Iron-Activated Carbon Systems to Enhance Aboriginal Aerobic Denitrifying Bacterial Consortium for Improved Treatment of Micro-Polluted Reservoir Water: Performances, Mechanisms, and Implications

Novel insights in seasonal dynamics and co-existence patterns of phytoplankton and micro-eukaryotes in drinking water reservoir, Northwest China: DNA data and ecological model

Although associations between phytoplankton and micro-eukaryotes have been studied in aquatic ecosystems, there are still knowledge gaps in comprehending their dynamics and interactions in drinking water reservoirs. Here, the seasonal dynamics of phytoplankton and micro-eukaryotic diversities and their co-existence patterns were studied in a drinking water reservoir, Northwest China. The highest phytoplankton diversity was observed in summer, and Chlorella sp. that belongs to Chlorophyta was the most abundant genus. The highest eukaryotic diversity was also detected in summer, and Rimostrombidium sp. that belongs to Ciliophora was the most dominant genus. Mantel test showed that the phytoplankton diversity was significantly correlated with ammonia nitrogen (r = 0.561, p = 0.001) and dissolved organic carbon (r = 0.267, p = 0.017), while the eukaryotic diversity was significantly associated with ammonia nitrogen (r = 0.265, p = 0.034) and temperature (r = 0.208, p = 0.046). PLS-PM (Partial Least Squares Path Modeling) further revealed that nutrients (P < 0.01) significantly affected the phytoplankton diversity, while nutrients (P < 0.01) and temperature (P < 0.01) significantly influenced the eukaryotic diversity. Co-occurrence network displayed the primarily positive interactions (77.66% positive and 22.34% negative) between phytoplankton and micro-eukaryotes. These findings could deepen our understanding of interactions between phytoplankton and micro-eukaryotes and their driving factors under changing aquatic environments of drinking water reservoirs.

Transcriptome analysis reveals the molecular mechanisms of Phragmites australis tolerance to CuO-nanoparticles and/or flood stress induced by arbuscular mycorrhizal fungi

The molecular mechanism of arbuscular mycorrhizal fungi (AMF) in vertical flow constructed wetlands (VFCWs) for the purification of copper oxide nanoparticles (CuO-NPs) contaminated wastewater remains unclear. In this study, transcriptome analysis was used to explore the effect of AMF inoculation on the gene expression profile of Phragmites australis roots under different concentrations of CuO-NPs and/or flood stress. 551, 429 and 2281 differentially expressed genes (DEGs) were specially regulated by AMF under combined stresses of CuO-NPs and flood, single CuO-NPs stress and single flood stress, respectively. Based on the results of DEG function annotation and enrichment analyses, AMF inoculation under CuO-NPs and/or flood stress up-regulated the expression of a number of genes involved in antioxidant defense systems, cell wall biosynthesis and transporter protein, which may contribute to plant tolerance. The expression of 30 transcription factors (TFs) was up-regulated by AMF inoculation under combined stresses of CuO-NPs and flood, and 44 and 44 TFs were up-regulated under single CuO-NPs or flood condition, respectively, which may contribute to the alleviating effect of symbiosis on CuO-NPs and/or flood stress. These results provided a theoretical basis for enhancing the ecological restoration function of wetland plants for metallic nanoparticles (MNPs) by mycorrhizal technology in the future.

Enhanced anaerobic digestion of waste activated sludge with periodate-based pretreatment

The potential of periodate (PI) in sludge anaerobic digestion is not tapped, although it has recently attracted great research interest in organic contaminants removal and pathogens inactivation in wastewater treatment. This is the first work to demonstrate significant improvement in methane generation from waste activated sludge (WAS) with PI pretreatment and to provide underlying mechanisms. Biochemical methane potential tests indicated that methane yield enhanced from 100.2 to 146.3 L per kg VS (VS, volatile solids) with PI dosages from 0 to 100 mg per g TS (TS, total solids). Electron spin resonance showed PI could be activated without extra activator addition, which might be attributed to the native transition metals (e.g., Fe²⁺) in WAS, thereby generating hydroxyl radical (•OH), superoxide radicals (•O₂ ^-), and singlet oxygen (¹O₂). Further scavenging tests demonstrated all of them synergistically promoted WAS disintegration, and their contributions were in the order of •O₂ ^- > •OH > ¹O₂, leading to the release of substantial biodegradable substances (i.e., proteins and polysaccharides) into the liquid phase for subsequent biotransformation. Moreover, fluorescence and ultraviolet spectroscopy analyses indicated the recalcitrant organics (especially lignocellulose and humus) could be degraded by reducing their aromaticity under oxidative stress of PI, thus readily for methanogenesis. Microbial community analysis revealed some microorganisms participating in hydrolysis, acidogenesis, and acetoclastic methanogenesis were enriched after PI pretreatment. The improved key enzyme activities and up-regulated metabolic pathways further provided direct evidence for enhanced methane production. This research was expected to broaden the application scope of PI and provide more diverse pretreatment choices for energy recovery through anaerobic digestion.

