Electrolyte Cations Binding with Extracellular Polymeric Substances Enhanced Microcystis Aggregation: Implication for Microcystis Bloom Formation in Eutrophic Freshwater Lakes

Dissolved organic matter, calcium ion and extracellular polymeric substances on living associated bacteria of Microcystis colony are crucial for unicellular Microcystis to efficiently form colonies

Microcystis typically forms colonies under natural conditions, which contributes to occurrence and prevalence of algal blooms. The colonies consist of Microcystis and associated bacteria (AB), embedded in extracellular polymeric substances (EPS). Previous studies indicate that AB can induce Microcystis to form colonies, however the efficiency is generally low and results in a uniform morphotype. In this study, by using filtrated natural water, several AB strains induced unicellular M. aeruginosa to form colonies resembling several Microcystis morphotypes. The mechanisms were investigated with Methylobacterium sp. Z5. Ca2+ was necessary for Z5 to induce Microcystis to form colonies, while dissolved organic matters (DOM) facilitated AB to agglomerate Microcystis to form large colonies. EPS of living Z5, mainly the aromatic protein components, played a key role in colony induction. Z5 initially aggregated Microcystis via the bridging effects of Ca2+ and DOM, followed by the induction of EPS synthesis and secretion in Microcystis. In this process, the colony forming mode shifted from cell adhesion to a combination of cell adhesion and cell division. Intriguingly, Z5 drove the genomic rearrangement of Microcystis by upregulating some transposase genes. This study unveiled a novel mechanism about Microcystis colony formation and identified a new driver of Microcystis genomic evolution.

Effect of surfactant's charge properties on behavior, physiology, and biochemistry and the release of microcystins of Microcystis aeruginosa

Surfactant pollution is escalatitheng in eutrophic waters, but the effect of surfactant charge properties on the physiological and biochemical properties of toxin-producing microalgae remains inadequately explored. To address this gap, this study explores the effects and mechanisms of three common surfactants-cetyltrimethylammonium bromide (CTAB, cationic), sodium dodecyl sulfate (SDS, anionic), and Triton X-100 (nonionic)-found in surface waters, on the agglomeration behavior, physiological indicators, and Microcystin-LR (MC-LR) release of Microcystis aeruginosa (M. aeruginosa) by using UV-visible spectroscope, Malvern Zetasizer, fluorescence spectrometer, etc. Results suggest that charge properties significantly affect cyanobacterial aggregation and cellular metabolism. The CTAB-treated group demonstrates a ∼5.74 and ∼9.74 times higher aggregation effect compared to Triton X-100 and SDS (300 mg/L for 180 min) due to strong electrostatic attraction. Triton X-100 outperforms CTAB and SDS in polysaccharide extraction, attributed to its higher water solubility and lower critical micelle concentration. CTAB stimulates cyanobacteria to secrete proteins, xanthohumic acid, and humic acids to maintain normal physiological cells. Additionally, the results of SEM and ion content showed that CTAB damages the cell membrane, resulting in a ∼90% increase in the release of intracellular MC-LR without cell disintegration. Ionic analyses confirm that all three surfactants alter cell membrane permeability and disrupt ionic metabolic pathways in microalgae. This study highlights the relationship between the surface charge properties of typical surfactants and the dispersion/agglomeration behavior of cyanobacteria. It provides insights into the impact mechanism of exogenous surfactants on toxic algae production in eutrophic water bodies, offering theoretical references for managing surfactant pollution and treating algae blooms.

Preferential deposition of buoyant small microplastics in surface sediments of the Three Gorges Reservoir, China: Insights from biomineralization

Sediments act as sinks of microplastics (MPs) derived from terrestrial ecosystems. However, the fate and transport of MPs at the zone of sediment-overlying water in reservoir environment are poorly understood. Here, the MPs distribution patterns in surface sediments of the Three Gorges Reservoir (TGR) and dominant mechanisms responsible for the sinking of MPs at the zone of sediment-overlying water were comprehensively investigated. The predominant occurrence of small microplastics (<300 µm, SMPs) in surface sediments of the TGR was found, with buoyant polyethene (PE) was dominant polymer types. Interestingly, the high abundance of SMPs in sediments correlated well with the Ca2+/Mg2+ in overlying water, suggesting that divalent cations in overlying water may enhance the preferential deposition of SMPs. Simulation sinking experiments under the presence of Microcystis aeruginosa and two divalent cations using different-sized PE MPs demonstrated that the greater deposition of SMPs was mainly the result of the formation of biogenic calcite on the surface of MPs rather than magnesium minerals, which provides stronger ballasting effects for SMPs than for large MPs. This study first highlights that the impact of biomineralization on preferential sinking of SMPs and enhances the understanding of the transport behaviour of MPs in aquatic environment.

Impacts of microplastic contamination on the rheology properties of sediments in a eutrophic shallow lake

Microplastic (MP) contamination is a growing global concern, with lake sediments serving as a significant sink for MP due to both anthropogenic and natural activities. Given the increasing evidence of MP accumulation in sediments, it was crucial to assess their influence on sediment erosion resistance, which directly affected sediment resuspension. To fill this gap, this study focused on the effect of MP on the sediments rheological properties. After 60-day experiments, it was found that MP addition into sediments reduced sediment viscosity, yield stress, and flow point shear stress. Meanwhile, MPs also significantly altered sediment properties and extracellular polymer composition. MP addition reduced extracellular polymeric substances production and cation exchange capacity, which then worked together and led to a weak sediment structure. Seemingly, MPs changed fluid sediment characteristics and caused stronger fluidity under less shear force. Consequently, the accumulation of MP might facilitate the resuspension of sediments under smaller wind and wave disturbances. This study provided novel insights into the direct impact of MPs on sediment physical properties using rheology, thereby enhancing our understanding of the environmental behavior of MPs in lake ecosystems.

Physiology, microcystin production, and transcriptomic responses of Microcystis aeruginosa exposed to calcium and magnesium

Calcium ions (Ca2+) and magnesium ions (Mg2+) are pivotal in the community composition and stability of harmful cyanobacteria, yet the physiological and molecular responses remains poorly understood. This study aims to explore these responses in the high microcystin producer Microcystis aeruginosa (M. aeruginosa). Results indicate that the growth of M. aeruginosa is inhibited by Ca2+/Mg2+ exposure (0.5-10 mM), while Fv/Fm photosynthetic parameters and extracellular microcystin-leucine-arginine (MC-LR) concentrations increase. Additionally, MC-LR release is significantly elevated under exposure to Ca2+/Mg2+, posing potential aquatic environmental risks. Transcriptomic analysis reveals downregulation of genes related to cell architecture, membrane transport, and metabolism, while the genes linked to photosynthesis electron transmission and heavy metal-responsive transcriptional regulators are upregulated to adapt to environmental changes. Further analysis reveals that Ca2+ and Mg2+ primarily impact sulfur metabolism and transport of amino acids and mineral within cells. These findings provide insights into M. aeruginosa cells responses to Ca2+ and Mg2+ exposure.

Spatial and temporal distribution of the microbial community structure in the receiving rivers of the middle and lower reaches of the Yangtze River under the influence of different wastewater types

The gradual intensification of human activity has caused severe negative impacts on the ecosystems of the Yangtze River Basin. Treated effluents still affect the environment and health of receiving rivers, particularly in terms of microbial community structure. However, relatively few studies have been conducted on the differences in the effects of wastewater types on microbial community structure. Three sampling campaigns (237 samples) were conducted in the Nanjing and Wuhan sections of the Yangtze River Basin. Our results showed that the microbial community structure differed significantly among the water periods and could recover to its original state at > 500 m downstream of the outfall. The diversity of the receiving rivers under the influence of industrial wastewater was higher than that of the other wastewater types, although the number of taxa was lower than that of other wastewater types. Cyanobium_PCC-6307 and Rhodoferax were screened for biomarkers in samples affected by domestic and industrial wastewater, respectively. Although different kinds of wastewater influenced the microbial community structure, environmental factors, and geographical distance were still the main drivers. This study suggests that treated wastewater still poses a risk to ecosystems and highlights the importance of effective management strategies for assessing ecosystem health.

