Denitrification and dissimilatory nitrate reduction to ammonium in freshwater lakes of the Eastern Plain, China: Influences of organic carbon and algal bloom

Nitrate removal by a novel aerobic denitrifying Pelomonas puraquae WJ1 in oligotrophic condition: Performance and carbon source metabolism

Reducing nitrate contamination in drinking water has become a critical issue in urban water resource management. Here a novel oligotrophic aerobic denitrifying bacterium, Pelomonas puraquae WJ1, was isolated and purified from artificial lake sediments. For the first time, excellent aerobic denitrification capabilities were demonstrated. At a carbon-to‑nitrogen ratio of 5.0, strain WJ1 achieved 100.0 % nitrate removal and 84.92 % total nitrogen removal within 24 h, with no nitrite accumulation. PCR amplification and sequencing confirmed the presence of the denitrification genes napA, nirS, and nosZ in the strain. The nitrogen balance demonstrated that approximately 74.95 % of the initial nitrogen was eliminated as gaseous products under aerobic conditions. Furthermore, carbon balance analysis showed that most electron donors from strain WJ1 were directed towards oxygen, with limited availability for nitrate reduction. A combination of bio-ECO analysis and network modeling indicated that strain WJ1 has robust metabolic capabilities for diverse carbon sources and exhibits high adaptability to complex carbon environments. Overall, Pelomonas puraquae WJ1 removed approximately 45.89 % of the nitrates in raw water, demonstrating significant potential for practical applications in oligotrophic denitrification.

Microbial nitrogen cycling in Microcystis colonies and its contribution to nitrogen removal in eutrophic Lake Taihu, China

Cyanobacterial blooms induced by excessive loadings of nitrogen (N) and other nutrients are a severe ecological problem in aquatic ecosystems. Previous studies of N removal have primarily focused on sediment-water interface, yet the role of cyanobacterial colonies has recently been attracting more research attention. In this study, N cycling processes were quantified for cyanobacterial colonies (primarily Microcystis colonies) and their contribution to N removal was estimated for a large, shallow eutrophic lake in China, Lake Taihu. Various N cycling processes were determined via stable 15N isotope, together with 16S rRNA gene sequencing and quantitative microbial element cycling (QMEC) chip. Denitrification was found to be the most prominent process, estimated to be 36.63, 9.85, 3.35, and 3.15 times higher than dissimilatory nitrate reduction to ammonium (DNRA), nitrification, ammonium (NH4+) uptake and nitrate (NO3-) uptake rates, respectively. Denitrifiers accounted for a large part of the bacterial taxa (35.50 ± 24.65%), and the nirS gene was the most abundant among N cycling-related genes, with (2.54 ± 0.51) × 109 copies g-1Microcystis colonies. A field investigation revealed a positive correlation between the potential denitrification rate and the Chl-a concentration (mostly derived from Microcystis colonies). Based on a multiple stepwise regression model and historical data from 2007 to 2015 for Lake Taihu, the total amount of N removed via denitrification by Microcystis colonies was estimated at 171.72 ± 49.74 t yr-1; this suggests that Microcystis colonies have played an important role in N removal in Lake Taihu since the drinking water crisis in 2007. Overall, this study revealed the importance of denitrification within Microcystis colonies for N removal in eutrophic lakes, like Lake Taihu.

Does invasive submerged macrophyte diversity affect dissimilatory nitrate reduction processes in sediments with varying microplastics?

Nitrogen removal is essential for restoring eutrophic lakes. Microorganisms and aquatic plants in lakes are both crucial for removing excess nitrogen. However, microplastic (MP) pollution and the invasion of exotic aquatic plants have become increasingly serious in lake ecosystems due to human activity and plant-dominant traits. This field mesocosm study explored how the diversity of invasive submerged macrophytes affects denitrification (DNF), anammox (ANA), and dissimilatory nitrate reduction to ammonium (DNRA) in lake sediments with varying MPs. Results showed that invasive macrophytes suppressed DNF rates, but DNRA and ANA were less sensitive than DNF to the diversity of invasive species. Sediment MPs increased the biomass of invasive species more than native species, but did not affect microbial processes. The effects of MPs on nitrate dissimilatory reduction were process-specific. MPs increased DNF rates and the competitive advantage of DNF over DNRA by changing the sediment environment. The decoupling of DNF and ANA was also observed, with increased DNF rates and decreased ANA rates. The study findings suggested new insights into how the invasion of exotic submerged macrophytes affects the sediment nitrogen cycle complex environments.

Optimization of "sulfur-iron-nitrogen" cycle in constructed wetlands by adjusting siderite/sulfur (Fe/S) ratio

Sulfur-siderite autotrophic denitrification (SSAD) has been proved to solve the key problem of low nitrogen removal efficiency caused by the shortage of carbon source in constructed wetlands (CWs). In this study, five vertical flow constructed wetlands (VFCWs) were constructed with different Fe/S ratios (0/0, 0/1, 1/1, 2/1 and 1/2) to optimizing SSAD process, labeled S.0, S.1, S.2, S.3 and S.4. The results showed that the best NO3--N and TN removal rates were achieved with a Fe/S ratio of 2:1 (S.3), which were 96.26 ± 1.40% and 93.63 ± 3.12%, respectively. The abundance of denitrification genes (nirS, nirK and nosZ) in S.3 was significantly increased. Illumina high-throughput sequencing analysis indicated that the abundance and diversity of microorganisms involved in the "Sulfur-Iron-Nitrogen" cycle were enriched in S.3. The current study provided that the "Sulfur-Iron-Nitrogen" cycle in CWs was optimized by adjusting Fe/S ratio, and more types of denitrifying bacteria could be enriched, thereby enhancing nitrogen removal.