Sediment nitrogen contents controlled by microbial community in a eutrophic tributary in Three Gorges Reservoir, China

Nitrogen pollution caused serious environmental problems in reservoir ecosystems. Reducing nitrogen pollution by enhancing nitrogen removal in river sediments deserved intensive research. Distributions of nitrogen contents in sediment-water interface were characterized along the Xiangxi bay (XXB), a eutrophic tributary in Three Gorges Reservoir, China. More than 47% of total Kjeldahl nitrogen (TKN) and 67% of total organic nitrogen (TON) were degraded during burial. Higher TN, TON and NH₄⁺ consuming at downstream sites indicated stronger nitrogen mineralization and release due to higher turbulence of the overlying density currents. Nitrifying bacteria, denitrifying bacteria, anaerobic ammonium oxidizing (anammox) bacteria and nitrite/nitrate-dependent anaerobic methane oxidation (N-DAMO) bacteria were detected in nitrate-ammonium transition zone. Nitrogen contents transitions were responded to microbial stakeholders indicated microbially mediated nitrogen cycling in sediments. The dissolved oxygen and nitrate availabilities were the key limits of denitrification and associated reactions. These results suggested microbial mediated nitrogen cycling processes in sediments were critical for nitrogen removal in aquatic ecosystems, and replenishing dissolved oxygen and nitrate was expected to enhance sediment denitrification and strengthen potential environmental self-purification.

The scale-dependence of spatial distribution of reservoir plankton communities in subtropical and tropical China

Distance-decay relationships (DDRs) represent a very useful approach to describing the spatial distribution of biological communities. However, plankton DDR patterns and community assembly mechanisms are still poorly understood at different spatial scales in reservoir ecosystems. We collected phytoplankton, zooplankton and water samples in 24 reservoirs from subtropical and tropical China from July to August 2018. We examined DDR patterns across three distinct spatial scales, i.e., within-reservoir, within-drainage (but between reservoirs) and between drainages. We tested whether the rate of change (i.e., slope) of DDRs is consistent across different spatial scales. We assessed the relative importance of spatial and environmental variables in shaping the community distribution of plankton and quantitatively distinguished the community assembly mechanisms. We observed significant DDR curves in phytoplankton and zooplankton communities, in which slopes of the DDRs were steepest at the smallest spatial scale. Both spatial and environmental factors had significant impacts on DDR and dispersal assembly was a slightly stronger process in reservoir phytoplankton and zooplankton community assembly than niche-based process. We conclude that DDRs of reservoir phytoplankton and zooplankton vary with spatial scale. Our data shed light on how spatial and environmental variables contribute to plankton community assembly together. However, we revealed that dispersal process contributes to the biogeography of reservoir plankton slightly more strongly than environmental filtering. Collectively, this study enhances the understanding of plankton biogeography and distribution at multiple spatial scales.

Bacterial community structure and metabolic activity of drinking water pipelines in buildings: A new perspective on dual effects of hydrodynamic stagnation and algal organic matter invasion

Eutrophication and algal blooms have become global issues. The drinking water treatment process suffers from pollution by algal organic matter (AOM) through cell lysis during the algal blooms. Nevertheless, it remains unclear how AOM invasion affects water quality and microbial communities in drinking water, particularly in the stagnant settings. In this study, the addition of AOM caused the residual chlorine to rapidly degrade and below the limit of 0.05 mg/L, while the NO₂^--N concentration ranged from 0.11 to 3.71 mg/L. Additionally, total bacterial counts increased and subsequently decreased. The results of Biolog demonstrated that the AOM significantly improved the utilization capacity of carbon sources and changed the preference for carbon sources. Full-length 16S rRNA gene sequencing and network modeling revealed a considerable reduction in the abundance of Proteobacteria, whereas that of Bacteroidetes increased significantly under the influence of AOM. Furthermore, the species abundance distributions of the Microcystis group and Scenedesmus group was most consistent with the Mandelbrot model. According to redundancy analysis and structural equation modeling, the bacterial community structure of the control group was most positively regulated by the free residual chlorine concentrations, whereas the Microcystis group and Scenedesmus group were positively correlated with the total organic carbon (TOC) concentration. Overall, these findings provide a scientific foundation for the evolution of drinking water quality under algae bloom pollution.

