Comprehensive evaluation of nitrogen contamination in water ecosystems of the Miyun reservoir watershed, northern China: distribution, source apportionment and risk assessment.

Miyun Reservoir plays a vital role as a source of drinking water for Beijing, however it grapples with nitrogen contamination issues that have been poorly understood in terms of their distribution, source, and associated health risks. This study addresses this knowledge gap by employing data on nitrate nitrogen (NO3--N), chloride (Cl-), dual isotopic compositions of NO3- (δ15N-NO3- and δ18O-NO3-) data in water ecosystems, systematically exploring the distribution, source and health risk of nitrogen contaminants in Miyun reservoir watersheds. The results showed that over the past 30 years, surface water runoff has exhibited a notable decrease and periodic fluctuations due to the combined influence of climate and anthropogenic activities, while the total nitrogen (TN) concentration in aquatic ecosystems presented an annual fluctuating upward trend. The TN concentration in the wet season was predominantly elevated because a large amount of nitrogen contaminants migrated into water ecosystems through heavy rainfall or river erosion. The concentration of NO3--N, the main contaminant of the water ecosystems, showed distinct variations across different watersheds, followed as rivers over the Miyun reservoir. Moreover, NO3--N levels gradually increased from upstream to downstream in different basins. NO3--N in surface water was mainly derived from the mixture of agricultural ammonia fertilizer and sewage and manure, with a minority of samples potentially undergoing denitrification. Comparatively, the main sources of NO3--N in groundwater were soil N and sewage and manure, while the denitrification process was inactive. The carcinogenic risks caused by NO3--N in groundwater were deemed either nonexistent or minimal, while the focus should predominantly be on potential non-carcinogenic risks, particularly for infants and children. Therefore, it is crucial to perform proactive measures aimed at safeguarding water ecosystems, guided by an understanding of the distribution, sources, and associated risks of nitrogen contamination.

Inhibition of Extracellular Enzyme Activity by Reactive Oxygen Species upon Oxygenation of Reduced Iron-Bearing Minerals.

The dual roles of minerals in inhibiting and prolonging extracellular enzyme activity in soils and sediments are governed by enzyme adsorption to mineral surfaces. Oxygenation of mineral-bound Fe(II) generates reactive oxygen species (ROS), yet it is unknown whether and how this process alters the activity and functional lifespan of extracellular enzymes. Here, the effect of mineral-bound Fe(II) oxidation on the hydrolytic activity of a cellulose-degrading enzyme ß-glucosidase (BG) was studied using two pre-reduced Fe-bearing clay minerals (nontronite and montmorillonite) and one pre-reduced iron oxide (magnetite) at pH 5 and 7. Under anoxic conditions, BG adsorption to mineral surfaces decreased its activity but prolonged its lifespan. Under oxic conditions, ROS was produced, with the amount of •OH, the most abundant ROS, being positively correlated with the extent of structural Fe(II) oxidation in reduced minerals. •OH decreased BG activity and shortened its lifespan via conformational change and structural decomposition of BG. These results suggest that under oxic conditions, the ROS-induced inhibitory role of Fe(II)-bearing minerals outweighed their adsorption-induced protective role in controlling enzyme activity. These results disclose a previously unknown mechanism of extracellular enzyme inactivation, which have pivotal implications for predicting the active enzyme pool in redox-oscillating environments.

Mineral-Bound Trace Metals as Cofactors for Anaerobic Biological Nitrogen Fixation.

Nitrogenase is the only known biological enzyme capable of reducing N2 to bioavailable NH3. Most nitrogenases use Mo as a metallocofactor, while alternative cofactors V and Fe are also viable. Both geological and bioinformatic evidence suggest an ancient origin of Mo-based nitrogenase in the Archean, despite the low concentration of dissolved Mo in the Archean oceans. This apparent paradox would be resolvable if mineral-bound Mo were bioavailable for nitrogen fixation by ancient diazotrophs. In this study, the bioavailability of mineral-bound Mo, V, and Fe was determined by incubating an obligately anaerobic diazotroph Clostridium kluyveri with Mo-, V-, and Fe-bearing minerals (molybdenite, cavansite, and ferrihydrite, respectively) and basalt under diazotrophic conditions. The results showed that C. kluyveri utilized mineral-associated metals to express nitrogenase genes and fix nitrogen, as measured by the reverse transcription quantitative polymerase chain reaction and acetylene reduction assay, respectively. C. kluyveri secreted chelating molecules to extract metals from the minerals. As a result of microbial weathering, mineral surface chemistry significantly changed, likely due to surface coating by microbial exudates for metal extraction. These results provide important support for the ancient origin of Mo-based nitrogenase, with profound implications for coevolution of the biosphere and geosphere.

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.

Distribution, source investigation, and risk assessment of topsoil heavy metals in areas with intensive anthropogenic activities using the positive matrix factorization (PMF) model coupled with self-organizing map (SOM).

