Sustainable pollutant removal by periphytic biofilm via microbial composition shifts induced by uneven distribution of CeO2 nanoparticles

Nano-enabled microalgae bioremediation: Advances in sustainable pollutant removal and value-addition

Microalgae-assisted bioremediation, enriched by nanomaterial integration, offers a sustainable approach to environmental pollution mitigation while harnessing microalgae's potential as a biocatalyst and biorefinery resource. This strategy explores the interaction between microalgae, nanomaterials, and bioremediation, advancing sustainability objectives. The potent combination of microalgae and nanomaterials highlights the biorefinery's promise in effective pollutant removal and valuable algal byproduct production. Various nanomaterials, including metallic nanoparticles and semiconductor quantum dots, are reviewed for their roles in inorganic and organic pollutant removal and enhancement of microalgae growth. Limited studies have been conducted to establish nanomaterial's (CeO2, ZnO, Fe3O4, Al2O3, etc.) role on microalgae in pollution remediation; most studies cover inorganic pollutants (heavy metals and nutrients) remediation, exhibited 50-300% bioremediation efficiency improvement; however, some studies cover antibiotics and toxic dyes removal efficiency with 19-95% improvement. These aspects unveil the complex mechanisms underlying nanomaterial-pollutant-microalgae interactions, focusing on adsorption, photocatalysis, and quantum dot properties. Strategies to enhance bioremediation efficiency are discussed, including pollutant uptake improvement, real-time control, tailored nanomaterial design, and nutrient recovery. The review assesses recent advancements, navigates challenges, and envisions a sustainable future for bioremediation, underlining the transformative capacity of nanomaterial-driven microalgae-assisted bioremediation. This work aligns with Sustainable Development Goals 6 (Clean Water and Sanitation) and 12 (Responsible Consumption and Production) by exploring nanomaterial-enhanced microalgae bioremediation for sustainable pollution management and resource utilization.

Antifouling characteristics and mechanisms in visible-light photocatalytic membrane bioreactor based on g-C3N4 modified membrane

A novel visible-light photocatalytic membrane bioreactor (R3) was constructed for membrane fouling control and effluent quality improvement. Specially, g-C3N4 modified membrane was evaluated for the performance of synergistic separation and photocatalysis. Another two parallel reactors, MBRs with ceramic membrane (R1) and g-C3N4 membrane in dark condition (R2), were operated synchronously for comparison. A satisfactory effluent quality was obtained in R3 with COD and NH4+-N around 22.0 mg/L and 1.02 mg/L during 60-day operation, which was superior to R1 (27.8, 1.42 mg/L) and R2 (29.9, 2.26 mg/L). The thickness of cake layer on membranes in R3 (2.46 μm) was thinner than R1 (3.52 μm) and R2 (4.97 μm) after operation, indicating the introduction of visible light could effectively mitigate membranes fouling. Moreover, microorganism community analysis revealed that visible light increased the relative abundance of Bacteroidetes and Chryseolinea, which not only enhanced the activity of microorganisms in metabolizing organic nutrients, but also improved the transfer and utilization of photogenerated electrons on the semiconductor-microorganism interface. The active aromatic protein metabolism and the upregulated related enzymes further demonstrated the synergistic effect of photocatalysis and microbial communities on the membrane fouling mitigation. This work provides a novel application of photocatalysis into antibiofouling effect in MBRs, and opens a strategy for bacteria inactivation and foulants removal with eco-friendly solar energy.

The interplay of hematite and photic biofilm triggers the acceleration of biotic nitrate removal

The soil-water interface is replete with photic biofilm and iron minerals; however, the potential of how iron minerals promote biotic nitrate removal is still unknown. This study investigates the physiological and ecological responses of photic biofilm to hematite (Fe2O3), in order to explore a practically feasible approach for in-situ nitrate removal. The nitrate removal by photic biofilm was significantly higher in the presence of Fe2O3 (92.5%) compared to the control (82.8%). Results show that the presence of Fe2O3 changed the microbial community composition of the photic biofilm, facilitates the thriving of Magnetospirillum and Pseudomonas, and promotes the growth of photic biofilm represented by the extracellular polymeric substance (EPS) and the content of chlorophyll. The presence of Fe2O3 also induces oxidative stress (•O2-) in the photic biofilm, which was demonstrated by electron spin resonance spectrometry. However, the photic biofilm could improve the EPS productivity to prevent the entrance of Fe2O3 to cells in the biofilm matrix and mitigate oxidative stress. The Fe2O3 then promoted the relative abundance of Magnetospirillum and Pseudomonas and the activity of nitrate reductase, which accelerates nitrate reduction by the photic biofilm. This study provides an insight into the interaction between iron minerals and photic biofilm and demonstrates the possibility of combining biotic and abiotic methods to improve the in-situ nitrate removal rate.