Patterns of internal nitrogen and phosphorus loadings in a cascade reservoir with a large water level gradient: Effects of reservoir operation and water depth

Internal nutrient loadings pose a high risk of being an additional N and P source, exacerbating eutrophication and deteriorating water quality. In this study, we selected the Daheiting Reservoir (DHTR) in North China, with a pronounced water level gradient, to investigate internal N and P loadings, estimate N and P fluxes across the sediment‒water interface based on the pore water profiles, and reveal the potential effects of water discharge from an upstream reservoir and high-intensity cage aquaculture on the risks of internal N and P release. The results indicated that DHTR presented with severe internal nutrient loadings, and the N and P fluxes showed significant spatiotemporal variations. NH₄⁺-N and soluble reactive phosphorus (SRP) fluxes were higher in deep areas (averages of 26.14 and 9.9 mgm^-2d^-1, respectively) than in shallow areas near inflows (averages of 5.0 and 1.24 mgm^-2d^-1, respectively). Unexpectedly, the estimated NH₄⁺-N and SRP fluxes were the lowest in summer (averages of 3.94 and 0.33 mgm^-2d^-1, respectively), which may have been influenced by seasonal thermal stratification and copious discharge from the hypolimnion of the upstream reservoir (Panjiakou Reservoir). Comparison of annual internal and external N and P loadings revealed that water discharge from the upstream Panjiakou Reservoir was the dominant source of N and P to the reservoir, contributing up to 83.6% of N input and 55.4% of P input. The internal P loading also contributed to water eutrophication to a great extent, accounting for 34.7% of the total P input. Our results highlight the impact of upstream reservoir discharge operation on downstream reservoir water quality and the importance of controlling the internal nutrient loading in cascade reservoirs, and further provide theoretical and practical foundations for the development of policies and strategies to conserve reservoir ecosystems.

Autochthonous sources and drought conditions drive anomalous oxygen-consuming pollution increase in a sluice-controlled reservoir in eastern China

Freshwater reservoirs are an important type of inland waterbody. However, they can suffer from oxygen-consuming pollution, which can seriously threaten drinking water safety and negatively impact the health of aquatic ecosystems. Oxygen-consuming pollutants originate from both allochthonous and autochthonous sources, and have temporally and spatially heterogeneous drivers. Datanggang Reservoir, China, is located in a small agricultural watershed; it is controlled by multiple sluice gates. Anomalously high oxygen consumption indicators were observed in this reservoir in March 2021. Here, it was hypothesized that autochthonous sources were the primary drivers of oxygen-consuming pollution in the reservoir under drought conditions. Datasets of water quality, precipitation, primary productivity, and sediment were used to analyze water quality trends in the reservoir and inflow rivers, demonstrating the effects of allochthonous inputs and autochthonous pollution. No correlation was found between reservoir oxygen consumption indicators and allochthonous inputs; reservoir oxygen consumption indicators and chlorophyll-a concentration were significantly positively correlated (p < 0.05). Substantially lower precipitation and higher water temperature and pH (compared to historical levels) were also observed before the pollution event. Therefore, during this period the hydrological conditions, water temperature, pH, and other variables caused by short-term drought conditions may have facilitated phytoplankton growth in the reservoir. This contributed to a large increase in autochthonous oxygen-consuming pollutants, as reflected by the abnormally high indicators. Sediments contaminated with organic matter may also have been an important contributor. As the effects of environmental management and pollution control continue to emerge, exogenous pollutants imported from the land to reservoirs are currently effectively controlled. However, endogenous pollutants driven by a variety of factors, such as meteorology and hydrology, will likely become the main drivers of short-term changes in oxygen-consuming pollution in freshwater reservoirs in the foreseeable future.