Contrasting effects of climate change on denitrification and nitrogen load reduction in the Po River (Northern Italy)

An increase in water temperature is one of the main factors that can potentially modify biogeochemical dynamics in lowland rivers, such as the removal and recycling of nitrogen (N). This effect of climate change on N processing deserves attention, as it may have unexpected impacts on eutrophication in the coastal zones. Intact sediment cores were collected seasonally at the closing section of the Po River, the largest Italian river and one of the main N inputs to the Mediterranean Sea. Benthic oxygen fluxes, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) rates were measured using laboratory dark incubations. Different temperature treatments were set up for each season based on historical data and future predictions. Higher water temperatures enhanced sediment oxygen demand and the extent of hypoxic conditions in the benthic compartment, favoring anaerobic metabolism. Indeed, warming water temperature stimulated nitrate (NO3-) reduction processes, although NO3- and organic matter availability were found to be the main controlling factors shaping the rates between seasons. Denitrification was the main process responsible for NO3- removal, mainly supported by NO3- diffusion from the water column into the sediments, and much more important than N recycling via DNRA. The predicted increase in the water temperature of the Po River due to climate change may exert an unexpected negative feedback on eutrophication by strongly controlling denitrification and contributing to partial buffering of N export in the lagoons and coastal areas, especially in spring.

Deterministic factors modulating assembly of groundwater microbial community in a nitrogen-contaminated and hydraulically-connected river-lake-floodplain ecosystem

The river-lake-floodplain system (RLFS) undergoes intensive surface-groundwater mass and energy exchanges. Some freshwater lakes are groundwater flow-through systems, serving as sinks for nitrogen (N) entering the lake. Despite the threat of cross-nitrogen contamination, the assembly of the microbial communities in the RLFS was poorly understood. Herein, the distribution, co-occurrence, and assembly pattern of microbial community were investigated in a nitrogen-contaminated and hydraulically-connected RLFS. The results showed that nitrate was widely distributed with greater accumulation on the south than on the north side, and ammonia was accumulated in the groundwater discharge area (estuary and lakeshore). The heterotrophic nitrifying bacteria and aerobic denitrifying bacteria were distributed across the entire area. In estuary and lakeshore with low levels of oxidation-reduction potential (ORP) and high levels of total organic carbon (TOC) and ammonia, dissimilatory nitrate reduction to ammonium (DNRA) bacteria were enriched. The bacterial community had close cooperative relationships, and keystone taxa harbored nitrate reduction potentials. Combined with multivariable statistics and self-organizing map (SOM) results, ammonia, TOC, and ORP acted as drivers in the spatial evolution of the bacterial community, coincidence with the predominant deterministic processes and unique niche breadth for microbial assembly. This study provides novel insight into the traits and assembly of bacterial communities and potential nitrogen cycling capacities in RLFS groundwater.

Relationships between environmental factors and N-cycling microbes reveal the indirect effect of further eutrophication on denitrification and DNRA in shallow lakes

Traditional views indicate that eutrophication and subsequent algal blooms favor denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in lake ecosystems. However, lakes tend to show an increasing propensity for inorganic nitrogen (N) limitation as they become more eutrophic. Thus, the influence of further eutrophication on denitrification and DNRA in eutrophic lakes are unclear due to the uncertainty of N availability. To fill this gap, we investigated the genes abundance (AOA, AOB, nirS, nirK and nrfA) and the composition of N-cycling microbes through quantitative PCR and 16S rRNA sequencing analysis, respectively, in 15 shallow eutrophic lakes of the Yangtze-Huaihe River basin, China. The results indicated that denitrification and DNRA rates could be modulated mainly by their functional gene abundances (nirS, nirK and nrfA), followed by the environmental factors (sediment total organic carbon and nitrogen). Denitrification rates significantly increased from slightly to highly eutrophic lakes, but DNRA rates were not. An explanation is that nitrification provided ample nitrate for denitrification, and this cooperative interaction was indicated by the positive correlation of their gene abundances. In addition, Pseudomonas and Anaeromyxobacter was the dominant genus mediated denitrification and DNRA, showing the potential to perform facultative anaerobic and strict anaerobic nitrate reduction, respectively. High level of dissolved oxygen might favor the facultatively aerobic denitrifiers over the obligately anaerobic fermentative DNRA bacteria in these shallow lakes. Chlorophyll a had a weak but positive effect on the gene abundances for nitrification (AOA and AOB). Further eutrophication had an indirect effect on denitrification and DNRA rates through modulating the genes abundances of N-cycling microbes.

Impact of seasonal change on dissimilatory nitrate reduction to ammonium (DNRA) triggering the retention of nitrogen in lake

Nitrogen (N) reduction processes including denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are critical for the eutrophication in the lake water. However, the understanding about the dominant pathways of N cycling keep limited due to the high complexity of N cycle processes in lacustrine environment. The N fractions in sediments collected from Shijiuhu Lake were measured using high-resolution (HR)-Peeper technique and chemical extraction method in varied seasons. The abundance and microbial community compositions of functional genes involved in various N-cycling processes were also obtained using high-throughput sequencing. The results showed that NH4+ concentrations in the pore water remarkably increased from the upper layer toward the deeper layer and from winter to spring. This trend suggested that higher temperature facilitated the accumulation of NH4+ in the water. Decreased NO3- concentrations were also detected at deeper sediment layers and higher temperature, indicating the intensification of N reduction on anaerobic conditions. The NH4+-N concentrations reduced in spring along with the slight change of NO3--N in solid sediment, indicating the desorption and release of mobile NH4+ from solid phase to the solution. Remarkably decreased absolute abundances of functional genes were found in spring with DNRA bacteria nrfA gene as dominant genus and Anaeromyxobacter as the most dominant bacterium (21.67 ± 1.03%). Higher absolute abundance (146.2-788.1 × 105 Copies/g) of nrfA gene relative to other genes was mainly responsible for the increase of bio-available NH4+ in the sediments. Generally, microbial DNRA pathway predominated the N reduction and retention processes in the lake sediment at higher temperature and water depth even experiencing the suppression of DNRA bacteria abundance. These results suggested the existence of ecological risk via N retention by the action of the DNRA bacteria in the sediment on the condition of higher temperature, further provided valuable information for N management of eutrophic lakes.