The extracellular polysaccharide determine the physico-chemical surface properties of Microcystis

Microcystis possesses the capacity to form colonies and blooms in lakes and reservoirs worldwide, causing significant ecological challenges in aquatic ecosystems. However, little is known about the determining factors of physico-chemical surface properties that govern the competitive advantage of Microcystis. Here, The physico-chemical surface properties of Microcystis wesenbergii and Microcystis aeruginosa, including specific surface area (SSA), hydrophobicity, zeta potential, and functional groups were investigated. Additionally, the extracellular polysaccharide (EPS) were analyzed. Laboratory-cultured Microcystis exhibited hydrophilic, a negative zeta potential and negatively charged. Furthermore, no significant relationship was shown between these properties and the cultivation stage. Microcystis wesenbergii exhibited low free energy of cohesion, high surface free energy, high growth rate, and high EPS content during the logarithmic phase. On the other hand, M. aeruginosa displayed lower free energy of cohesion, high surface free energy, high EPS content, and high growth rate during the stationary phase. These characteristics contribute to their respective competitive advantage. Furthermore, the relationship between EPS and surface properties was investigated. The polysaccharide component of EPS primarily influenced the SSA and total surface energy of Microcystis. Likewise, the protein component of EPS influenced hydrophobicity and surface tension. The polysaccharide composition, including glucuronic acid, xylose, and fructose, mainly influenced surface properties. Additionally, hydrophilic groups such as O-H and P-O-P played a crucial role in determining hydrophobicity in Microcystis. This study elucidates that EPS influenced the SSA, hydrophobicity, and surface free energy of Microcystis cells, which in turn impact the formation of Microcystis blooms and the collection.

Investigation and prediction of the biotoxicity of Cu2+ to Chlorella vulgaris: modification of the biotic ligand model

The increased copper ion (Cu2+) concentrations in aquatic ecosystem significantly influence the environmental quality and ecosystem safety, while information on the Cu2+ biotoxicity to aquatic microorganisms and the models for biotoxicity prediction are still unclear. In this study, the toxicities of Cu2+ to Chlorella vulgaris under different environmental conditions (e.g., Na+, K+, Ca2+, Mg2+, pH, and dissolved organic matter) were explored, with the experimental results in comparison with those predicted by the biotic ligand model (BLM). Results showed that increased Cu2+ concentration caused obvious toxicities to C. vulgaris, whereas the commonly occurring cations and dissolved organic matters can protect the metabolism system of C. vulgaris. The presence of extracellular polymeric substances (EPS) matrix can alleviate the biotoxicity via increasing the surface biosorption but decreasing cell internalization of Cu2+ in C. vulgaris. Due to the presence of EPS matrix, the experimental biotoxicity results under each condition were significantly lower than those predicted by the BLM model, which was thus modified via taking the EPS matrix as the supplement of allochthonous organic matters. After that, the modified BLM was characterized with a higher degree of precision and can be used in natural waters for biotoxicity prediction. Results obtained can enhance our insights into the ecological effects and biotoxicity prediction of heavy metals in natural aquatic ecosystems.

Biodegradability of algal-derived dissolved organic matter and its influence on methylmercury uptake by phytoplankton

Methylmercury (MeHg) uptake by phytoplankton represents a key step in determining the exposure risks of aquatic organisms and human beings to this potent neurotoxin. Phytoplankton uptake is believed to be negatively related to dissolved organic matter (DOM) concentration in water. However, microorganisms can rapidly change DOM concentration and composition and subsequent impact on MeHg uptake by phytoplankton has rarely been tested. Here, we explored the influences of microbial degradation on the concentrations and molecular compositions of DOM derived from three common algal sources and tested their subsequent impacts on MeHg uptake by the widespread phytoplankton species Microcystis elabens. Our results indicated that dissolved organic carbon was degraded by 64.3‒74.1% within 28 days of incubating water with microbial consortia from a natural meso‑eutrophic river. Protein-like components in DOM were more readily degraded, while the numbers of molecular formula for peptides-like compounds had increased after 28 days' incubation, probably due to the production and release of bacterial metabolites. Microbial degradation made DOM more humic-like which was consistent with the positive correlations between changes in proportions of Peaks A and C and bacterial abundance in bacterial community structures as illustrated by 16S rRNA gene sequencing. Despite rapid losses of the bulk DOM during the incubation, we found that DOM degraded after 28 days still reduced the MeHg uptake by Microcystis elabens by 32.7‒52.7% relative to a control without microbial decomposers. Our findings emphasize that microbial degradation of DOM would not necessarily enhance the MeHg uptakes by phytoplankton and may become more powerful in inhibiting MeHg uptakes by phytoplankton. The potential roles of microbes in degrading DOM and changing the uptakes of MeHg at the base of food webs should now be incorporated into future risk assessments of aquatic Hg cycling.

Effects of variable-sized polyethylene microplastics on soil chemical properties and functions and microbial communities in purple soil

Microplastic contamination of soil has drawn increased attention due to the ecological harm it poses to the soil ecosystem. However, little is known about how microplastic particle sizes affect soil chemical properties and microbial communities, particularly in purple soil. In this study, a four-week incubation experiment was conducted to evaluate the effect of polyethylene microplastics (PE MPs) with different particle sizes (i.e., 300 and 600 μm) on soil properties, extracellular polymeric substances (EPS), enzyme activities, and microbial communities in purple soil. When compared to 600 μm-PE MPs, 300 μm-PE MPs reduced contents of dissolved organic matter (DOM), EPS, and β-1,4-N-acetylglucosaminidase (NAG) activity, but increased the cation exchange capacity (CEC). High-throughput 16S rRNA gene sequencing revealed that the 300 μm-PE MPs resulted in an increase in the phylum Nitrospirae, which is associated with microplastic degradation. The data implied that smaller PE MPs improved the growth of polyethylene-degrading bacteria by adsorbing more EPS and DOM, resulting in the degradation of microplastics. Co-occurrence network analysis revealed that smaller PE MPs had lower toxicity to microbial populations than larger PE MPs, increasing the stability of the network. CEC and β-1,4-glucosidase (BG) were found to be the two major factors affecting the microbial communities by redundancy analysis (RDA). The study highlighted how microplastic particle sizes affect soil bacterial communities and soil functions.

Phosphorus Accumulation in Extracellular Polymeric Substances (EPS) of Colony-Forming Cyanobacteria Challenges Imbalanced Nutrient Reduction Strategies in Eutrophic Lakes

Extracellular polymeric substances (EPS) are crucial for cyanobacterial proliferation; however, certain queries, including how EPS affects cellular nutrient processes and what are the implications for nutrient management in lakes, are not well documented. Here, the dynamics of cyanobacterial EPS-associated phosphorus (EPS-P) were examined both in a shallow eutrophic lake (Lake Taihu, China) and in laboratory experiments with respect to nitrogen (N) and phosphorus (P) availability. Results indicated that 40-65% of the total cyanobacterial aggregate/particulate P presented as EPS-P (mainly labile P and Fe/Al-P). Phosphorus-starved cyanobacteria rapidly replenished their EPS-P pools after the P was resupplied, and the P concentration in this pool was stable for long afterward, although the environmental P concentration decreased dramatically. A low-N treatment enhanced the EPS production alongside two-fold EPS-P accumulation (particularly labile P) higher than the control. Such patterns occurred in the lake where EPS and EPS-P contents were high under N limitation. EPS-P enrichment increased the P content in cyanobacteria; subsequently, it could hold the total P concentration higher for longer and make bloom mitigation harder. The findings outline a new insight into EPS functions in the P process of cyanobacterial aggregates and encourage consideration of both N and P reductions in nutrient management.