Over the past decade, heavy metal (HMs) contamination in soil environments has become severe worldwide. However, their resulting ecological and health risks remained elusive across a variety of soil ecosystems due to the complicated distributions and sources. This study investigated the HMs (Cr, As, Cu, Pb, Zn, Ni, Cd, and Hg) in areas with multi-mineral resources and intensive agricultural activities to study their distribution and source apportionment using a positive matrix factorization (PMF) model coupled with self-organizing map (SOM). The potential ecological and health risks were assessed in terms of distinct sources of HMs. The results disclosed that the spatial distribution of HM contaminations in the topsoil was region-dependent, primarily located in areas with high population intensity. The geo­accumulation index (Igeo) and enrichment factor (EF) values collectively displayed that the topsoils were severely contaminated by Hg, Cu, and Pb, particularly in residential farmland areas. The comprehensive analysis combined with PMF and SOM identified both geogenic and anthropogenic sources of HMs including natural, agricultural, mining, and mixed sources (caused by multi-anthropogenic factors), accounting for 24.9%, 22.6%, 45.9%, and 6.6% contribution rates, respectively. The potential ecological risk was predominantly due to the enrichment of Hg, followed by Cd. The non-carcinogenic risks were mostly below the acceptable risk level, while the potential carcinogenic health risks caused by As and Cr should be paid prime attention to, particularly for children. In addition to the 40% geogenic sources, agricultural activities contributed to 30% of the non-carcinogenic risk, whereas mining activities contributed to nearly half of the carcinogenic health risks.

Disentangling Microbial Syntrophic Mechanisms for Hexavalent Chromium Reduction in Autotrophic Biosystems.

Hexavalent chromium [Cr(VI)] is one of the common heavy-metal contaminants in groundwater, and the availability of electron donors is considered to be a key parameter for Cr(VI) biotransformation. During the autotrophic remediation process, however, much remains to be illuminated about how complex syntrophic microbial communities couple Cr(VI) reduction with other elemental cycles. Two series of Cr(VI)-reducing groundwater bioreactors were independently amended by elemental sulfur and iron and inoculated with the same inoculum. After 160 days of incubation, both bioreactors showed similar archaea-dominating microbiota compositions, whereas a higher Cr(VI)-reducing rate and more methane production were detected in the Fe0-driven one. Metabolic reconstruction of 23 retrieved genomes revealed complex symbiotic relationships driving distinct elemental cycles coupled with Cr(VI) reduction in bioreactors. In both bioreactors, these Cr(VI) reducers were assumed to live in syntrophy with oxidizers of sulfur, iron, hydrogen, and volatile fatty acids and methane produced by carbon fixers and multitrophic methanogens, respectively. The significant difference in methane production was mainly due to the fact that the yielded sulfate greatly retarded acetoclastic methanogenesis in the S-bioreactor. These findings provide insights into mutualistic symbioses of carbon, sulfur, iron, and chromium metabolisms in groundwater systems and have implications for bioremediation of Cr(VI)-contaminated groundwater.

Promotion of Microbial Oxidation of Structural Fe(II) in Nontronite by Oxalate and NTA.

Iron redox cycling occurs extensively in soils and sediments. Previous research has focused on microbially mediated redox cycling of aqueous Fe. At circumneutral pH, most Fe occurs in solid phase, where Fe and organic ligands interact closely. However, the role of organic ligands in microbial oxidation of solid-phase Fe(II) is not well understood. Here, we incubated reduced nontronite NAu-2 (rNAu-2) with an iron-oxidizing bacterium and in the presence of oxalate and nitrilotriacetic acid. These ligands significantly enhanced the rate and extent of microbial oxidation of structural Fe(II) in rNAu-2. Aqueous and solid-phase analyses, coupled with biogeochemical modeling, revealed a pathway for ligand-enhanced bio-oxidation of solid-phase Fe(II): (1) dissolution of rNAu-2 to form aqueous Fe(II)-ligand complex; (2) bio-oxidation to Fe(III)-ligand complex; (3) rapid reduction of Fe(III)-ligand complex to Fe(II)-ligand complex by structural Fe(II) in rNAu-2. In this process, the Fe(II)-ligand and Fe(III)-ligand complexes effectively serve as electron shuttle to expand the bioavailable pool of solid-phase Fe(II). These results have important implications for a better understanding of the bioavailability and reactivity of solid-phase Fe pool in the environment.

Distinct functional microbial communities mediating the heterotrophic denitrification in response to the excessive Fe(II) stress in groundwater under wheat-rice stone and rock phosphate amendments.