Effect of river-lake connectivity on ecological stoichiometry of lake and carbon storage status in Eastern Plain, China

C, N, and P in lake sediment are the basis of material and energy cycle, reflecting the economic development, ecological function, and environmental effect. Current research on the effect of lake eutrophication on carbon storage and the river-lake connectivity on nutrient diffusion is lack. This work investigated the accumulation, distribution, correlations, and stoichiometric ratios of C, N, and P of 82 lakes (≥ 10 km²) in Eastern China, analyzed the nutrient limitation, sediment carbon sink, and effect of river-lake connectivity, and discussed the relationships between eutrophication and sediment carbon storage. The average concentrations and ranges of total C, N, and P in lake sediments were (23.26 mg/g, 0.08-153.45 mg/g), (2.32 mg/g, 0.29-14.17 mg/g), and (0.86 mg/g, 0.23-2.64 mg/g), respectively. The ecological stoichiometry of C: N: P in lake sediments was 32: 3.2: 1. P can be easily accumulated in lakes connected from the Yangtze River, while C and N can be easily accumulated in disconnected lakes. The soil-water erosion in runoff is an important factor for P diffusion. The C/N and C/N/P weren't affected by the river-lake connectivity but depended on the plant type. The Eastern Plain Lake Region of China is C and N co-depletion, and P enrichment. The lake eutrophication leading to algal bloom is unfavorable to the goal of carbon storage and carbon neutrality. Outcome of this study will provide a significant reference and strategies for carbon sequestration research, eco-environmental protection, and watershed nutrient management.

Arrhenius equation construction and nitrate source identification of denitrification at the Lake Taihu sediment - water interface with 15 N isotope

Total nitrogen in Taihu Lake, China has gradually decreased since 2015 while the total phosphorus concentration has exhibited an increasing trend, indicating an asynchronous change. The dominant nitrogen removal process in freshwater ecosystems is denitrification which primarily occurs at the sediment-water interface. In this study, ¹⁵ N isotope incubation experiments were attempted to analyze the effect of water temperature on denitrification, to construct the regional denitrification Arrhenius equations considering water temperature, and to identify the nitrate source of denitrification in Lake Taihu sediments. The results indicated that the potential N₂ production rates and denitrification rates generally decreased in the west to east direction, which was significantly positively correlated with the nitrate concentration of overlying water by Pearson correlation coefficient analysis (P < 0.05). In addition, when the water temperature was lower than 30 °C, the rates of the potential N₂ production and denitrification were higher with an increase in water temperature, but when the water temperature was overhigh, denitrification was inhibited. The ratio of the total denitrification rate of nitrate from the water column in the sediment to the total denitrification rate during the incubation experiment was above 0.5 at each sampling site. This indicated that the denitrification in the Lake Taihu sediment primarily occurred at the expense of nitrate from the water column. The research results of Arrhenius equation construction and nitrate source identification of denitization can be applied to improve the accuracy of water quality model of Taihu Lake, which is of great significance to improve Taihu Lake water quality, and can act as a reference for the water environment treatment of other shallow eutrophic lakes in China and abroad.

Microbial one‑carbon and nitrogen metabolisms are beneficial to the reservoir recovery after cyanobacterial bloom

Cyanobacterial blooms have profound effects on the structure and function of plankton communities in inland waters, but few studies have focused on the effects of microbial-based processes in one‑carbon and nitrogen cycling on water quality improvement following the bloom. Here, we compared the structure and function of the bacterial community, focusing on microbial one‑carbon and nitrogen metabolisms during and after a cyanobacterial Microcystis bloom in a deep subtropical reservoir. Our data showed that microbial one‑carbon and nitrogen cycles were closely related to different periods of the bloom, and the changes of functional genes in microbial carbon and nitrogen cycling showed the same consistent trend as that of Methylomonas sp. With the receding of the bloom, the abundance of Methylomonas as well as the functional genes of microbial one‑carbon and nitrogen cycling reached the peak and then recovered. Our results indicate that microbial one‑carbon and nitrogen metabolisms were beneficial to the recovery of water quality from the cyanobacterial bloom. This study lays a foundation for a deep understanding of the cyanobacterial decomposition mediated by microbes in one‑carbon and nitrogen cycles in inland freshwaters.

Climate-induced salinization may lead to increased lake nitrogen retention

Salinization caused by climate change and nitrogen (N) pollution are both important environmental threats for inland lakes. However, evaluating their interactive effects continues to be challenging. Here, field observation and microcosmic experiments were conducted in six lakes of East Asia with the different salinity and climate characteristics, to explore the response of the key N cycle processes related to N fate to the climate-induced change in salinity. The results indicated that increased salinity inhibited denitrification, which was the outcome of two cumulative effects: the long-term microbial adaptation effect and the direct salinity stress. Whereas increased salinity had unsignificant or positive effects on dissimilatory nitrate reduction to ammonium. It had caused that N retention capacity is relatively stronger in saline than freshwater lakes. Inland lakes are long-term basin-wide integrators of climatic conditions that drying (salinization) and wetting (desalination) with climate change. In semi-arid regions of East Asia, lake shrinkage, salinization and increasing temperature driven by climate warming and drying may exert a negative impact on N pollution through concentrating, decreasing denitrification and increasing ammonium release from sediment. The threat of climate change on these lakes is not just the quantity of water, but its quality.