Insight into biofilm-forming patterns: biofilm-forming conditions and dynamic changes in extracellular polymer substances

The microbial biofilm adheres to the surface of the carrier, which protects the pollutant-degrading bacteria and resists harsh environments; thus, research on biofilm-forming patterns will help promote the application of biofilms in wastewater treatment. Herein, univariate analysis and response surface methodology (RSM) confirmed that glucose and mannose at 3-5 g/L promoted biofilm formation. Notably, the microplate method demonstrated that compared to trivalent cations, divalent cations could more greatly enhance the activity (especially magnesium) of the biofilm matrix, and the period of biofilm formation in the three strains was divided into the following stages: initial attachment (0-10 h), microcolony (10-24 h), maturation (24-48 h), and dispersion (36-72 h). During maturation, large amounts of extracellular polysaccharides (EPs) and extracellular DNA (eDNA) were distributed in the extracellular and intracellular spaces, respectively, as observed by super-resolution structured illumination microscopy (SR-SIM). This study enhances the understanding of the characteristics and patterns of biofilm formation and can facilitate the application of biofilms in wastewater treatment.

Shift of calcium-induced Microcystis aeruginosa colony formation mechanism: From cell adhesion to cell division

Colony formation is an essential stage of cyanobacterial blooms. High calcium concentration can promote Microcystis aeruginosa aggregation behavior, but the mechanism of colony formation caused by calcium has rarely been reported. In this study, high calcium-induced colony formation was identified as a shift from cell adhesion to cell division, rather than only cell adhesion as previously thought. Algae responded to this calcium-induced environmental pressure by aggregating and forming colonies. Algal cells initially secreted large quantities of extracellular polysaccharides (EPS) and rapidly aggregated by cell adhesion. The highest aggregation proportion was up to 68.93%. However, high calcium concentrations cannot completely inhibit algal cell growth, but only delay the algae into the rapid growth phase. With adaption to calcium and existing high EPS content, the daughter cells reduced EPS synthesis and the aggregation proportion decreased. The increasing growth rate was also responsible for the decreased xylose content in EPS. The mechanism of colony formation changed to cell division. The downregulation of genes related to EPS secretion also supported this hypothesis. Overall, these results can benefit for our understanding of cyanobacterial bloom formation.

Comparison of organic matter (OM) pools in water, suspended particulate matter, and sediments in eutrophic Lake Taihu, China: Implication for dissolved OM tracking, assessment, and management

Suspended particulate matter (SPM) and sediments are important sources of dissolved organic matter (DOM) in lake water. However, studies on what extent and how both sources affect DOM composition are lacking, which hampers DOM management. Herein, DOM, SPM-extracted particulate organic matter (POM), and sediment-extracted organic matter (SOM) were characterized and compared in terms of absorption spectral properties and chemical composition in Lake Taihu, a large cyanobacterial bloom-affected shallow lake. A statistical method was proposed to quantify the similarity of organic matter (OM) in the different states and to evaluate the potential effects of SPM and sediments on DOM. Results showed that POM and DOM were mainly composed of small-molecular-size and low-humified organic components (i.e., 27 %-38 % tryptophan-like and ~30 % protein-like substances), and most of them were derived from autochthonous sources. While tyrosine-like (57 %) and humic-like (27 %) substances were dominant in SOM. The OM similarity between POM and DOM was approximately 1.5 times higher than that between SOM and DOM, indicating the greater effect of SPM than sediments on DOM composition. High pH and low nitrogen (e.g., nitrate and ammonia) were positively correlated to the OM similarity between POM and DOM. Further, the findings indicated that nitrogen limitation enhanced the OM exchange between POM and DOM by promoting the production of extracellular polymeric substances (EPS) in cyanobacterial aggregates. The obtained findings highlighted the importance of SPM in shaping the DOM composition relative to sediments and facilitating the DOM management in bloom-affected lakes.

Effects of soil colloids on the aggregation and degradation of engineered nanoparticles (Ti3C2Tx MXene)

Soil colloid is a nonnegligible factor when evaluating the environmental risk of engineered nanoparticles (ENPs) in the groundwater. In this study, the environmental fate of an emerging ENP (Ti₃C₂T_x MXene) in the groundwater was investigated for the first time, which currently poses a severe environmental risk due to its cytotoxicity but has received little attention. The colloidal dispersion stability and degradation kinetics of Ti₃C₂T_x MXene in the groundwater were evaluated by considering the effects of soil colloids prepared from sodium humate (SH), montmorillonite (MT), and a natural soil (NS) under variable solution chemistry. The results showed that the affinity of soil colloids with Ti₃C₂T_x followed an SH > MT > NS sequence. Increasing SH concentration led to Ti₃C₂T_x disaggregation by enhancing the electrical and steric repulsive forces, while MT and NS resulted in hetero-aggregation because of the elevated collision frequency. SH and MT enhanced the critical coagulation concentrations of Ti₃C₂T_x by 100 and 10 folders, respectively, via surface coating process, while NS slightly reduced due to the bridging effects induced by the soluble cations. The soil colloids promoted Ti₃C₂T_x degradation compared with their absence and in an SH > MT ≫ NS sequence. SH and MT were through forming Ti-O-C and Si-O-Ti bonds with Ti₃C₂T_x via their carboxyl and hydroxyl groups, respectively, rendering the Ti₃C₂T_x surface more reactive and faster degradation. NS showed a weak promotion effect because of its less affinity with Ti₃C₂T_x and limited organic matter and clay contents with hydroxyl and carboxyl groups. This study demonstrated the unstable environmental behaviors of Ti₃C₂T_x in the groundwater and mitigated its environmental risk concerns.

Microcystis colony formation: Extracellular polymeric substance, associated microorganisms, and its application

Microcystis sp., amongst the most prevalent bloom-forming cyanobacteria, is typically found as a colonial form with multiple microorganisms embedded in the mucilage known as extracellular polymeric substance. The colony-forming ability of Microcystis has been thoroughly investigated, as has the connection between Microcystis and other microorganisms, which is crucial for colony development. The following are the key subjects to comprehend Microcystis bloom in depth: 1) key issues related to the Microcystis bloom, 2) features and functions of extracellular polymeric substance, as well as diversity of associated microorganisms, and 3) applications of Microcystis-microorganisms interaction including bloom control, polluted water bioremediation, and bioactive compound production. Future research possibilities and recommendations regarding Microcystis-microorganism interactions and their significance in Microcystis colony formation are also explored. More information on such interactions, as well as the mechanism of Microcystis colony formation, can bring new insights into cyanobacterial bloom regulation and a better understanding of the aquatic ecosystem.

