if present at high concentrations. By contrast, studies dealing with influence that organisms could exert on the fate and thus effects of NPs are still very rare. Drawing on the existing up-to-date knowledge we critically discuss the formation of biomolecular corona as one of the mechanisms by which organisms exerted control on the NPs fate in the aquatic and biotic environments. We focused the formation of corona by exogeneous and endogenous biomolecules and illustrated the discussion with the specific example of phytoplankton and aquatic invertebrate species. We highlighted the necessity to incorporate the concept of biomolecular corona within more general framework considering the feedback of aquatic organisms and the control they exert in shaping the fate and impact of NPs in the aquatic and biological environment. In our view such broader perspective will contribute to get novel insights into the drivers of environmental transformations of NPs and their mechanisms, which are important in environmental risk assessment.

Soil indigenous microorganisms alleviate soluble vanadium release from industrial dusts

Vanadium-bearing dusts from industrial processes release abundant toxic vanadium, posing imminent ecological and human health concerns. Although the precipitation of these dusts has been recognized as the main source of soil vanadium pollution, little is known regarding the interrelationships between industrial dusts and soil inherent compositions. In this study, the interactions between dusts from vanadium smelting and soil indigenous microorganisms were investigated. Soluble vanadium (V) [V(V)] released from industrial dusts was reduced by 41.5 ± 0.39% with soil addition, compared to water leaching. Reducible fraction accounted for the highest proportion (55.1 ± 1.73%) of vanadium speciation in the resultant soils, while residual vanadium fraction increased to 83.7 ± 3.22% in the leached dusts. Functional genera (e.g., Aliihoeflea, Actinotalea) that transformed V(V) to insoluble vanadium (IV) alleviated dissolved vanadium release. Nitrate/nitrite reduction and glutathione metabolisms contributed to V(V) immobilization primarily. Structural equation model analysis indicated that V(V) reducers had significant negative impacts on soluble V(V) in the leachate. This first-attempt study highlights the importance of soil microorganisms in immobilizing vanadium from industrial dusts, which is helpful to develop novel strategies to reduce their environmental risks associated to vanadium smelting process.

The Inhibition of Microcystin Adsorption by Microplastics in the Presence of Algal Organic Matters

Microplastics (MPs) could act as vectors of synthetic chemicals; however, their influence on the adsorption of chemicals of natural origin (for example, MC-LR and intracellular organic matter (IOM), which could be concomitantly released by toxic Microcystis in water) is less understood. Here, we explored the adsorption of MC-LR by polyethylene (PE), polystyrene (PS), and polymethyl methacrylate (PMMA). The results showed that the MPs could adsorb both MC-LR and IOM, with the adsorption capability uniformly following the order of PS, PE, and PMMA. However, in the presence of IOM, the adsorption of MC-LR by PE, PS, and PMMA was reduced by 22.3%, 22.7% and 5.4%, respectively. This is because the benzene structure and the specific surface area of PS facilitate the adsorption of MC-LR and IOM, while the formation of Π-Π bonds favor its interaction with IOM. Consequently, the competition for binding sites between MC-LR and IOM hindered MC-LR adsorption. The C=O in PMMA benefits its conjunction with hydroxyl and carboxyl in the IOM through hydrogen bonding; thus, the adsorption of MC-LR is also inhibited. These findings highlight that the adsorption of chemicals of natural origin by MPs is likely overestimated in the presence of metabolites from the same biota.

Flocculation with heterogeneous composition in water environments: A review

Flocculation is a key process for controlling the fate and transport of suspended particulate matter (SPM) in water environments and has received considerable attention in the field of water science (e.g., oceanography, limnology, and hydrology), remaining an active area of research. The research on flocculation has been conducted to elucidate the SPM dynamics and to diagnose various environmental issues. The flocculation, sedimentation, and transportation of SPM are closely linked to the compositional and structural properties of flocs. In fact, flocs are highly heterogeneous in terms of composition. However, the lack of comprehensive research on floc composition and structure has led to misconceptions regarding the temporal and spatial dynamics of SPM. This review summarizes the current understanding of the heterogeneous composition of flocs (e.g., minerals, organic matter, metals, microplastic, engineered nanoparticles) and its effect on their structure and on their fate and transport within aquatic environments. Furthermore, the effects of human activities (e.g., pollutant discharge, construction) on floc composition are discussed.

Advances in application of ultraviolet irradiation for biofilm control in water and wastewater infrastructure

Biofilms are ubiquitous in aquatic environment. While so far, most of the ultraviolet (UV) disinfection studies focus on planktonic bacteria, and only limited attention has been given to UV irradiation on biofilms. To enrich this knowledge, the present paper reviews the up-to-date studies about applying UV to control biofilms in water and wastewater infrastructure. The development of UV light sources from the conventional mercury lamp to the light emitting diode (LED), and the resistance mechanisms of biofilms to UV are summarized, respectively. Then the feasibility to control biofilms with UV is discussed in terms of three technical routes: causing biofilm slough, inhibiting biofilm formation, and inactivating bacteria in the established biofilm. A comprehensive evaluation of the biofilm-targeted UV technologies currently used or potentially useful in water industry is provided as well, after comparative analyses on single/combined wavelengths, continuous/pulsed irradiation, and instant/chronic disinfection effects. UV LEDs are emerging as competitive light sources because of advantages such as possible selection of wavelengths, adjustable emitting mode and the designable configuration. They still, however, face challenges arising from the low wall plug efficiency and power output. At last, the implementation of the UV-based advanced oxidation processes in controlling biofilms on artificial surfaces is overviewed and their synergistic mechanisms are proposed, which further enlightens the prospective of UV in dealing with the biofilm issue in water infrastructure.