Insights into microbial interactive mechanism regulating dissimilatory nitrate reduction processes in riparian freshwater aquaculture sediments

Aquaculture can substantially alter the accumulation and cycling of nutrients in sediments. However, the microbial mechanisms mediating sediment dissimilatory nitrate (NO₃^-) reduction in freshwater aquaculture ponds are still unclear, which rule the removal and retention of N element. In the present study, three microbial NO₃^- reduction processes in riparian aquaculture pond sediments (i.e., crab, shrimp and fish ponds) and natural freshwater sediments (i.e., lakes and rivers) were investigated via isotopic tracing and molecular analyses. The potential rates of denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) significantly increased in the aquaculture ponds compared with the natural freshwaters. Denitrification contributed 90.40-94.22% to the total NO₃^- reduction (product as N₂), followed by 2.49-5.82% of anammox (product as N₂) and 2.09-5.18% of DRNA (product as NH₄⁺). The availability of C and N substrates, rather than functional gene abundance, regulated the activities of NO₃^- reductions and microbiome composition. Microbial mechanism based on network analysis indicated that heterotrophic denitrifiers and DNRA bacteria (e.g., Bacillus, Micromonospora, Mycobacterium and Brachybacterium) determined the community structure and function for N conversions in aquaculture ponds, whereas the such microbial network in natural freshwater sediments was manipulated by autotrophic denitrifiers (e.g., Desulfuromonas, Polaromonas, Solitalea). Collectively, this study provides an in-depth exploration of microbial nitrogen removal in freshwater aquaculture areas and supports management strategies for N pollution caused by reclamation for aquaculture in riparian zones.

Sediment organic matter properties facilitate understanding nitrogen transformation potentials in East African lakes

East African lakes include the most productive and alkaline lake group in the world. Yet, they generally receive fewer nutrient inputs than the densely populated subtropical and temperate lakes in the northern hemisphere. In these lakes with insufficient supplies of inorganic nitrogen, the mineralization of benthic organic matter can play an important role in driving the nutrient cycle and nitrogen loss. Using a suite of stable ¹⁵N isotope dilution and tracer techniques, we examined five main processes of the sediment nitrogen cycle in 16 lakes and reservoirs of Tanzania and Kenya, East Africa: gross nitrogen mineralization, ammonium immobilization, dissimilatory nitrate reduction to ammonium (DNRA), and the dinitrogen (N₂) production via denitrification and anaerobic ammonium oxidation (anammox). Gross nitrogen mineralization and ammonium immobilization showed the maximum values of 9.84 and 12.39 μmol N kg^-1 h^-1, respectively. Potential DNRA rates ranged from 0.22 to 8.15 μmol N kg^-1 h^-1 and accounted for 10 %-74 % (average 25 %) of the total dissimilatory nitrate reduction. Potential nitrate reduction rates in most lakes were dominated by denitrification with a contribution of 26 %-85 % and a mean of 65 %. We further found that the sediment nitrogen transformations were driven mainly by benthic organic matter properties and water column phosphate concentrations, reflecting microbial metabolic responses to the changing carbon and nutrients availability. For instance, autochthonous production of protein-like organic matter attributed to active sediment nitrogen mineralization, DNRA, and denitrification. In contrast, the high degree of humification caused by the inputs of terrestrial humic-like substances slowed down the sediment nitrogen transformations. The contribution of DNRA to total dissimilatory nitrate reduction was significantly positively correlated to sediment C: N ratios. These results indicate that predictions of sediment N supply and loss in East African lakes can be improved by incorporating sediment organic matter properties.

Ammonia level influences the assembly of dissimilatory nitrate reduction to ammonia bacterial community in soils under different heavy metal remediation treatments

Heavy metal remediation treatments might influence functional microbial community assembly. Dissimilatory nitrate reduction to ammonia (DNRA) contributes to the nitrogen retention processes in soil ecosystems. We assumed that remediation might reduce heavy metal toxicity and increase some available nutrients for the DNRA microbes, thus balancing the deterministic and stochastic process for DNRA community assembly. Here, we investigated the process of DNRA bacterial community assembly under different heavy metal remediation treatments (including control, biochar, limestone, rice straw, rice straw + limestone, and biochar + limestone) in an Alfisol soil. The abundance of DNRA bacteria diverged across treatments. The α-diversity of the DNRA bacterial community was correlated with pH, available phosphorus (AP), ammonium (NH₄⁺), and extractable Fe (EFe). Metal Cd and Fe significantly affected the abundance of the nrfA gene. The β-diversity was associated with pH, NH₄⁺, and EFe. Deterministic processes dominantly drove the assembly processes of the DNRA bacterial community. NH₄⁺ level played an essential role in the assembly processes than the other soil physicochemical properties and metal availability. High, moderate, and low levels of NH₄⁺ could advocate stochastic process plus selection, heterogeneous selection to stochastic process, and heterogeneous selection, respectively. Network analysis highlighted a predominant role of NH₄⁺ in regulating DNRA bacterial community assembly. However, the relative abundance of modules and some keystone species also were influenced by pH and EFe, respectively. Therefore, the DNRA bacterial community assembly under different heavy metal remediation treatments in this study was dominantly driven by nitrogen availability. pH, phosphorus, and metal availability were auxiliary regulators on DNRA bacterial community.