UV/ozone induced physicochemical transformations of polystyrene nanoparticles and their aggregation tendency and kinetics with natural organic matter in aqueous systems

Once discharged into the environment, plastics debris are unavoidably subjected to natural weathering processes. Unfortunately, the impact of weathering on the aggregation tendency and kinetics of nanoplastics in complex environmental matrices is poorly understood. Here, we investigated the influence of weathering as induced by UV and O₃ treatments, on the aggregation of polystyrene nanoparticles (PSNPs) in simulated waters containing representative organic molecules (humic acid, lysozyme, and alginate) and in natural waters. Results showed that UV/O₃ weathering-induced physicochemical transformations of PSNPs, particularly the formation of oxygen-containing functional groups and the increase in hydrophilicity, altered the aggregation state of PSNPs to different extents. The presence of organic molecules destabilized the UV-aged PSNPs with strength of lysozyme > alginate > humic acid, owing to the decrease of sorption of macromolecules on their surface. Differently, the O₃-aged PSNPs displayed strong stability in the absence or presence of organic molecules (except for lysozyme), probably due to steric repulsion arising from the leakage of endogenous organic matters. This work demonstrates that the aggregation behavior of PSNPs is determined by the complex interplays among weathering, natural organic matter, and solution chemistry, and provides significant insights into the fate and transport of PSNPs in realistic scenarios.

Production of bio-stable fluid sediment from accumulation of cyanobacterial bloom biomass under various water depths

While fluid sediments normally formed through hydrodynamic erosion and transport was well known, the fluid sediments caused by organic matter accumulation and degradation in eutrophic lakes was rarely investigated. Here, the effects of cyanobacterial bloom biomass (CBB) accumulation and water depth on the occurrence of fluid sediments were studied. Within 30 days of experiments, the variation of sediment height firstly increased to the maximum with rising in water depth, then decreased due to the high hydraulic pressure. While the surface sediments density decreased slightly from 1.35 g cm^-3 to around 1.32 g cm^-3 without CBB accumulation, and CBB accumulation led to lower density (around 1.02 g cm^-3) but higher shear stress of sediments. Through analyzing the extracellular polymeric substances (EPS), it was found that CBB accumulation improved the polysaccharide/protein ratios of sediment. The infrared analysis further indicated that the bound-EPS could protect fluid sediments bio-stabilization. Meanwhile, the enriched Acinetobacter, Pseudomonas in sediments with CBB accumulation might play roles in EPS production, which benefited the bio-stabilization of fluid sediments. Furthermore, the stability of fluid sediments increased with increase in water depth, and the resuspension of biological fluid sediments would occur more likely in the low water depth area. Altogether, this study reported the formation and stability of the biological fluid sediments in eutrophic shallow lakes, and could help provide clues against sediment resuspension in lake ecosystems.

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.

Photogeochemistry of particulate organic matter in aquatic systems: A review

Photochemical transformation of natural organic matter in aquatic environments strongly impacts the environmental behaviors of carbon, nutrients, and pollutants by affecting their solubility, toxicity, bioavailability, and mobility. However, the role of particulate organic matter (POM) in environmental photogeochemistry has received much less attention than that of dissolved organic matter (DOM). In this study, a systematic overview was conducted to summarize the photodissolution and photoflocculation of POM in aquatic systems. The photodissolution of various POM, such as resuspended sediments and algal detritus, could be a potential and important source of DOM in the overlying waters, and these photoreleased DOM were dominated by humic-like components. The photogeochemistry of POM is thought to proceed via direct photochemical reactions and reactive radical-dominated indirect processes. Photodissolution can modify the bioavailability of organic matter and influence the biogeochemical cycling of nutrients, heavy metals, and organic pollutants. In addition, the photo-induced flocculation of DOM to POM could also influence the transport and transformation of organic matter and its associated pollutants. The photochemistry of POM can be significantly influenced by several environmental factors, including irradiation wavelength and intensity, organic matter properties, and radical oxygen species. POM photogeochemistry is one of the most important components of the global cycling of natural organic matter. Further studies regarding photogeochemistry should be conducted to overcome the potential problems arising from the concurrent photodegradation of organic matter and to further develop more filed investigations and analytical methods.

Humic acid inhibits colony formation of the cyanobacterium Microcystis at high level of iron

Colony formation is a key process for the occurrence of Microcystis blooms. In order to inhibit colony formation of Microcystis at high level of iron using humic acid, unicellular Microcystis aeruginosa was cultivated in laboratory treated with varying concentrations of iron and humic acid. Our results showed that the extracellular polysaccharides (EPS) content and average colony size increased from 0.57 pg cells^-1 and 4.0 μm to 0.93 pg cells^-1 and 26.1 μm, respectively, while iron concentration increased from 0.68 mg L^-1 to 6.8 mg L^-1, suggesting that high level of iron stimulated EPS secretion and induced unicellular Microcystis to form colonies. Transcriptome analysis showed that two genes described as glycosyltransferases (BH695-2217 and BH695-3696) were significantly up-regulated while EPS content increased with increasing iron concentration indicating that iron may regulate the expression of genes involved in polysaccharide synthesis. When treated with 10 mg C L^-1 humic acid at high level of iron, the EPS content and average colony size decreased by 35.5% and 56.3%, respectively, revealing that humic acid inhibited EPS secretion under high level of iron condition, and ultimately inhibited colony formation of Microcystis. Our results suggested that humic acid could be used as an agentia inhibiting large colony formation of Microcystis and thereby reducing the occurrence of Microcystis blooms.

Effects of cations on biofilm formation and characteristics in integrated fixed film activated sludge process at different carbon and nitrogen loadings

Both divalent cations including calcium and magnesium play important roles for microbial aggregates in binding to negatively charged functional groups on bacterial surfaces, in extracellular polymeric substances (EPS), and on inorganic materials in flocs and biofilms. Monovalent cations such as sodium and potassium deteriorate the floc structure and physical properties. The Integrated Fixed Activated Sludge (IFAS) process employs fixed film media in the aerobic zone; therefore, both monovalent and divalent cations are involved in the process performances. In this study, the effects of cations indicated as the monovalent to divalent cations (M/D) ratio on the biofilm formation and characteristics, and on the IFAS performances for carbon and ammonium removals were evaluated. The experiments were conducted in three IFAS systems feeding with the same wastewater but different M/D ratios and two carbon and nitrogen loadings. The findings revealed that high monovalent with low divalent cations at the M/D ratios higher than 2.0 produced excessive polysaccharides in EPS resulting in high viscosity of activated sludge flocs causing viscous bulking with high SVI values, decreasing the biofilm formation, and increasing the biofilm sloughing. Increasing of both monovalent and carbon loading increased more polysaccharides in the EPS leading to the failures of IFAS system. Nitrification failed at higher M/D ratios because of less nitrifiers in flocs and biofilm. The M/D ratio less than 2.0 is suggested to minimize the excessive EPS production in the IFAS system, especially at high organic loading.

A Novel Jumbo Phage PhiMa05 Inhibits Harmful Microcystis sp

Microcystis poses a concern because of its potential contribution to eutrophication and production of microcystins (MCs). Phage treatment has been proposed as a novel biocontrol method for Microcystis. Here, we isolated a lytic cyanophage named PhiMa05 with high efficiency against MCs-producing Microcystis strains. Its burst size was large, with approximately 127 phage particles/infected cell, a short latent period (1 day), and high stability to broad salinity, pH and temperature ranges. The PhiMa05 structure was composed of an icosahedral capsid (100 nm) and tail (120 nm), suggesting that the PhiMa05 belongs to the Myoviridae family. PhiMa05 inhibited both planktonic and aggregated forms of Microcystis in a concentration-dependent manner. The lysis of Microcystis resulted in a significant reduction of total MCs compared to the uninfected cells. A genome analysis revealed that PhiMa05 is a double-stranded DNA virus with a 273,876 bp genome, considered a jumbo phage. Out of 254 predicted open reading frames (ORFs), only 54 ORFs were assigned as putative functional proteins. These putative proteins are associated with DNA metabolisms, structural proteins, host lysis and auxiliary metabolic genes (AMGs), while no lysogenic, toxin and antibiotic resistance genes were observed in the genome. The AMGs harbored in the phage genome are known to be involved in energy metabolism [photosynthesis and tricarboxylic acid cycle (TCA)] and nucleotide biosynthesis genes. Their functions suggested boosting and redirecting host metabolism during viral infection. Comparative genome analysis with other phages in the database indicated that PhiMa05 is unique. Our study highlights the characteristics and genome analysis of a novel jumbo phage, PhiMa05. PhiMa05 is a potential phage for controlling Microcystis bloom and minimizing MC occurrence.