Biological nitrogen removal and metabolic characteristics of a novel cold-resistant heterotrophic nitrification and aerobic denitrification Rhizobium sp. WS7

For improving the poor de-nitrogen efficiency and effluent quality faced by wastewater treatment plants in winter, a novel cold-resistant strain, Rhizobium sp. WS7 was isolated. Strain WS7 presented dramatic de-nitrogen efficiencies including 98.73 % of NH₄⁺-N, 99.98 % of NO₃^--N, 100 % of NO₂^--N and approximately 100 % of mixed nitrogen (NH₄⁺-N and NO₃^--N) at 15 °C. Optimum parameters of WS7 for aerobic denitrification were determined. Additionally, functional genes (amoA, napA, nirK, norB, and nosZ) and key enzymes (nitrate reductase and nitrite reductase) activities were determined. Nitrogen balance analysis suggested that assimilation played a dominant role in de-nitrogen by WS7, the NH₄⁺-N metabolic pathway was deduced as NH₄⁺-N → NH₂OH → NO → N₂O → N₂, and the NO₃^--N/NO₂^--N metabolic pathway was deduced as NO₃^--N → NO₂^--N → NO → N₂O → N₂. The cold-resistant Rhizobium sp. WS7 has great application feasibility in cold sewage treatment.

Iron-Based Nanocatalysts for Electrochemical Nitrate Reduction

Nitrate has a high level of stability and persistence in water, endangering human health and aquatic ecosystems. Due to its high reliability and efficiency, the electrochemical nitrate reduction reaction (NO₃ RR) is regarded as the best available option for mitigating excess nitrate in water and wastewater, especially for the removal of trace levels of nitrate. One of the most critical factors in the electrochemical reduction are the catalysts, which directly affect the reaction efficiency of nitrate removal. Iron-based nanocatalysts, which have the advantages of nontoxicity, wide availability, and low cost, have emerged as a promising electrochemical NO₃ RR material in recent years. This review covers major aspects of iron-based nanocatalysts for electrochemical NO₃ RR, including synthetic methods, structural design, performance enhancement, electrocatalytic nitrate reduction test, and reduction mechanism. The recent progress of iron-based nanocatalysts for electrochemical NO₃ RR and the mechanism of functional advantages for modified structures are reviewed from the perspectives of loading, doping, and assembly strategies, in order to realize the conversion from pollutant nitrate to harmless nitrogen or ammonia and other sustainable products. Finally, challenges and future directions for the development of low-cost and highly-efficient iron-based nanocatalysts are explored.

Synchronous N and P Removal in Carbon-Coated Nanoscale Zerovalent Iron Autotrophic Denitrification─The Synergy of the Carbon Shell and P Removal

Fe⁰ is a promising electron donor for autotrophic denitrification in the simultaneous removal of nitrate and phosphorus in low C/N wastewater. However, P removal may inevitably inhibit bio-denitrification. It has not been well recognized and led to an overdose of iron materials. This study employed carbon-coated zerovalent iron (Fe⁰@C) to support autotrophic denitrification to mitigate the inhibition effects of P removal and enhance both N and P removal. The critical role of the carbon shell in Fe⁰@C was to block the direct contact between Fe⁰ and P and NO₃^--N, to maintain the Fe⁰ activity. Besides, P inhibited the chemical reduction of NO₃^--N by competing for Fe⁰ active sites. This indirectly boosted H₂ generation and promoted bio-denitrification. P removal displayed negligible effects on microbial species but indirectly enhanced the nitrogen metabolic activities because of promoted H₂ in Fe⁰@C-based autotrophic denitrification. Bio-denitrification, in turn, strengthened Fe-P co-precipitation by promoting the formation of ferric hydroxide as a secondary adsorbent for P removal. This study demonstrated an efficient method for simultaneous N and P removal in autotrophic denitrification and revealed the synergistic interactions among N and P removal processes.