Salinity-Linked Denitrification Potential in Endorheic Lake Bosten (China) and Its Sensitivity to Climate Change

Endorheic lakes in arid regions of Northwest China are generally vulnerable and sensitive to accelerated climate change and extensive human activities. Therefore, a better understanding of the self-purification capacity of ecosystems, such as denitrification, is necessary to effectively protect these water resources. In the present study, we measured unamended and amended denitrification rates of Lake Bosten by adding the ambient and extra nitrate isotopes in slurry incubations. Meanwhile, we investigated the abundances and community structure of nitrous oxide-reducing microorganisms using qPCR and high-throughput sequencing, respectively, in the surface sediments of Lake Bosten to study denitrification potential in endorheic lakes of arid regions as well as the response of those denitrifiers to climatically induced changes in lake environments. Amended denitrification rates increased by one order of magnitude compared to unamended rates in Lake Bosten. The great discrepancy between unamended and amended rates was attributed to low nitrate availability, indicating that Lake Bosten is not operating at maximum capacity of denitrification. Salinity shaped the spatial heterogeneity of denitrification potential through changes in the abundances and species diversity of denitrifiers. Climate change had a positive effect on the water quality of Lake Bosten so far, through increased runoff, decreased salinity, and enhanced denitrification. But the long-term trajectories of water quality are difficult to predict alongside future glacier shrinkage and decreased snow cover.

Classifying diurnal changes of cyanobacterial blooms in Lake Taihu to identify hot patterns, seasons and hotspots based on hourly GOCI observations

Occurrence of cyanobacterial blooms in most lakes has dramatic changes in time and space. However, most current studies only focused on daily or seasonal scales to obtain a relatively coarse resolution result. To explore the possibility of fine changes occurring within a day in Lake Taihu (China), the area coverage of surface cyanobacterial blooms was quantified from the hourly Geostationary Ocean Color Imager (GOCI) data using a GOCI-derived cyanobacterial index. Based on that, diurnal change characteristics were explored at two scales, and the environmental impacts were investigated. For that, an classification method was first designed to identify the types of diurnal change patterns of cyanobacterial blooms automatically. This method classified the patterns into four types, including the decreasing (Type1), decreasing first and then increasing (Type2), increasing (Type3), increasing first and then decreasing (Type4). Based on that, the types of diurnal change patterns of blooms in Lake Taihu (from April 1, 2011 to October 31, 2020) were identified at pixel (500 m) and synoptic scales. Results indicated that Type1 and Type3 were two hot diurnal change patterns of blooms, and lakeshore was the hotspot occurring severe diurnal changes, and autumn was the hot season occurring frequent diurnal changes. Specifically, hotspot of Type1 was lakeshore, while hotspot of Type3 was Central Regions. Environmental impacts were analyzed at two scales. At pixel scale (500 m), diurnal variation of temperature affected the regional occurence of each type ofdiurnal changes patterns of blooms, and the afternoon temperature played the most critical role (p < 0.001, N = 8316). The occurrence frequency of Type1 was positively (R = 0.41) related with the afternoon temperature, and the occurrence frequency of Type3 was negatively (R = -0.37) related with it. Diurnal variation of wind speed was another key factor impacting the occurrence of obvious diurnal blooms changes, and the wind impacts should be distinguished when the wind speed was over or below 3.5 m/s. At synoptic scale, the interaction of multi environmental factors influenced the diurnal change degree of blooms area, and the environmental contributions were 71%.Comparing with the existing manual classifying workat synoptic scale, the designed classification method can identify the types of diurnal change patterns of blooms at a higher spatial resolution (500 m). These explorations on diurnal dynamics of cyanobacterial blooms in Lake Taihu provide a new insight for advanced cyanobacteria dynamics studies and regional water management.

The influences of soil sulfate content on the transformations of nitrate and sulfate during the reductive soil disinfestation (RSD) process

The transformations and products of sulfate (SO₄^2-) and nitrate (NO₃^-), especially the influences of SO₄^2- content on the transformations during RSD process, are unclear. In this study, a series of soil SO₄^2- contents (from 333 to 3000 mg S kg^-1) were prepared before RSD treatment. The results indicated that nearly all the cumulative NO₃^- (>98.6%) was removed and not affected by the soil SO₄^2- content. The ¹⁵N recovery results showed that 0.57-1.24% and 2.94-4.59% of NO₃^- translated into ammonium (NH₄⁺) and organic N, respectively, and high SO₄^2- contents stimulated the processes of NO₃^- dissimilatory reduction and NO₃^- immobilization. The soluble SO₄^2- contents decreased by 397-922 mg S kg^-1, but the contents of total sulfur, sulfide, and sulfate precipitation varied slightly after RSD, indicating that the decreased SO₄^2- was mainly immobilized into organic sulfur in all soils. In addition, a fraction of decreased SO₄^2- was adsorbed to the soil with a relatively high SO₄^2- content. The leaching of SO₄^2- was high (42.9-602 mg S kg^-1) during the RSD process, and the leaching amounts increased with increasing soil SO₄^2- content. In terms of the gases emitted from the transformations of NO₃^- and SO₄^2-, the cumulative emissions of nitrous oxide (N₂O) and six sulfurous gases (hydrogen sulfide, carbonyl sulfide, carbon disulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide) were in the ranges of 17.1-21.2 mg N kg^-1 and 7.78-23.5 μg S kg^-1, respectively, during the whole RSD process. The emissions of sulfurous gases were inhibited by high soil SO₄^2- content, but the N₂O emissions were unaffected. In conclusion, the soil SO₄^2- content influenced the transformations of NO₃^- and SO₄^2- during RSD process, and the SO₄^2- leaching and N₂O emissions might threaten the environment which should be concerned.