Interaction of cyanobacteria with calcium facilitates the sedimentation of microplastics in a eutrophic reservoir

Low-density microplastics are frequently found in sediments of many lakes and reservoirs. The processes leading to sedimentation of initially buoyant polymers are poorly understood for inland waters. This study investigated the impact of biofilm formation and aggregation on the density of buoyant polyethylene microplastics. Biofilm formation on polyethylene films (4 × 4 × 0.15 mm) was studied in a eutrophic reservoir (Bautzen, Saxony, Germany). Additionally, aggregation dynamics of small PE microplastics (~85 µm) with cyanobacteria were investigated in laboratory experiments. During summer phototrophic sessile cyanobacteria (Chamaesiphon spp. and Leptolyngbya spp.) precipitated calcite while forming biofilms on microplastics incubated in Bautzen reservoir. Subsequently the density of the biofilms led to sinking of roughly 10% of the polyethylene particles within 29 days of incubation. In the laboratory experiments planktonic cyanobacteria (Microcystis spp.) formed large and dense cell aggregates under the influence of elevated Ca²⁺ concentrations. These aggregates enclosed microplastic particles and led to sinking of a small portion (~0.4 %) of polyethylene microplastics. This study showed that both sessile and planktonic phototrophic microorganisms mediate processes influenced by calcium which facilitates densification and sinking of microplastics in freshwater reservoirs. Loss of buoyancy leads to particle sedimentation and could be a prerequisite for the permanent burial of microplastics within reservoir sediments.

The complexation with proteins in extracellular polymeric substances alleviates the toxicity of Cd (II) to Chlorella vulgaris

The complexation with extracellular polymeric substances (EPS) greatly reduces the toxicity of heavy metals towards organisms in the environment. However, the molecular mechanism of EPS-metal complexation remains unclear owing to the limitation of precise analysis for key fractions and functionalities in EPS that associate with metals. Herein, we explored the EPS-Cd (II) complexation by fluorescence excitation emission matrix coupled with parallel factor (EEM-PARAFAC), two-dimensional Fourier transform infrared correlation spectroscopy (2D-FTIR-COS) and X-ray photoelectron spectroscopy (XPS), attempting to explain the mechanisms of EPS in alleviating Cd (II) toxicity toward a green alga Chlorella vulgaris (C. vulgaris). When the algal EPS were removed, the cell internalizations of Cd (II), growth inhibition rate and chlorophyll autofluorescence increased, but the surface adsorption and esterase activities decreased, indicating that the sorption of Cd (II) by EPS was crucial in alleviating the algal toxicity. Moreover, the complexation with proteins in EPS controlled the sorption of Cd (II) to algal EPS, resulting in the chemical static quenching of the proteins fluorescence by 47.69 ± 2.37%. Additionally, the complexing capability of the main functionalities, COO^- and C-OH in proteins with Cd (II) was stronger than that of C-O(H) and C-O-C in polysaccharides or C-OH in the humus-related substances. Oxygen atom in protein carboxyl C-O might be the key site of EPS-Cd (II) complexation, supported by the modified Ryan-Weber complexation model and the obvious shift of oxygen valence-electron signal. These findings provide deep insights into understanding the interaction of EPS with heavy metals in aquatic environment.

Interactions of CeO2 nanoparticles with natural colloids and electrolytes impact their aggregation kinetics and colloidal stability

The aggregation kinetics and colloidal stability of CeO₂-NPs in the presence of monovalent or divalent electrolytes, as well as inorganic (kaolin and goethite) and organic (humic acid, HA) colloids were evaluated using time-resolved dynamic light scattering, advanced spectroscopic tools, and theoretical calculations. Critical coagulation concentrations for CeO₂-NPs were generally lower in CaCl₂ than that in NaCl electrolyte. The negatively charged kaolin accelerated CeO₂-NPs aggregation due to electrostatic attraction, whilst opposite phenomenon was observed for the positively charged goethite in NaCl solution. In CaCl₂ solution, goethite destabilized CeO₂-NPs because of its well-crystal structure and specific adsorption of Ca²⁺. The presence of 0.1 mg C/L HA decreased the surface charge of CeO₂-NPs, resulting in lower critical coagulation concentrations. Increasing the HA concentration from 0.1 to 1 mg C/L improved CeO₂-NPs stability, mainly via electrostatic and steric repulsion. The Ca²⁺-bridging and complexation contributed significantly to CeO₂-NPs aggregation. Additional aggregation experiments in seven natural waters revealed that CeO₂-NPs remained stable in water types with high contents of organic colloids and low levels of salts, thus having higher transport potential. These findings provided new insights into the interactive influence of naturally occurring colloids and ions on the heteroaggregation behavior and fate of CeO₂-NPs.

Calcium promotes formation of large colonies of the cyanobacterium Microcystis by enhancing cell-adhesion

Large Microcystis colonies can lead to the rapid formation of surface accumulations, which are a globally significant environmental issue. Laboratory studies have shown that Ca²⁺ can quickly promote non-classical Microcystis colony formation via cell-adhesion, but our knowledge of the changes in the morphology of these colonies during subsequent long-term culture with Ca²⁺ is limited. In this study, a 72-day cultivation experiment was conducted to determine the long-term effects of Ca²⁺ on Microcystis colony formation. Laboratory results indicate that Ca²⁺ causes Microcystis to rapidly aggregate and form a colony through cell adhesion, then colony formation by cell-adhesion lost dominance, owing to the decrease in Ca²⁺ concentrations caused by precipitation/complexation. Although the initial colony morphology by cell adhesion is sparse, the newly divided cells, without separating from the mother cells, constantly fill the gaps in the original colony at Ca²⁺ concentrations >40 mg L^-1 for a long time, which creates colonies on day 72 with a morphology similar to that of M. ichthyoblabe in Lake Taihu. If the Ca²⁺ levels in Lake Taihu continue to increase, Microcystis growth rate will decrease only slightly, while the colony proportion of total biovolume and biomass will increase. Moreover, higher Ca²⁺ concentrations do not affect microcystin content, but promote the content of bound extracellular polysaccharides (bEPS), enabling formation of larger colonies, which may promote Microcystis surface accumulation.

Characterization of Microcystis morphotypes: Implications for colony formation and intraspecific variation

Groundworks on Microcystis colony formation and morphological variation are critical to understanding the whole eco-cycle of Microcystis blooms. In this study, we tested the cell adhesion effect, an important pathway for colony formation, among Microcystis colonies of different morphotypes, and examined the potential linkage between cell properties and morphological plasticity. Results showed that cell adhesion significantly contributed to the aggregation of Microcystis colonies, but such adhesion only occurred in colonies belonging to the same morphotype. This suggests that Microcystis cannot form large colonies through a direct adhesion effect among different morphotypes, possibly due to substantial differences in the chemical structures and compositions of their extracellular polymeric substances (EPS). Cell functional features also varied substantially with morphotypes, implying high intraspecific variation in competitive and defensive strategies of Microcystis. Our results offer new insights into colony formation of Microcystis and substantiate the importance of fundamental chemical characteristics of EPS in determining the morphological plasticity.