Cyanobacterial bloom intensities determine planktonic eukaryote community structure and stability

The intensity of cyanobacterial blooms that predominate in the world's lakes and reservoirs is variable, which may lead to differing effects on the freshwater ecosystem. Planktonic eukaryotes play key roles in the structure and function of freshwater ecosystems; however, little is known about the influence of cyanobacterial blooms on eukaryotic plankton communities and their function. Herein, the dynamics of eukaryotic plankton communities in Hongze Lake, which is the fourth largest freshwater lake in China, with a range of bloom levels occurred, from low to high, were studied to reveal the effect of cyanobacterial blooms' spatial heterogeneity on planktonic eukaryotes. Results showed that the diversity, richness, and evenness of eukaryotic plankton community were not affected by low level of bloom; however, they were decreased obviously by high level of bloom. Metazoa, Ochrophyta, Chloroplastida, Cryptomonadales, and Ciliophora were the main planktonic eukaryotes in this lake. Metazoa relative abundance declined 25.1% and relative abundance of eukaryotic phytoplankton (mainly Ochrophyta, Chloroplastida, and Cryptomonadales) and Ciliophora increased 17.4% and 2.0%, respectively, during the period with low level of bloom; conversely, the site with the high bloom level manifested the opposite changes. The linkage density of planktonic eukaryotic network was 0.188 and 0.138 with low and high level of bloom, respectively, indicating the stability of planktonic eukaryotes was lower when a high level of bloom occurred compared to that of a low bloom level. Our findings indicate that cyanobacterial blooms should be controlled at low level to avoid their obvious negative impact on microeukaryotes in lakes or reservoirs.

Cooperation triggers nitrogen removal and algal inhibition by actinomycetes during landscape water treatment: Performance and metabolic activity

The actinomycetes strain Streptomyces sp. XD-11-9-3 and Streptomyces sp. 5 were isolated and presented poor denitrification performance. Co-culture of actinomycetes triggers nitrogen removal capacity under aerobic conditions (reduced 96% of total nitrogen). Nitrogen balance analysis presented that 71% of initial nitrogen converted as gaseous nitrogen. Moreover, co-culture increased the concentrations of adenosine triphosphate (>2.1 folds) and electron-transmission system activity (>1.5 folds) significantly. The co-culture presented excellent carbon source metabolism activity (especially amines and carboxylic acids) compared with monoculture. The removal efficiency of total nitrogen in the micro-polluted landscape water water reached 61% in the co-culture system, and the algal survival could be inhibited significantly. However, the dominant niche of the co-culture system restrained the diversity of the indigenous nirS-type denitrifying bacterial community. This study provided a novel pathway to the research of co-culture inefficiency aerobic denitrifier and further application in the restoration of polluted water.

Clarifying the beneficial effects of plant growth-promoting rhizobacteria for reducing abundances of antibiotic resistance genes during swine manure composting

This study investigated the effects on antibiotic resistance genes (ARGs) and the related mechanisms of different plant growth-promoting rhizobacteria (PGPR) inoculation strategies during composting: no inoculation (CK), inoculation in initial phase (T1), inoculation in cooling phase (T2), and inoculation in both initial and cooling phases (T3). After composting, the total relative abundances (RAs) of ARGs decreased by 0.26 and 0.03 logs under T3 and T2, respectively, but increased by 0.05 and 0.22 logs under T1 and CK. The abundances of eight ARGs were lowest under T3, including some high risk ARGs with clinical importance. Bioavailable Cu significantly affected the readily removed ARGs, and PGPR inoculation decreased the bioavailability of Cu. T3 reduced the abundances of potential pathogen hosts, inhibited horizontal gene transfer by reducing the RAs of mobile gene elements (0.48 logs), and downregulated the expression of genes related to ARG propagation, thereby decreasing the ecological risk of ARGs.

Nitrogen removal from low carbon/nitrogen polluted water is enhanced by a novel synthetic micro-ecosystem under aerobic conditions: Novel insight into abundance of denitrification genes and community interactions

The main limiting factor in treatment of wastewater with a low carbon/nitrogen ratio is insufficient electron donors for aerobic denitrification. A novel synthetic micro-ecosystem (SM) with functional materials as the core structure was prepared to enhance nitrate removal during wastewater treatment. Nitrate removal in the reactors with SM increased by more than 40 % and reached 97.43 % under aerobic conditions. The abundance of denitrification functional genes in activated sludge increased by 2.7 folds after adding SM. Network analysis showed that the denitrifying bacterial community in the reactors with SM displayed a more abundant symbiotic structure. In the reactors with SM, bacteria with both denitrification and inorganic electron transfer capabilities (such as Paracoccus sp., Thaurea sp., and Achromobacter sp.) occupied dominant niche. A species abundance distribution model indicated more intense competition for the dominant niche for the denitrification community in the reactor with SM. Thus, SM promotes denitrification in polluted water bodies under aerobic conditions.