Monitoring Spatio-Temporal Dynamics in the Eastern Plain Lakes of China Using Long-Term MODIS UNWI Index

Monitoring the spatio-temporal dynamics of the Eastern Plain Lake (EPL) is vital to the local environment and economy. However, due to the limitations and efficiency of traditional image formats in storing and processing large amounts of images and optimal threshold adjustments are often necessary for water/non-water separation based on traditional multi-band/spectral water indexes over large areas and in the long-term, previous studies have either been on a short period or mainly focused on water inundation dynamics of several lakes. To address these issues, a multi-dimensional dataset (MDD) storage format was used to efficiently organize more than ~7000 time series composite MODIS images. Furthermore, a universal normalized water index (UNWI) was developed based on full-spectrum information to simplify optimal threshold adjustments. Consequently, the present study analyzed the patterns of spatio-temporal water dynamic patterns and potential driving factors of inundation changes at large lakes (>5 km2) in the EPL during 2000–2020 through MDD and UNWI. In terms of annual inundation patterns, the numbers of lakes that experienced significant (p < 0.05) decreases (17 lakes) and increases (43 lakes) were highest for Class IV lakes among six geographical classes. Variation in intra-annual inundation in Classes I and II is correlated with consumption of chemical fertilizers (CCF), while precipitation accounted for the most change in lake area in Class III. This spatio-temporal analysis of lakes provides a necessary foundation for the sustainable development and continuous investigations of the EPL.

Aromatic compounds releases aroused by sediment resuspension alter nitrate transformation rates and pathways during aerobic-anoxic transition

Aromatic compounds (ACs) releases aroused by sediment resuspension would certainly change the concentrations of suspended sediment (SPS) and organic carbon, which may alter nitrate-N transformation during aerobic-anoxic transition. To prove this, three typical ACs (aniline, nitrobenzene, and methylbenzene) with different octanol-water partition coefficients (K_ow) were selected to investigate the effects of ACs releases aroused by sediment resuspension on nitrate-N transformation during aerobic-anoxic transition. ACs releases aroused by sediment resuspension accelerated nitrate-N transformation and enhanced the potential for dissimilatory nitrate reduction to ammonium (DNRA), compared to that without sediment resuspension. With sediment resuspension, methylbenzene releases affected nitrate-N transformation rates and pathways more significantly than aniline and nitrobenzene releases. Microbial analysis indicated that sediment resuspension created complicated microbial co-occurrence networks and changed the associations among bacteria; dominant bacteria abundance varied with different ACs releases. Further analysis revealed that ACs distributed in SPS, which increased with logK_ow, indirectly affected nitrate-N transformation rates and pathways via altering dominant bacteria abundance and electron transport system activity (ETSA). Especially, ETSA, which was positively associated with ACs distributed in SPS, affected nitrate-N transformation most directly. Overall, ACs release fate played important roles in nitrate-N transformation, causing ammonia-N retention and alterations in nitrogen cycle during aerobic-anoxic transition.

An Underestimated Contribution of Deltaic Denitrification in Reducing Nitrate Export to the Coastal Zone (Po River–Adriatic Sea, Northern Italy)

In transitional environments, the role of sediments biogeochemistry and denitrification is crucial for establishing their buffer potential against nitrate (NO3−) pollution. The Po River (Northern Italy) is a worldwide hotspot of eutrophication. However, benthic N dynamics and the relevance of denitrification in its delta have not yet been described. The aim of the present study was to quantify the contribution of denitrification in attenuating the NO3− loading transported to the sea during summer. Benthic fluxes of dissolved inorganic nitrogen (N) and denitrification rates were measured in laboratory incubations of intact sediment cores collected, along a salinity gradient, at three sections of the Po di Goro, the southernmost arm of the Po Delta. The correlation between NO3− consumption and N2 production rates demonstrated that denitrification was the main process responsible for reactive N removal. Denitrification was stimulated by both NO3− availability in the Po River water and organic enrichment of sediment likely determined by salinity-induced flocculation of particulate organic load, and inhibited by increasing salinity, along the river–sea gradient. Overall, denitrification represented a sink of approximately 30% of the daily N loading transported in middle summer, highlighting a previously underestimated role of the Po River Delta.

Cyanobacterial community succession and associated cyanotoxin production in hypereutrophic and eutrophic freshwaters

Cyanobacterial harmful algal blooms (cyanoHABs) in freshwater bodies are mainly attributed to excess loading of nutrients [nitrogen (N) and phosphorus (P)]. This study provides a comprehensive review of how the existing nutrient (i.e., N and P) conditions and microbial ecological factors affect cyanobacterial community succession and cyanotoxin production in freshwaters. Different eutrophic scenarios (i.e., hypereutrophic vs. eutrophic conditions) in the presence of (i) high levels of N and P, (ii) a relatively high level of P but a low level of N, and (iii) a relatively high level of N but a low level of P, are discussed in association with cyanobacterial community succession and cyanotoxin production. The seasonal cyanobacterial community succession is mostly regulated by temperature in hypereutrophic freshwaters, where both temperature and nitrogen fixation play a critical role in eutrophic freshwaters. While the early cyanoHAB mitigation strategies focus on reducing P from water bodies, many more studies show that both N and P have a profound contribution to cyanobacterial blooms and toxin production. The availability of N often shapes the structure of the cyanobacterial community (e.g., the relative abundance of N₂-fixing and non-N₂-fixing cyanobacterial genera) and is positively linked to the levels of microcystin. Ecological aspects of cyanotoxin production and release, related functional genes, and corresponding nutrient and environmental conditions are also elucidated. Research perspectives on cyanoHABs and cyanobacterial community succession are discussed and presented with respect to the following: (i) role of internal nutrients and their species, (ii) P- and N-based control vs. solely P-based control of cyanoHABs, and (iii) molecular investigations and prediction of cyanotoxin production.