Characterizing Properties and Environmental Behaviors of Dissolved Organic Matter Using Two-Dimensional Correlation Spectroscopic Analysis

Dissolved organic matter (DOM) exists ubiquitously in environments and plays critical roles in pollutant mitigation, transformation, and organic geochemical cycling. Understanding its properties and environmental behaviors is critically important to develop water treatment processes and environmental remediation strategies. Generalized two-dimensional correlation spectroscopy (2DCOS), which has numerous advantages, including enhancing spectral resolution and discerning specific order of structural change under an external perturbation, could be used as a powerful tool to interpret a wide range of spectroscopic signatures relating to DOM. A suite of spectroscopic signatures, such as UV-vis, fluorescence, infrared, and Raman spectra that can be analyzed by 2DCOS, is able to provide additional structural information hiding behind the conventional one-dimensional spectra. In this article, the most recent advances in 2DCOS applications for analyzing DOM-related environmental processes are reviewed, and the state-of-the-art novel spectroscopic techniques in 2DCOS are highlighted. Furthermore, the main limitations and requirements of current approaches for exploring DOM-related environmental processes and how these limitations and drawbacks can be addressed are explored. Finally, suggestions and new approaches are proposed to significantly advance the development of 2DCOS in analyzing the properties and behaviors of DOM in natural and engineered environments.

Exploring Utilization of Recycled Agricultural Biomass in Constructed Wetlands: Characterization of the Driving Force for High-Rate Nitrogen Removal

Improper treatment of various wastewaters with a low C/N ratio and management of abundant agricultural wastes may pose a serious threat to bodies of water and agricultural ecosystems in rural areas, especially in developing countries. Thus, a potential alternative for simultaneous mitigation of this pollution is needed to protect rural environments. This study investigated the feasibility and enhanced performance of applying typical agricultural wastes (such as wheat straw, apricot pits, and walnut shells) as carbon sources for nitrogen removal in constructed wetlands (CWs). The leaching experiment employed fluorescence excitation-emission spectrophotometry and revealed that the wheat straw material had the highest capability of carbon release with an average dissolved organic carbon release content and rate of 27.88 mg g^-1 and 5.24 mg g^-1 day^-1, respectively. Dissolved organic matter released from different agricultural wastes mainly consisted of humic acid-like and fulvic acid-like compounds. Long-term assessment of lab-scale intermittent aeration CWs receiving agricultural wastes revealed a high total nitrogen removal of 66.75-93.67% in low carbon/nitrogen ratio wastewaters (C/N = 3). These findings can contribute to a better understanding of the driving mechanism through which agricultural wastes enhance nitrogen removal in CW wastewater treatments.

Dynamic molecular size transformation of aquatic colloidal organic matter as a function of pH and cations

Knowledge of the dynamic changes in molecular size of natural colloidal organic matter (COM) along the aquatic continuum is of vital importance for a better understanding of the environmental fate and ecological role of dissolved organic matter and associated contaminants in aquatic systems. We report here the pH- and cation-dependent size variations of COMs with different sources (river and lake) quantified using flow field-flow fractionation (FIFFF), fluorescence spectroscopy and parallel factor analysis (PARAFAC), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and zeta potential analysis. Increasing pH caused a decline in molecular sizes and an obvious size transformation from the >10 kDa to 5-10 kDa and further to 1-5 kDa size fraction, whereas the opposite trend was observed for increasing cation (e.g., Ca²⁺ and Cu²⁺) abundance. Compared with lakewater COM, the riverwater COM exhibited a greater pH-dependent dispersion but less extent in cation-induced aggregation, demonstrating that the dispersion and aggregation dynamics were highly dependent on COM source and solution chemistry (e.g., pH and cations). Based on ATR-FTIR analysis, the extensive dissolution of C=O and C-O functional groups resulted in a greater pH-dependent dispersion for river COM. Fluorescence titration revealed that, despite their similar cation-induced aggregation behavior, the binding constants of all the PARAFAC-derived components for Cu²⁺ were 1-2 orders of magnitude higher than those for Ca²⁺ (logK_M: 4.54-5.45 vs. 3.35-3.70), indicating a heterogeneous nature in cation-DOM interactions. The greater extent of decline in zeta potential for lake COM suggested a Ca-induced charge neutralization and aggregation mechanism. However, for Cu-induced aggregation, chemical complexation was the predominant pathway for the river COM, with higher binding constants, while charge neutralization and chemical complexation co-induced the aggregation of lake COM. Thus, natural COMs may have different environmental behavior along the aquatic continuum and further affect the fate and transport of contaminants in aquatic environments.

Dissolved organic matter binding with Pb(II) as characterized by differential spectra and 2D UV-FTIR heterospectral correlation analysis

Dissolved organic matter (DOM) in aquatic environment significantly influences the behavior and fate of heavy metals via binding, complexation and thus changes the metal speciation; however detailed interfacial processes and mechanisms are still unclear. Here, differential absorbance and fluorescence spectra and two dimensional UV-FTIR heterospectral correlation analysis were applied to probe into the Pb(II)-DOM interaction at a wide range of pH and ionic strength (IS). The absorbance of DOM molecules under all conditions increased with metal addition, while the different extents of absorbance variations along the wavelength range in the differential zero-order and log-transformed absorbance spectra indicated the site heterogeneity within the DOM pool for metal binding. Spectral parameters, namely differential fluorescent components 1 and 2 (DFC1 and DFC2) and differential slopes of log-transformed absorbance in the range of wavelength 350-400 nm (DSlope_350-400) were found to be highly correlated with the total amounts of DOM-bound Pb(II) predicted by the NICA-Donnan model, while the differential absorbance spectra at 235 nm (DA₂₃₅) was related to the extent of Pb(II) bound by carboxylic groups. Thus, these parameters are an indicator or proxy for the in situ Pb(II)-DOM interaction extent. Aryl C-H gave the fastest response to Pb(II) binding at lower pH and IS (e.g., pH 4.7 and IS = 0.01 M), followed by carboxyl C=O and polysaccharide C-OH and then chromophoric groups at 265 nm (CDOM₂₆₅). However, the CDOM₂₆₅ bound to Pb(II) prior to aryl C-H and polysaccharide C-OH groups at higher pH and IS (6.0 and 0.1 M, respectively), showing that the binding sequences were highly dependent on solution chemistry. Differential spectra combined with two dimensional UV-FTIR heterospectral correlation analysis can be used as a promising approach to elucidate metal-DOM interaction processes, including site heterogeneity, binding sensitivity and sequence at the functional group level.

Nanoparticle tracking analysis versus dynamic light scattering: Case study on the effect of Ca2+ and alginate on the aggregation of cerium oxide nanoparticles

The effect of Ca²⁺ and alginate on the stability of CeO₂ nanoparticles (NPs) was investigated using dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA); the two methods were then compared. DLS showed rapid aggregation of CeO₂ NPs in 8 mM Ca²⁺ solution; however, NTA showed that some primary aggregates (200-400 nm) still remained-a result that was obscured in DLS measurements. Aggregation of alginate molecules was also studied using DLS and NTA, where NTA particle concentration and video provided additional information on the alginate aggregation progress. Finally, DLS showed that in the presence of alginate, the aggregation rate and size of CeO₂ NPs increased. NTA intensity measurements provided insight into a heteroaggregation and bridging mechanism. NTA particle concentration and video also showed CeO₂ NPs were linked by alginate gel in high Ca²⁺ concentration (>4 mM). the DLS and NTA had different advantages in measuring particle size. DLS could better study the initial aggregation stage and large aggregates, while NTA could better detect small aggregates. NTA also measured particle number concentration, individual intensity, and particle motion video, which provided additional insight. Combining these two methods could help us to better understand the behavior and fate of NPs in water.