Influence of double-layer filling structure on nitrogen removal and internal microbial distribution in bioretention cells

The nitrogen removal effect of traditional bioretention cells on runoff rainwater is not stable. The nitrogen removal effect of bioretention cells can be improved by setting up a layered filling structure, but the effect of changes in filling structure on the nitrogen removal process and microbial community characteristics is still unclear. Two types of porosity fillers were set up in the experiment, and a homogeneous bioretention cell and three bioretention cells with layered fillers were constructed by changing the depth range of the upper and lower layers to analyze the influence of the pore variation of different depth fillers on the nitrogen removal process and microbial community characteristics. The experimental results showed that, compared with the homogeneous filing structure, the layered filling structure can strengthen the adsorption of NH₄⁺-N and the conversion of NO₃^--N, so as to increase the removal rates of NH₄⁺-N and NO₃^--N by 20.71-81.56% and 9.25%-78.19%, respectively. Although the low porosity filler structure will reduce the nitrification activity and urease activity by 48.63%-66.68% and 8.00%-20.64% respectively, it can increase the denitrification activity by 19.14%-31.92%, thus significantly reducing the nitrate content in the filler. The low porosity filler structure can affect the growth and reproduction of various phylum bacteria such as Proteobacteria, Chloroflexi, Acidobacteria, and genus bacteria such as Nitrospira, Ellin6067, Rhizobacter, Pseudomonas, which can improve the diversity and richness of microorganisms.

Response of microbial communities to nutrient removal in coastal sediment by using ecological concrete

Ecological concrete (eco-concrete) is a kind of environment-friendly material with porous characteristics. In this study, the eco-concrete was used to remove the total nitrogen (TN), total phosphorus (TP), and total organic carbon (TOC) in marine coastal sediment. The bacterial communities in sediment and on eco-concrete surface were also investigated by using high-throughput sequencing and quantitative PCR of 16S rRNA gene. We found that the mean removal efficiencies of TN, TP, and TOC in treatment group were 8.3%, 8.4%, and 12.3% after 28 days. The bacterial community composition in the treatment group was significantly different from that in the control group on day 28. In addition, the bacterial community composition on eco-concrete surface was slightly different from that in sediment, and the copy numbers of 16S rRNA gene were higher on eco-concrete surface than in sediment. The types of eco-concrete aggregates (gravel, pebble, and zeolite) also had effects on the bacterial community composition and 16S rRNA gene copy numbers. Furthermore, we found the abundant genus Sulfurovum increased significantly on eco-concrete surface in the treatment group after 28 days. Bacteria belonging to this genus were found having denitrification ability and were commonly detected in bioreactors for nitrate removal. Overall, our study expands the application scopes of eco-concrete and suggests that the bacterial communities in eco-concrete can potentially enhance the removal efficiency of nutrients in coastal sediment.

Research trends and hotspots of aquatic biofilms in freshwater environment during the last three decades: a critical review and bibliometric analysis

Freshwater periphytic biofilms (FPBs), existing widely in various aquatic environments, have attracted extensive attention for many years. In the present study, a bibliometric analysis based on Web of Science Core Collection (WoSCC) was used to understand the research progress, trends, and hot topics of FPBs qualitatively and quantitatively. The results indicated that publications on FPBs have increased from 1991 to 2020 rapidly, and researchers have focused more on the areas of environmental sciences, microbiology, and marine freshwater biology. The most influential countries were mainly the USA, Spain, France, and Germany. Cooperation network analysis reflected that the USA and its affiliated institutions played crucial roles in the research of FPB cooperation, but the collaboration between core author groups still fell short. Based on the analysis of top 20 high-cited FPB documents over the last 30 years, research hotspots mainly included micro-observation and assembly mechanisms of FPBs; interactions of FPBs and pollutants including heavy metals, antibiotic resistance genes, pathogens, organic pollutants, and nanoparticles; and the role of FPBs for biogeochemical cycling, especially nitrogen cycling. Additionally, future research directions were proposed. Overall, this study provides a comprehensive and systematic overview of FPBs, which is useful for research development and researchers who are interested in this area.