A machine learning approach for early warning of cyanobacterial bloom outbreaks in a freshwater reservoir

Understanding the dynamics of harmful algal blooms is important to protect the aquatic ecosystem in regulated rivers and secure human health. In this study, artificial neural network (ANN) and support vector machine (SVM) models were used to predict algae alert levels for the early warning of blooms in a freshwater reservoir. Intensive water-quality, hydrodynamic, and meteorological data were used to train and validate both ANN and SVM models. The Latin-hypercube one-factor-at-a-time (LH-OAT) method and a pattern search algorithm were applied to perform sensitivity analyses for the input variables and to optimize the parameters of the models, respectively. The results indicated that the two models well reproduced the algae alert level based on the time-lag input and output data. In particular, the ANN model showed a better performance than the SVM model, displaying a higher performance value in both training and validation steps. Furthermore, a sampling frequency of 6- and 7-day were determined as efficient early-warning intervals for the freshwater reservoir. Therefore, this study presents an effective early-warning prediction method for algae alert level, which can improve the eutrophication management schemes for freshwater reservoirs.

Spectroscopic fingerprinting of dissolved organic matter in a constructed wetland-reservoir ecosystem for source water improvement-a case study in Yanlong project, eastern China

The coupling between constructed wetlands and reservoir (CWs-R) afforded a novel ecosystem to improve the water quality and increase the emergency storage capacity of micro-polluted river drinking water source. In this study, spectroscopic characteristics of DOM in YL CWs-R ecosystem were first systematic studied based on a three-year field monitoring to investigate the chemical composition, sources and track the involved biogeochemical processes in the ecosystem. Three humic-like components (C1, C2, and C4, em >380 nm) and one protein-like component (C3, em < 380 nm) were identified by PARAFAC model. Significant spatiotemporal variations in concentration and composition of FDOM were observed in YL CWs-R ecosystem. The improved water transparency (SD) and, the increased hydraulic retention time (HRT) along YL CWs-R ecosystem enhance photochemical processes, leading to significant decreases in the intensities of humic-like components in effluent (P < 0.05) with lower degrees of aromaticity, molecular weights, and humification (decrease in HIX and increases in S_R and BIX). In contrast, no significant spatial difference was observed for protein-like component (P > 0.05), which implies that the biodegradation and production of protein-like component may balance each other in the CWs-R ecosystem. The ecological pond unit plays a major role in the removal and transformation of DOM, especially in summer, while wetland purification unit contributes little to DOM reduction. In addition, the decay of aquatic macrophytes in wetland purification unit and the risk of algal bloom in the ecological pond unit might become important autochthonous sources of DOM, especially in summer and autumn. These findings are critical for further understanding the transformation processes of DOM in large-scale CWs-R ecosystems, and could provide important implications to improve sustainable safety of drinking water sources.

Indicator species drive the key ecological functions of microbiota in a river impacted by acid mine drainage generated by rare earth elements mining in South China

Acid mine drainage (AMD) generated by rare earth elements (REEs) deposits exploration contains high concentrations of REEs, ammonium and sulfates, which is quite different from typical metallic AMD. Currently, microbial responses and ecological functions in REEs-AMD impacted rivers are unknown. Here, 16S rRNA analysis and genome-resolved metagenomics were performed on microbial community collected from a REEs-AMD contaminated river. The results showed that REEs-AMD significantly changed river microbial diversity and shaped unique indicator species (e.g. Thaumarchaeota, Methylophilales, Rhodospirillales and Burkholderiales). The main environmental factors regulating community were pH, ammonium and REEs, among which high concentration of REEs increased REEs-dependent enzyme-encoding genes (XoxF and ExaF/PedH). Additionally, we reconstructed 566 metagenome-assembled genomes covering 70.4% of identifying indicators. Genome-centric analysis revealed that the abundant archaea Thaumarchaeota and Xanthomonadaceae were often involved in nitrification and denitrification, while family Burkholderiaceae were capable of sulfide oxidation coupled with dissimilatory nitrate reduction to ammonium. These indicators play crucial roles in nitrogen and sulfur cycling as well as REEs immobilization in REEs-AMD contaminated rivers. This study confirmed the potential dual effect of REEs on microbial community at the functional gene level. Our investigation on the ecological roles of indicators further provided new insights for the development of REEs-AMD bioremediation.

Identifying the Mechanisms behind the Positive Feedback Loop between Nitrogen Cycling and Algal Blooms in a Shallow Eutrophic Lake

Algal blooms have increased in frequency, intensity, and duration in response to nitrogen (N) cycling in freshwater ecosystems. We conducted a high-resolution sedimentary study of N transformation and its associated microbial activity in Lake Taihu to assess the accumulation rates of the different N fractions in response to algal blooms, aiming to understand the mechanisms of N cycling in lacustrine environments. Downcore nitrification and denitrification processes were measured simultaneously in situ via diffusive gradients in thin-films technique, peeper, and microelectrode devices in a region of intensified algal blooms of shallow lake. The decomposition of different biomasses of algal blooms did not change the main controlling factor on different N fractions in profundal sediment. However, the decomposition of different algal biomasses led to significant differences in the nitrification and denitrification processes at the sediment–water interface (SWI). Low algal biomasses facilitated the classic process of N cycling, with the balanced interaction between nitrification and denitrification. However, the extreme hypoxia under high algal biomasses significantly limited nitrification at the SWI, which in turn, restricted denitrification due to the lack of available substrates. Our high-resolution results combined with estimates of apparent diffusion fluxes of the different N fractions inferred that the lack of substrates for denitrification was the main factor influencing the positive feedback loop between N and eutrophication in freshwater ecosystems. Moreover, this positive feedback can become irreversible without technological intervention.