Temporal and spatial distribution of Microcystis biomass and genotype in bloom areas of Lake Taihu

Cyanobacterial blooms as a global environmental issue are of public health concern. In this study, we investigated the spatial (10 sites) and temporal (June, August and October) variations in: 1) their biomass based on chlorophyll-a (chl-a) concentration, 2) their toxic genotype based on gene copy ratio of mcyJ to cpcBA, and 3) their cpcBA genotype composition of Microcystis during cyanobacterial bloom in Lake Taihu. While spatial-temporal variations were found in chl-a and mcyJ/cpcBA ratio, only spatial variation was observed in cpcBA genotype composition. Samples from northwestern part had a higher chl-a, but mcyJ/cpcBA ratio didn't vary among the sites. High chl-a was observed in August, while mcyJ/cpcBA ratio and genotypic richness increased with time. The spatial variations in chl-a and mcyJ/cpcBA ratio and temporal variation in cpcBA genotype were correlated negatively with dissolved N and positively with dissolved P. Spatial distribution of Microcystis biomass was positively correlated with nitrite and P excluding October, but no correlation was found for spatial distribution of mcyJ/cpcBA ratio and cpcBA genotype. Spatial distribution of toxic and cpcBA genotypes may result from horizontal transport of Microcystis colonies, while spatial variation in Microcystis biomass was probably controlled by both nutrient-mediated growth and horizontal transport of Microcystis. The temporal variation in Microcystis biomass, toxic genotype and cpcBA genotype composition were related to nutrient levels, but cause-and-effect relationships require further study.

Characterization, origin and aggregation behavior of colloids in eutrophic shallow lake

Stability of colloidal particles contributes to the turbidity in the water column, which significantly influences water quality and ecological functions in aquatic environments especially shallow lakes. Here we report characterization, origin and aggregation behavior of aquatic colloids, including natural colloidal particles (NCPs) and total inorganic colloidal particles (TICPs), in a highly turbid shallow lake, via field observations, simulation experiments, ultrafiltration, spectral and microscopic, and light scattering techniques. The colloidal particles were characterized with various shapes (spherical, polygonal and elliptical) and aluminum-, silicon-, and ferric-containing mineralogical structures, with a size range of 20-200 nm. The process of sediment re-suspension under environmentally relevant conditions contributed 78-80% of TICPs and 54-55% of NCPs in Lake Taihu, representing an important source of colloids in the water column. Both mono- and divalent electrolytes enhanced colloidal aggregation, while a reverse trend was observed in the presence of natural organic matter (NOM). The influence of NOM on colloidal stability was highly related to molecular weight (MW) properties with the high MW fraction exhibiting higher stability efficiency than the low MW counterparts. However, the MW-dependent aggregation behavior for NCPs was less significant than that for TICPs, implying that previous results on colloidal behavior using model inorganic colloids alone should be reevaluated. Further studies are needed to better understand the mobility/stability and transformation of aquatic colloids and their role in governing the fate and transport of pollutants in natural waters.

Contrasting effects of photochemical and microbial degradation on Cu(II) binding with fluorescent DOM from different origins

Effects of photochemical and microbial degradation on variations in composition and molecular-size of dissolved organic matter (DOM) from different sources (algal and soil) and the subsequent influence on Cu(II) binding were investigated using UV-Vis, fluorescence excitation-emission matrices coupled with parallel factor analysis, flow field-flow fractionation (FlFFF), and metal titration. The degradation processes resulted in an initial rapid decline in the bulk dissolved organic carbon and chromophoric and fluorescent DOM components, followed by a small or little decrease. Specifically, photochemical reaction decreased the aromaticity, humification and apparent molecular weights of all DOM samples, whereas a reverse trend was observed during microbial degradation. The FlFFF fractograms revealed that coagulation of both protein- and humic-like DOM induced an increase in molecular weights for algal-DOM, while the molecular weight enhancement for allochthonous soil samples was mainly attributed to the self-assembly of humic-like components. The Cu(II) binding capacity of algal-derived humic-like and fulvic-like DOM consistently increased during photo- and bio-degradation, while the soil-derived DOM exhibited a slight decline in Cu(II) binding capacity during photo-degradation but a substantial increase during microbial degradation, indicating source- and degradation-dependent metal binding heterogeneities. Pearson correlation analysis demonstrated that the Cu(II) binding potential was mostly related with aromaticity and molecular size for allochthonous soil-derived DOM, but was regulated by both DOM properties and specific degradation processes for autochthonous algal-derived DOM. This study highlighted the coupling role of inherent DOM properties and external environmental processes in regulating metal binding, and provided new insights into metal-DOM interactions and the behavior and fate of DOM-bound metals in aquatic environments.

The antibiotic resistome of free-living and particle-attached bacteria under a reservoir cyanobacterial bloom

In freshwater systems, both antibiotic resistance genes (ARGs) and cyanobacterial blooms attract global public health concern. Cyanobacterial blooms can greatly impact bacterial taxonomic communities, but very little is known about the influence of the blooms on antibiotic resistance functional community. In this study, the ARGs in both free-living (FL) and particle-attached (PA) bacteria under bloom and non-bloom conditions were simultaneously investigated in a subtropical reservoir using high-throughput approaches. In total, 145 ARGs and 9 mobile genetic elements (MGEs) were detected. The most diverse and dominant of which (68.93%) were multidrug resistance genes and efflux pump mechanism. The richness of ARGs in both FL and PA bacteria was significantly lower during the bloom period compared with non-bloom period. The abundance of ARGs in FL bacteria was significantly lower under bloom condition than in the non-bloom period, but the abundance of ARGs in PA bacteria stayed constant. More importantly, the resistant functional community in PA bacteria was more strongly influenced by the cyanobacterial bloom than in the FL bacteria, although >96% ARGs were shared in both FL and PA bacteria or both bloom and non-bloom periods. We also compared the community compositions between taxonomy and function, and found antibiotic resistant communities were highly variable and exhibited lower similarity between bloom and non-bloom periods than seen in the taxonomic composition, with an exception of FL bacteria. Altogether, cyanobacterial blooms appear to have stronger inhibitory effect on ARG abundance in FL bacteria, and stronger influence on antibiotic resistant community composition in PA bacteria. Our results further suggested that both neutral and selective processes interactively affected the ARG composition dynamics of the FL and PA bacteria. However, the antibiotic resistant community of FL bacteria exhibited a higher level of temporal stochasticity following the bloom event than PA bacteria. Therefore, we emphasized the bacterial lifestyles as an important mechanism, giving rise to different responses of antibiotic resistant community to the cyanobacterial bloom.