Chronic effects of cerium dioxide nanoparticles on biological nitrogen removal and nitrous oxide emission: Insight into impact mechanism and performance recovery potential

The influence of cerium dioxide nanoparticles (CeO₂ NPs) on biological nitrogen removal and associated nitrous oxide (N₂O) emission has seldom been addressed yet. Herein, the chronic effect of CeO₂ NPs on the nitrogen transformation processes during wastewater treatment and the impacted system's self-recovery potential after CeO₂ NP stress removal were investigated. CeO₂ NP of 10-50 mg/L induced significant declines of the ammonia nitrogen (NH₄⁺-N) and the total nitrogen removal efficiencies, but triggered the nitrite accumulation and the N₂O emission. The N₂O reductase (NOS) activity was negatively correlated with the N₂O emission level, and the inhibition of NOS activity under CeO₂ NP stress was probably due to the depressions of the sludge denitrifiers' metabolic activities. The NH₄⁺-N removal efficiency was successfully regained after the recovery period although the N₂O emission level was still higher than the pre-exposure period, which was probably due to the residual CeO₂ NPs inside the activated sludge.

Performance of basalt fiber-periphyton in deep-level nutrient removal: A study concerned periphyton cultivation, characterization and application

Nutrients in centralized discharge area of treated sewage can cause high ecological risks to aquatic systems, thus a deep-level nutrient removal is necessary. Recently, periphyton has attracted increasing interests for its excellent performance in nutrient removal. In this study, the suitability and durability of basalt fiber (BF) as a new green carrier of periphyton was evaluated, and development process of basalt fiber-periphtyon (BFP) was tracked with bacterial community succession and physiological indicators. Then, well-developed BFP was applied to deeply purify water containing the same concentration of nutrient as the treated sewage. Results showed the periphyton could adapt to BF and formed in large quantities. In addition, the tensile strength of BF after being used as a carrier was still strong. Bacterial community and physiological indicators indicated that BFP was well developed in 40-50 days. LEfSE and random forest analysis revealed that Deinococcus-Deinococci, Spartobacteria and Chlamydiia at class-level, Rhizobiales and Rhodobacterales at order-level were the biomarkers for development of BFP. Moreover, application results showed BFP efficiently removed nitrogen and phosphorus from water and promoted the transformation of ammonia to nitrate. The concentration of ammonia and phosphorus severely decreased from 4.90 ± 0.11 mg/L to 0.51 ± 0.20 mg/L, from 0.66 ± 0.016 mg/L to 0.023 ± 0.013 mg/L, respectively. The efficient nutrient removal was attributed to accumulation of nitrogen and phosphorus metabolism related organisms in BFP as well as favorable water physic-chemical conditions created by BFP. These results suggest that BF is a suitable and durable green carrier of periphyton, and BFP could efficiently reduce ecological risk to aquatic systems receiving treated sewage.

Contribution of periphytic biofilm of paddy soils to carbon dioxide fixation and methane emissions

Rice paddies are major contributors to anthropogenic greenhouse gas emissions via methane (CH₄) flux. The accurate quantification of CH₄ emissions from rice paddies remains problematic, in part due to uncertainties and omissions in the contribution of microbial aggregates on the soil surface to carbon fluxes. Herein, we comprehensively evaluated the contribution of one form of microbial aggregates, periphytic biofilm (PB), to carbon dioxide (CO₂) and CH₄ emissions from paddies distributed across three climatic zones, and quantified the pathways that drive net CH₄ production as well as CO₂ fixation. We found that PB accounted for 7.1%-38.5% of CH₄ emissions and 7.2%-12.7% of CO₂ fixation in the rice paddies. During their growth phase, PB fixed CO₂ and increased the redox potential, which promoted aerobic CH₄ oxidation. During the decay phase, PB degradation reduced redox potential and increased soil organic carbon availability, which promoted methanogenic microbial community growth and metabolism and increased CH₄ emissions. Overall, PB acted as a biotic converter of atmospheric CO₂ to CH₄, and aggravated carbon emissions by up to 2,318 kg CO₂ equiv ha^-1 season^-1. Our results provide proof-of-concept evidence for the discrimination of the contributions of surface microbial aggregates (i.e., PB) from soil microbes, and a profound foundation for the estimation and simulation of carbon fluxes in a potential novel approach to the mitigation of CH₄ emissions by manipulating PB growth.