Multi-factor analysis of algal blooms in gate-controlled urban water bodies by data mining

Intense human disturbance has made algal bloom a prominent environmental problem in gate-controlled urban water bodies. Urban water bodies present the characteristics of natural rivers and lakes simultaneously, whose algal blooms may manifest multi-factor interactions. Hence, effective regulation strategies require a multi-factor analysis to understand local blooming mechanisms. This study designed a holistic multi-factor analysis framework by integrating five data mining techniques. First, the Kolmogorov-Smirnov test was conducted to screen out the possible explanatory variables. Then, correlation analyses and principal component analyses were performed to identify variable collinearity and mutual causality, respectively. After collinearity and mutual causality were treated prudently by using orthogonalization and instrumental variables, multilinear regression can be properly conducted to quantify factor contributions to algae growth. Lastly, a decision tree was used innovatively to depict the limiting threshold curves of each driving factor that restricts algae growth under different circumstances. The driving factors, their contributions, and the limiting threshold curves compose the complete blooming mechanisms, thus providing a clear direction for the targeted regulation task. A typical case study was performed in Suzhou, a Chinese city with an intricate gate-controlled river network. Results confirmed that climatic factors (i.e., water temperature and solar radiation), hydrodynamic factors (i.e., flow velocity), nutrients (i.e., phosphorus and nitrogen), and external loadings contributed 49.3%, 21.7%, 21.3%, and 7.7%, respectively, to algae growth. These results indicate that a joint regulation strategy is urgently required. Future studies can focus on coupling the revealed mechanisms with an ecological model to provide a comprehensive toolkit for the optimization of an adaptive joint regulation plan under the background of global warming.

Hypoxia, acidification and nutrient accumulation in the Yellow Sea Cold Water of the South Yellow Sea

Hypoxia and acidification in coastal waters are of global concern. However, the complex hydrodynamic processes and human interferences are major challenges for the diagnosis of their mechanism. The role of seasonal water masses involved still remains unknown. We herein investigated the dynamics of dissolved oxygen (DO), pH, inorganic and organic nutrients in the South Yellow Sea (SYS) in autumn, aiming for a better understanding of the biogeochemical processes of the Yellow Sea Cold Water (YSCW). Low DO, pH and organic nutrients were observed in the YSCW, while the apparent oxygen utilization and dissolved inorganic nutrients were relatively high. Quantitative assessment shows that although the water volume of the YSCW accounts for only 16.4% that of the SYS, the reservoirs of dissolved inorganic nitrogen, phosphate and silicate were 30.8%, 52.1% and 33.0%, respectively. Our results suggest that organic matter mineralization and water stratification are important driving forces for hypoxia, acidification and nutrient accumulation in the YSCW. The upwelling of the YSCW can bring abundant nutrients and stimulate the algal blooms, which are detrimental to the ecology. As global warming continues, the hypoxia and acidification in the YSCW will likely intensify in the near future in response to a projected slowdown of overturning circulation.

A risk assessment method for remote sensing of cyanobacterial blooms in inland waters

The widespread occurrence of Cyanobacterial blooms (CABs) in inland waters is a typical and severe challenge for water resources management and environment protection. An accurate and spatially continuous risk assessment of CABs is critical for prediction and preparedness in advance. In this study, a multivariate integrated risk assessment (MIRA) method of CABs in inland waters was proposed. MIRA was simplified with the trophic levels, cyanobacterial and other aquatic plant condition using remote sensing indexes, including the Trophic State Index (TSI), Floating Algae Index (FAI) and Cyanobacteria and Macrophytes Index (CMI). First, the dates of risk assessment were carefully selected based on TSI. Then, we obtained the trophic levels, cyanobacterial, and other aquatic plant condition of water using TSI, CMI and FAI on the selected date, and further scored them pixel by pixel to quantify the risk value. Finally, the risk of CABs in water was accurately assessed based on the pixel risk value. Based on Landsat 8 OLI dataset, MIRA was executed and validated in three different lakes of Wuhan urban agglomeration (WUA) with different trophic states. The results demonstrated that the risk of CABs in Lake LongGan was overall higher than that in Lake LiangZi and Lake FuTou. And the risk of CABs in the east part of Lake LongGan was higher than the other parts. Seasonally, the risk level ranking in Lake LiangZi was the highest in summer, while lowest in winter. However, the seasonal risk ranking was spring, summer, autumn, and winter in Lake LongGan. Based on the comparisons with monthly water quality classification data and results of the existing study, including trophic level, ecology risk, and algal extent, the MIRA method was valuable for accurate and spatially continuous identifying the risk of CABs in inland waters with potential eutrophication trends.

Role of algal accumulations on the partitioning between N2 production and dissimilatory nitrate reduction to ammonium in eutrophic lakes

Cyanobacterial blooms change benthic nitrogen (N) cycling in eutrophic lake ecosystems by affecting organic carbon (OC) delivery and changing in nutrients availability. Denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) are critical dissimilatory nitrate reduction pathways that determine N removal and N recycling in aquatic environments. A mechanistic understanding of the influence of algal accumulations on partitioning among these pathways is currently lacking. In the present study, a manipulative experiment in aquarium tanks was conducted to determine the response of dissimilatory nitrate reduction pathways to changes in algal biomass, and the interactive effects of OC and nitrate. Potential dinitrogen (N₂) production and DNRA rates, and related functional gene abundances were determined during incubation of 3-4 weeks. The results indicated that high algal biomass promoted DNRA but not N₂ production. The concentrations of dissolved organic carbon were the primary factor affecting DNRA rates. Low nitrate availability limited N₂ production rates in treatments with algal pellets and without nitrate addition. Meanwhile, the AOAamoA gene abundance was significantly correlated with the nrfA and nirS gene abundances, suggesting that coupled nitrification-denitrification/DNRA was prevalent. Partitioning between N₂ production and DNRA was positively correlated with the ratios of dissolved organic carbon to nitrate. Correspondingly, in Lake Taihu during summer to fall, the relatively high organic carbon/nitrate might favorably facilitate DNRA over denitrification, subsequently sustaining cyanobacterial blooms.