Influence of extracellular polymeric substance on the interaction between titanium dioxide nanoparticles and Chlorella pyrenoidosa cells

The presence of extracellular polymeric substance (EPS) plays a vital role in the accumulation and toxicity of nanoparticles to microorganisms, in which the involved processes and mechanisms are still waiting to be revealed. Herein, we specifically investigated the interfacial interaction between titanium dioxide nanoparticles (nTiO₂) and algae (Chlorella pyrenoidosa) with/without EPS and the effect of EPS on algal cell internalization of nTiO₂. Results showed that the presence of EPS on cell surface promoted heteroaggregation between nTiO₂ and algal cells, and induced more nTiO₂ accumulation on algal surface; however, algal cell internalization of nTiO₂ was limited by the presence of EPS. Pearson correlation analysis further proved that the presence of EPS had a positive effect on the surface accumulation of nTiO₂ and a negative effect on the internalization of nTiO₂. More than 60% of cell internalized nTiO₂ entered algal cells through the energy dependent endocytosis pathway. It is interesting to find that anatase nTiO₂ (nTiO₂-A) entered algal cells mainly through the clathrin dependent endocytosis, while rutile nTiO₂ (nTiO₂-R) mainly through the dynamin dependent endocytosis. This difference could be due to the different affinities of nTiO₂-A and nTiO₂-R to the mediating receptors referring to different endocytic pathways. The removal of EPS activated the associated mediating pathways, allowing more nTiO₂ to be internalized. These findings address the role of EPS on the interaction between nTiO₂ and algae and promote a deeper understanding of the ecological effect of nTiO₂.

Effects of microbial inactivation approaches on quantity and properties of extracellular polymeric substances in the process of wastewater treatment and reclamation: A review

Microbial extracellular polymeric substances (EPS) have a profound role in various wastewater treatment and reclamation processes, in which a variety of technologies are used for disinfection and microbial growth inhibition. These treatment processes can induce significant changes in the quantity and properties of EPS, and altered EPS could further adversely affect the wastewater treatment and reclamation system, including membrane filtration, disinfection, and water distribution. To clarify the effects of microbial inactivation approaches on EPS, these effects were classified into four categories: (1) chemical reactions, (2) cell lysis, (3) changing EPS-producing metabolic processes, and (4) altering microbial community. Across these different effects, treatments with free chlorine, methylisothiazolone, TiO₂, and UV irradiation typically enhance EPS production. Among the residual microorganisms in EPS matrices after various microbial inactivation treatments, one of the most prominent is Mycobacterium. With respect to EPS properties, proteins and humic acids in EPS are usually more susceptible to treatment processes than polysaccharides. The affected EPS properties include changes in molecular weight, hydrophobicity, and adhesion ability. All of these changes can undermine wastewater treatment and reclamation processes. Therefore, effects on EPS quantity and properties should be considered during the application of microbial inactivation and growth inhibition techniques.

Characterization of dissolved organic matter for understanding the adsorption on nanomaterials in aquatic environment: A review

Nanomaterials (NMs) have received tremendous attention as emerging adsorbents for environmental applications. The ever-increasing release into aquatic systems and the potential use in water treatment processes heighten the likelihood of the interactions of NMs with aquatic dissolved organic matter (DOM). Once DOM is adsorbed on NMs, it substantially modifies the surface properties, thus altering the fate and transport of NMs, as well as their toxic effects on (micro)organisms in natural and engineered systems. The environmental consequences of DOM-NMs interaction have been widely studied in the literature. In contrast, a comprehensive understanding of DOM-NM complexes, particularly regarding the controlling factors, is still lacking, and its significance has been largely overlooked. This gap in the knowledge mainly arises from the complex and heterogeneous structures of the DOM, which prompts the urgent need to further characterize the DOM properties to deepen the understanding associated with the adsorption processes on NMs. This review aims to provide in-depth insights into the complex DOM adsorption behavior onto NMs, whether they are metal- or carbon-based materials. First, we summarize the up-to-date analytical methods to characterize the DOM to unravel the underlying adsorption mechanisms. Second, the key DOM characteristics governing the adsorption processes are discussed. Next, the environmental factors, such as the nature of adsorbents and solution chemistry, affecting the DOM-NM interactions, are identified and discussed. Finally, future studies are recommended to fully understand the chemical traits of DOM upon its adsorption onto NMs.

Effects of Extracellular Polymeric Substances on the Formation and Methylation of Mercury Sulfide Nanoparticles

Growing evidence has suggested that microbial biofilms are potential environmental "hotspots" for the production and accumulation of a bioaccumulative neurotoxin, methylmercury. Here, we demonstrate that extracellular polymeric substances (EPS), the main components of biofilm matrices, significantly interfere with mercury sulfide precipitation and lead to the formation of nanoparticulate metacinnabar available for microbial methylation, a natural process predominantly responsible for the environmental occurrence of methylmercury. EPS derived from mercury methylating bacteria, particularly Desulfovibrio desulfuricans ND132, substantially increase the methylation potential of nanoparticulate mercury. This is likely due to the abundant aromatic biomolecules in EPS that strongly interact with mercury sulfide via inner-sphere complexation and consequently enhance the short-range structural disorder while mitigating the aggregation of nanoparticulate mercury. The EPS-elevated bioavailability of nanoparticulate mercury to D. desulfuricans ND132 is not induced by dissolution of these nanoparticles in aqueous phase, and may be dictated by cell-nanoparticle interfacial reactions. Our discovery is the first step of mechanistically understanding methylmercury production in biofilms. These new mechanistic insights will help incorporate microbial EPS and particulate-phase mercury into mercury methylation models, and may facilitate the assessment of biogeochemical cycling of other nutrient or toxic elements driven by EPS-producing microorganisms that are prevalent in nature.