Commonwealth of Soil Health: How Do Earthworms Modify the Soil Microbial Responses to CeO2 Nanoparticles?

Soil ecotoxicological assays on nanoparticles (NPs) have mainly investigated single components (e.g., plants, fauna, and microbes) within the ecosystem, neglecting possible effects resulting from the disturbance of the interactions between these components. Here, we investigated soil microbial responses to CeO₂ NPs in the presence and absence of earthworms from the perspectives of microbial functions (i.e., enzyme activities), the community structure, and soil metabolite profiles. Exposure to CeO₂ NPs (50, 500 mg/kg) alone decreased the activities of enzymes (i.e., acid protease and acid phosphatase) participating in soil N and P cycles, while the presence of earthworms ameliorated these inhibitory effects. After the CeO₂ NP exposure, the earthworms significantly altered the relative abundance of some microbes associated with the soil N and P cycles (Flavobacterium, Pedobacter, Streptomyces, Bacillus, Bacteroidota, Actinobacteria, and Firmicutes). This was consistent with the pattern found in the significantly changed metabolites which were also involved in the microbial N and P metabolism. Both CeO₂ NPs and earthworms changed the soil bacterial community and soil metabolite profiles. Larger alterations of soil bacteria and metabolites were found under CeO₂ NP exposure with earthworms. Overall, our study indicates that the top-down control of earthworms can drastically modify the microbial responses to CeO₂ NPs from all studied biological aspects. This clearly shows the importance of the holistic consideration of all soil ecological components to assess the environmental risks of NPs to soil health.

A review on plant-microbial interactions, functions, mechanisms and emerging trends in bioretention system to improve multi-contaminated stormwater treatment

Management and treatment of multi-polluted stormwater in bioretention system have gained significant attraction recently. Besides nutrients, recent source appointment studies found elevated levels of Potentially toxic metal(loid)s (PTMs) and contaminants of emerging concern (CECs) in stormwater that highlighted many limitations in conventional media adsorption-based pollutant removal bioretention strategies. The substantial new studies include biological treatment approaches to strengthen pollutants degradation and adsorption capacity of bioretention. The knowledge on characteristics of plants and their corresponding mechanisms in various functions, e.g., rainwater interception, retention, infiltration, media clogging prevention, evapotranspiration and phytoremediation, is scattered. The microorganisms' role in facilitating vegetation and media, plant-microorganism interactions and relative performance over different functions in bioretention is still unreviewed. To uncover the underneath, it was summarised plant and microbial studies and their functionality in hydrogeochemical cycles in the bioretention system in this review, contributing to finding their interconnections and developing a more efficient bioretention system. Additionally, source characteristics of stormwater and fate of associated pollutants in the environment, the potential of genetical engineered plants, algae and fungi in bioretention system as well as performance assessment of plants and microorganisms in non-bioretention studies to propose the possible solution of un-addressed problems in bioretention system have been put forward in this review. The present review can be used as an imperative reference to enlighten the advantages of adopting multidisciplinary approaches for the environment sustainability and pollution control.

Characterization of Epigallocatechin-Gallate-Grafted Chitosan Nanoparticles and Evaluation of Their Antibacterial and Antioxidant Potential

Nanoparticles based on chitosan modified with epigallocatechin gallate (EGCG) were synthetized by nanoprecipitation (EGCG-g-chitosan-P). Chitosan was modified by free-radical-induced grafting, which was verified by Fourier transform infrared (FTIR). Furthermore, the morphology, particle size, polydispersity index, and zeta potential of the nanoparticles were investigated. The grafting degree of EGCG, reactive oxygen species (ROS) production, antibacterial and antioxidant activities of EGCG-g-chitosan-P were evaluated and compared with those of pure EGCG and chitosan nanoparticles (Chitosan-P). FTIR results confirmed the modification of the chitosan with EGCG. The EGCG-g-chitosan-P showed spherical shapes and smoother surfaces than those of Chitosan-P. EGCG content of the grafted chitosan nanoparticles was 330 μg/g. Minimal inhibitory concentration (MIC) of EGCG-g-chitosan-P (15.6 μg/mL) was lower than Chitosan-P (31.2 μg/mL) and EGCG (500 μg/mL) against Pseudomonas fluorescens (p < 0.05). Additionally, EGCG-g-chitosan-P and Chitosan-P presented higher Staphylococcus aureus growth inhibition (100%) than EGCG at the lowest concentration tested. The nanoparticles produced an increase of ROS (p < 0.05) in both bacterial species assayed. Furthermore, EGCG-g-chitosan-P exhibited higher antioxidant activity than that of Chitosan-P (p < 0.05) in 2,2'-azino-bis (3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS), 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) and ferric-reducing antioxidant power assays. Based on the above results, EGCG-g-chitosan-P shows the potential for food packaging and biomedical applications.