Variations in size and composition of colloidal organic matter in a negative freshwater estuary

Dynamic variations in chemical composition and size distribution of dissolved organic matter (DOM) along the river-lake interface in the Fox River plume were investigated using ultrafiltration, flow field-flow fractionation, UV-Vis and fluorescence spectroscopy and parallel factor analysis. On average, ~67% of bulk dissolved organic carbon (DOC) were partitioned in the <1kDa (actual cutoff 2.5kDa) low molecular weight fraction, and the other 33% were in the 1kDa-0.7μm colloidal phase. Concentrations of DOC and chromophoric DOM in the bulk and size-fractionated samples decreased monotonously with decreasing conductivity from river to bay waters, demonstrating a dominant terrestrial source and quasi conservative mixing behavior. However, the percentages of colloidal fluorescent-DOM increased while those of carbohydrates decreased from river to bay waters, showing different mixing behavior in the river plume. Colloidal chromophores and humic-like fluorophores were mainly partitioned in the size range of 1-6nm, but a bimodal distribution (with peaks at 1-6 and 35-45nm) was observed for colloidal protein-like DOM. Along the river-lake transect, the peak locations of chromophores, humic-like and small-sized protein-like colloids remained almost constant, while the larger-sized protein-like colloids exhibited a slight peak shift from 38.3 to 40.4nm, showing a molecular size enhancement from high to low conductivity waters, with physical mixing, photochemical/microbial degradation, and disaggregation/repartitioning being the important processes affecting the variations of DOM size and composition. New results herein should enhance our understanding of the heterogeneity of DOM in size and composition and its fate, transport and transformation at the river-lake interface and along the aquatic continuum as a whole.

The effects of extracellular polymeric substances on magnetic iron oxide nanoparticles stability and the removal of microcystin-LR in aqueous environments

The behaviors of nanoparticles rely on the aqueous condition such as natural organic matter (NOM). Therefore the presence of NOM would influence the interaction of nanoparticles with other substances possibly. Here, microcystin-LR (MC-LR) adsorption on iron oxide nanoparticles (IONPs) was studied in an aqueous solution with different types of NOM, including extracellular polymeric substances (EPS) from cyanobacteria and alginic acid sodium salt (AASS) from brown algae. Results revealed that EPS played an important role in stabilizing IONPs and in the toxin adsorption efficiency. The stability of IONPs was heavily depended on the concentration and type of NOM, which can affect the surface charge of IONPs significantly in solution. The enhanced stability of IONPs was due to the electrostatic interactions. Adsorption kinetics and isotherm studies confirmed that NOM can affect the IONPs' adsorption efficiency, and pseudo-second-order kinetics better explained this process. The removal efficiency for MC-LR decreased in the presence of NOM (Control > EPS-M1 > AASS > EPS-M9), indicating that NOM and MC-LR compete for limited adsorption sites. The presence of NOM in a eutrophic environment stabilized the IONPs while inhibiting the MC-LR removal efficiency. This investigation emphasized the negative effect of cyanobacterial EPS on the removal of microcystins when using magnetic separation technology. And this results could also be used to model the transportation of iron minerals carrying toxic substances in aqueous environment.

Identifying Plant Stress Responses to Roxarsone in Soybean Root Exudates: New Insights from Two-Dimensional Correlation Spectroscopy

Roxarsone (ROX) is an organoarsenic feed additive of increasing interest used in the poultry industry. Soybean responses to ROX stress were investigated in root exudates (REs) using two-dimensional correlation spectroscopy (2D-COS) with fluorescence and Fourier transform infrared spectra. Environmentally relevant ROX concentrations caused negligible toxicity to crop growth and photosynthesis activity but blackened soybean roots at high concentrations. 2D-COS analysis revealed that the protein-like fluorophore and C═C and C═O, aliphatic OH, and polysaccharide C-O-H moieties in soybean REs were most sensitive to ROX stress. Heterospectral 2D-COS results suggested that aromatic, amide I, quinone, ketone, and aliphatic functional groups were the foundational components of protein-like and short-wavelength excited humic-like fluorophores in soybean REs. Carboxyl and phenolic moieties were related to the long-wavelength excited humic-like fluorophore. Overall, 2D-COS combined with molecular-based spectral analysis of REs provided an innovative approach to characterize the physiological responses of crops to contaminants at sublethal levels.

Responses of Periphyton to Fe2O3 Nanoparticles: A Physiological and Ecological Basis for Defending Nanotoxicity

The toxic effects of nanoparticles on individual organisms have been widely investigated, while few studies have investigated the effects of nanoparticles on ubiquitous multicommunity microbial aggregates. Here, periphyton as a model of microbial aggregates, was employed to investigate the responses of microbial aggregates exposed continuously to Fe₂O₃ nanoparticles (5.0 mg L^-1) for 30 days. The exposure to Fe₂O₃ nanoparticles results in the chlorophyll (a, b, and c) contents of periphyton increasing and the total antioxidant capacity decreasing. The composition of the periphyton markedly changes in the presence of Fe₂O₃ nanoparticles and the species diversity significantly increases. The changes in the periphyton composition and diversity were due to allelochemicals, such as 3-methylpentane, released by members of the periphyton which inhibit their competitors. The functions of the periphyton represented by metabolic capability and contaminant (organic matter, nitrogen, phosphorus and copper) removal were able to acclimate to the Fe₂O₃ nanoparticles exposure via self-regulation of morphology, species composition and diversity. These findings highlight the importance of both physiological and ecological factors in evaluating the long-term responses of microbial aggregates exposed to nanoparticles.

Aggregation kinetics of inorganic colloids in eutrophic shallow lakes: Influence of cyanobacterial extracellular polymeric substances and electrolyte cations

The stability/aggregation propensity of inorganic colloids in eutrophic shallow lakes is of great essence in governing the water transparency and contaminant behavior. In this study, time-resolved dynamic light scattering was employed to investigate the aggregation kinetics of Al₂O₃ inorganic colloids over a wide range of cyanobacterial extracellular polymeric substance (EPS) concentrations in the absence and presence of electrolyte cations. The results showed that EPS adsorption alone greatly decreased the hydrodynamic diameters of colloidal particles, whose stability behavior followed closely the predictions of the classical DLVO theory. Electrolyte cations, however, can induce the aggregation of colloidal particles, and divalent Ca²⁺ were found to be more efficient in destabilizing the colloids than monovalent Na⁺, as indicated by the considerably lower critical coagulation concentrations (2.5 mM for Ca²⁺ vs. 170 mM for Na⁺). Further addition of Ca²⁺, i.e., >2.5 mM, caused an extremely high aggregation degree and rate. High resolution transmission electron microscopy revealed that this enhanced aggregation should be attributed to the gel-like bridging between colloidal particles, which were verified to be the amorphous EPS-Ca²⁺ complexes. Field-emission scanning electron microscopy coupled with elemental mapping provided additional evidence that the bridging interaction of EPS with Ca²⁺ was the predominant mechanism for the aggregation enhancement.

Enrichment of Saccharides and Divalent Cations in Sea Spray Aerosol During Two Phytoplankton Blooms

Sea spray aerosol (SSA) is a globally important source of particulate matter. A mesocosm study was performed to determine the relative enrichment of saccharides and inorganic ions in nascent fine (PM_2.5) and coarse (PM_10-2.5) SSA and the sea surface microlayer (SSML) relative to bulk seawater. Saccharides comprise a significant fraction of organic matter in fine and coarse SSA (11 and 27%, respectively). Relative to sodium, individual saccharides were enriched 14-1314-fold in fine SSA, 3-138-fold in coarse SSA, but only up to 1.0-16.2-fold in SSML. Enrichments in SSML were attributed to rising bubbles that scavenge surface-active species from seawater, while further enrichment in fine SSA likely derives from bubble films. Mean enrichment factors for major ions demonstrated significant enrichment in fine SSA for potassium (1.3), magnesium (1.4), and calcium (1.7), likely because of their interactions with organic matter. Consequently, fine SSA develops a salt profile significantly different from that of seawater. Maximal enrichments of saccharides and ions coincided with the second of two phytoplankton blooms, signifying the influence of ocean biology on selective mass transfer across the ocean-air interface.