Study on optimization and performance of biological enhanced activated sludge process for pharmaceutical wastewater treatment

Simulated pharmaceutical wastewater was treated by moving bed biofilm reactor (MBBR) and total reflux sludge reactor process (STR) system. By cultivating specific bacterial groups, optimizing reactor process parameters, and comparatively analyzing the pollutant removal efficiency under stable operating conditions of the system, the treatment efficiency of the two systems under the combined impact load of organic pollutants on the target pollutants indole and naphthalene was studied. The optimal operation parameters of reactors: hydraulic retention time (HRT) was 8 h, aeration was 0.12 m³/h. The effect was better in 25 ± 1 °C than that in 20 ± 2 °C. During stable operation, the average removal rate of chemical oxygen demand (COD) and ammonia nitrogen (NH₄⁺-N) of the MBBR system was significantly higher than that of STR, and the two kinds of target pollutants concentration in water was lower than the detection limit. In the combined impact test of organic pollutants, the dominant bacterial group obtained by domestication had a high degradation ability, so the combined impact of indole and naphthalene had little effect on the two reactors. But in the fourth stage, the residual naphthalene concentration in the STR system effluent exceeded the target value. Therefore, the MBBR process has a stronger treatment effect on pharmaceutical wastewater than the STR system during the stable period and the impact load stage.

Metal Oxide Nanoparticles Against Bacterial Biofilms: Perspectives and Limitations

At present, there is an urgent need in medicine and industry to develop new approaches to eliminate bacterial biofilms. Considering the low efficiency of classical approaches to biofilm eradication and the growing problem of antibiotic resistance, the introduction of nanomaterials may be a promising solution. Outstanding antimicrobial properties have been demonstrated by nanoparticles (NPs) of metal oxides and their nanocomposites. The review presents a comparative analysis of antibiofilm properties of various metal oxide NPs (primarily, CuO, Fe3O4, TiO2, ZnO, MgO, and Al2O3 NPs) and nanocomposites, as well as mechanisms of their effect on plankton bacteria cells and biofilms. The potential mutagenicity of metal oxide NPs and safety problems of their wide application are also discussed.

Heterogeneous Diffusion of Polystyrene Nanoparticles through an Alginate Matrix: The Role of Cross-linking and Particle Size

Most bacteria in natural and engineered environments grow and exist in biofilms. Recent investigations have shown that nanoparticles (NPs) interact with environmental biofilms, but these interactions are still not well characterized. Extracellular polymeric substances (EPS) are polymers secreted by bacteria to establish the functional and structural integrity of biofilms, and EPS porosity is a major contributor to NP access to and diffusion in biofilms. We used a synergistic combination of total internal reflection fluorescence microscopy and image correlation spectroscopy to monitor and map diffusion of fluorescent NPs in alginate yielding a detailed picture of the heterogeneous structure and connectivity of pores within a model EPS polymer. Using different sizes (20, 100, and 200 nm) of carboxylated polystyrene NPs, we examined how NP diffusive behaviors change as a result of calcium-induced cross-linking of the alginate matrix. This study reveals that cross-linking decreases NP diffusion coefficients and pore accessibility in an NP size-dependent manner and that NP movement through alginate matrices is anisotropic and heterogeneous. These results on heterogeneous and size-dependent movement within biofilms have important implications for future studies and simulations of NP-biofilm interactions.

Potential Hazard of Lanthanides and Lanthanide-Based Nanoparticles to Aquatic Ecosystems: Data Gaps, Challenges and Future Research Needs Derived from Bibliometric Analysis

Lanthanides (Ln), applied mostly in the form of nanoparticles (NPs), are critical to emerging high-tech and green energy industries due to their distinct physicochemical properties. The resulting anthropogenic input of Ln and Ln-based NPs into aquatic environment might create a problem of emerging contaminants. Thus, information on the biological effects of Ln and Ln-based NPs is urgently needed for relevant environmental risk assessment. In this mini-review, we made a bibliometric survey on existing scientific literature with the main aim of identifying the most important data gaps on Ln and Ln-based nanoparticles' toxicity to aquatic biota. We report that the most studied Ln for ecotoxicity are Ce and Ln, whereas practically no information was found for Nd, Tb, Tm, and Yb. We also discuss the challenges of the research on Ln ecotoxicity, such as relevance of nominal versus bioavailable concentrations of Ln, and point out future research needs (long-term toxicity to aquatic biota and toxic effects of Ln to bottom-dwelling species).

Responses of Periphyton Microbial Growth, Activity, and Pollutant Removal Efficiency to Cu Exposure

Periphyton is an effective matrix for the removal of pollutants in wastewater and has been considered a promising method of bioremediation. However, it still needs to be verified whether periphyton can maintain microbial activity and pollutant removal efficiency when dealing with the influence with complex components, and the underlying mechanisms of periphyton need to be revealed further. Herein, this study investigated the microbial growth, activity and functional responses of periphyton after removal of Cu from wastewater. Results showed that the cultivated periphyton was dominated by filamentous algae, and high Cu removal efficiencies by periphyton were obtained after 108 h treatments. Although 2 mg/L Cu²⁺ changed the microalgal growth (decreasing the contents of total chlorophyll-a (Chla), the carbon source utilization and microbial metabolic activity in periphyton were not significantly affected and even increased by 2 mg/L Cu²⁺. Moreover, chemical oxygen demand (COD) removal rates were sustained after 0.5 and 2 mg/L Cu²⁺ treatments. Our work showed that periphyton had strong tolerance and resistance on Cu stress and is environmentally friendly in dealing with wastewater containing heavy metals, as the microbial functions in pollutant removal could be maintained.

Microscale and molecular analyses of river biofilm communities treated with microgram levels of cerium oxide nanoparticles indicate limited but significant effects

Cerium oxide (CeO₂) nanoparticles are used as in-fuel catalysts and in manufacturing processes, creating a potential for release to aquatic environments. Exposures at 1 and 10 μg/L CeO₂-nanoparticles were made to assess effects during the development of river biofilm communities. Scanning transmission x-ray microscopy (STXM) indicated extensive sorption of nanoparticles to the community and co-localization with lipid moieties. Following 8 weeks of development, polycarbonate coupons were removed from the reactors and used for molecular analyses, denaturing gradient gel electrophoresis analysis (DGGE-16S rRNA) and 16S rRNA amplicon sequencing. Microscopic imaging of the biofilm communities (bacterial, photosynthetic biomass, exopolymer composition, thickness, protozoan numbers), as well as carbon substrate utilization fingerprinting was performed. There was a trend toward reduced photosynthetic biomass, but no significant effects of CeO₂ exposure were found on photosynthetic and bacterial biomass or biofilm thickness. Sole carbon source utilization analyses indicated increased utilization of 10 carbon sources in the carbohydrate, carboxylic acid and amino acids categories related to CeO₂ exposures; however, predominantly, no significant effects (p < 0.05) were detected. Measures of microbial diversity, lectin binding affinities of exopolymeric substances and results of DGGE analyses, indicated significant changes to community composition (p < 0.05) with CeO₂ exposure. Increased binding of the lectin Canavalia ensiformis was observed, consistent with changes in bacterial-associated polymers. Whereas, no significant changes were observed in binding to residues associated with algal and cyanobacterial exopolymers. 16S rRNA amplicon sequencing of community DNA indicated changes in diversity and shifts in community composition; however, these did not trend with increasing CeO₂ exposure. Counting of protozoans in the biofilm communities indicated no significant effects on this trophic level. Thus, based on biomass and functional measures, CeO₂ nanoparticles did not appear to have significant effects; however, there was evidence of selection pressure resulting in significant changes in microbial community composition.

Effects of titanium dioxide nanoparticles on algal and bacterial communities in periphytic biofilms

The widespread application of commercial TiO₂ NPs inevitably leads to their release into environmental waters through various ways. TiO₂ NPs released into water might be absorbed by and react with periphytic biofilms, which are a kind of aquatic environmental media of important ecological significance, and influence the physiological activity and ecological function of periphytic biofilms. This study investigated the effects of exposure to 1 mg/L and 5 mg/L of TiO₂ NPs on periphytic biofilms cultured indoors. After a 10-day exposure to TiO₂ NPs, the growth (measured by chlorophyll-a content) of microalgal community was inhibited greatly (more than 60%); however, the primary production (indicated by quantum yield) of periphytic biofilms maintained changeless. As for bacteria, TiO₂ NP-exposure increased the bacterial diversity and altered the composition structure. Significant changes were observed in the bacterial communities at the class level, mainly including Alphaproteobacteria, Gammaproteobacteria, Cytophagia, Flavobacteriia, Sphingobacteriia, Synechococcophycideae and Oscillatoriophycideae. The enhancement of metabolic activities (the production of extracellular polymeric substances, especially proteins content increased by 48.51%) of periphytic biofilms was a resistance mechanism to toxicity of NPs. As for extracellular enzyme activities of periphytic biofilms, alkaline phosphatase activity was inhibited (22.43%) after exposed to 5 mg/L of TiO₂ NPs, which posed a threat to phosphorus metabolism of periphytic biofilms. Overall, this study demonstrated that 1 mg/L and 5 mg/L of TiO₂ NPs negatively influenced physiological activities and ecological functions of periphytic biofilms, highlighting that the ecological risks of TiO₂ NPs should be paid attention to.

Strategies and relative mechanisms to attenuate the bioaccumulation and biotoxicity of ceria nanoparticles in wastewater biofilms

Inhibitory effects of ceria nanoparticles (CeO₂ NPs) on biofilm were investigated individually and in combination with phosphate (P), ethylene diamine tetraacetic acid (EDTA), humic acid (HA) and citrate (CA) to further explore the toxicity alleviating solutions. Exposure to 20 mg/L CeO₂ NPs significantly decreased the performance of biofilm in nutrients removal. Distribution experiments suggested >98% of the CeO₂ NPs retained in microbial aggregates, leading to 51.26 μg/L Ce ions dissolution. The dissolved Ce^IV and its further being reduced to Ce^III stimulated the formation of O₂^- and OH, which increased lipid peroxidation level to 130.93% in biofilms. However, P/EDTA/CA captured or precipitated Ce ions, whereas EDTA/HA/CA shielded NPs-bacteria direct contacts, both disturbing the NPs adsorption, intercepting the redox transition between Ce^IV and Ce^III, reducing the generation of O₂^- and OH, thus mitigating the toxicity of CeO₂ NPs. These results illustrate the main drivers of CeO₂ NPs biotoxicity and provide safer-by-design strategies.

A New Concept of Promoting Nitrate Reduction in Surface Waters: Simultaneous Supplement of Denitrifiers, Electron Donor Pool, and Electron Mediators

The efficiency of biological nitrate reduction depends on the community composition of microorganisms, the electron donor pool, and the electron mediators participating in the biological reduction process. This study aims at creating an in situ system comprising of denitrifiers, electron donors, and electron mediators to reduce nitrate in surface waters. The ubiquitous periphytic biofilm in waters was employed to promote in situ nitrate reduction in the presence of titanium dioxide (TiO₂) nanoparticles (NPs). The nitrate removal rate in the periphytic biofilm and TiO₂ NPs system was significantly higher than the control (only periphytic biofilm or TiO₂ NPs). TiO₂ NPs optimized the community composition of periphytic biofilm for nitrate reduction by increasing the relative abundance of four dominant denitrifying bacteria. Periphytic biofilm showed a substantial increase in extracellular polymeric substance, especially the humic acid and protein content, due to the presence of TiO₂ NPs. The synergistic action of humic acid, protein, denitrifying bacteria of the periphytic biofilm, and TiO₂ NPs contributed to 80% of the nitrate reduction. The protein and humic acid, acting as electron mediators, facilitated the transfer of exogenous electrons from photoexcited TiO₂ NPs to periphytic biofilm containing denitrifiers, which enhanced nitrate reduction in surface waters.

Imaging the Microprocesses in Biofilm Matrices

Biofilms, which are aggregates of microorganisms and extracellular matrices, widely colonize natural water bodies, wastewater treatment systems, and body tissues, and have vital roles in water purification, biofouling, and infectious diseases. Recently, multiple imaging modalities have been developed to visualize the morphological structure and material distribution within biofilms and to probe the microprocesses in biofilm matrices, including biofilm formation, transfer and metabolism of substrates, and cell-cell communication. These technologies have improved our understanding of biofilm control and the fates of substrates in biofilms. In this review, we describe the principles of various imaging techniques and discuss the advantages and limitations of each approach to characterizing microprocesses in biofilm matrices.