Denitrifying microbial community can be utilized for eliminating nitrate and Fe(II) combined contamination in groundwater, while excessive amount of Fe(II) limit the process. Natural mineral can be additional substrate for the microbial growth, whereas how it influences the microbial community that mediating the denitrification coupling with Fe(II) oxidation and balancing inhibition of excessive Fe(II) on denitrification remain unclear. In the present study, we conducted a series of microcosm experiments to explore the denitrification and Fe(II) oxidation kinetic, and used RNA-based qPCR and DNA-based high-throughput sequencing to elucidate microbial diversity, co-occurrence and metabolic profiles amended by wheat-rice stone and rock phosphate. The results showed that both minerals could extensively improve and double the denitrification rates (2.0 ± 0.03 to 2.12 ± 0.13 times), decrease the nitrite accumulation and trigger the high resistance of the denitrifiers from the stress of Fe(II), whereas only wheat-rice stone with higher surface area increased the oxidation of Fe(II) (<10%). The addition of both minerals enhanced the microbial alpha-diversity, shaped the beta-diversity and co-occurrence network, and recovered the transcription of nitrate and nitrite reductase (Nar, Nap, NirS, NirK) from the Fe(II) inhibition. Accordingly, heterotroph Methyloversatilis sp., Methylotenra sp. might contribute to the denitrification under wheat-rice stone amendment, Denitratisoma sp. contribute to the denitrification for rock phosphate, and Fe oxidation was partially catalyzed by Dechloromonas sp. or abiotically by the nitrite/nitrous oxide. These findings would be helpful for better understanding the bioremediation of nitrate and Fe contaminated groundwater.

Effect of Fe(II) on reactivity of heterotrophic denitrifiers in the remediation of nitrate- and Fe(II)-contaminated groundwater.

Heterotrophic denitrifiers, capable of simultaneous nitrate reduction and Fe(II) oxidation, can be applied for the remediation of nitrate and Fe(II) combined contamination in groundwater. Under strictly anaerobic condition, denitrifying microbial communities were enriched with the coexistence of soluble nitrate, Fe(II) and associated nutrient elements to monitor the denitrification process. Low abundance of Fe(II) (e.g., 10 mg L-1 in this study) tended to stimulate the activity of denitrifying microbial communities. However, elevated Fe(II) concentration (50 and 100 mg L-1 in this study), acted as a stress, strongly inhibited the activity and reproduction of denitrifiers. Besides, through thermodynamics calculations, methanol rather than Fe(II) was proved to be the preferable electron donors for both energy metabolism and anabolism. Betaproteobacteria was found to be the most predominant (sub)phylum in all enriched microbial assemblages. Methylovesartilis was the most predominant group mainly catalyzed for methanol based denitrification, and others denitrifiers included Methylophilaceae, Dechloromonas and Denitratisoma. Excessive Fe(II) in the solution greatly reduced the proportions of these denitrifying groups, while the influence seemed to be less apparent on functional genes composition. As such, a conceptional metabolism pathway of the most dominant genus (i.e., Methylovesartilis) for nitrate reducing as well as methanol and Fe(II) oxidation confirmed that biotic nitrate reducing and Fe(II) oxidizing were potentially proceeded in cytoplasm by enzymes such as NarGHI. The Fe(II) oxidation rate depended on the rate of Fe(II) entering into the cell. These findings provide a clear mechanistic understanding of heterotrophic denitrification coupling with Fe(II) oxidation, and environmental implication for the bioremediation of nitrate and Fe(II) contaminated groundwater.

Geochemical and Temporal Influences on the Enrichment of Acidophilic Iron-Oxidizing Bacterial Communities.

UNLABELLED: Two acid mine drainage (AMD) sites in the Appalachian bituminous coal basin were selected to enrich for Fe(II)-oxidizing microbes and measure rates of low-pH Fe(II) oxidation in chemostatic bioreactors. Microbial communities were enriched for 74 to 128 days in fed-batch mode, then switched to flowthrough mode (additional 52 to 138 d) to measure rates of Fe(II) oxidation as a function of pH (2.1 to 4.2) and influent Fe(II) concentration (80 to 2,400 mg/liter). Biofilm samples were collected throughout these operations, and the microbial community structure was analyzed to evaluate impacts of geochemistry and incubation time. Alpha diversity decreased as the pH decreased and as the Fe(II) concentration increased, coincident with conditions that attained the highest rates of Fe(II) oxidation. The distribution of the seven most abundant bacterial genera could be explained by a combination of pH and Fe(II) concentration. Acidithiobacillus, Ferrovum, Gallionella, Leptospirillum, Ferrimicrobium, Acidiphilium, and Acidocella were all found to be restricted within specific bounds of pH and Fe(II) concentration. Temporal distance, defined as the cumulative number of pore volumes from the start of flowthrough mode, appeared to be as important as geochemical conditions in controlling microbial community structure. Both alpha and beta diversities of microbial communities were significantly correlated to temporal distance in the flowthrough experiments. Even after long-term operation under nearly identical geochemical conditions, microbial communities enriched from the different sites remained distinct. While these microbial communities were enriched from sites that displayed markedly different field rates of Fe(II) oxidation, rates of Fe(II) oxidation measured in laboratory bioreactors were essentially the same. These results suggest that the performance of suspended-growth bioreactors for AMD treatment may not be strongly dependent on the inoculum used for reactor startup. IMPORTANCE: This study showed that different microbial communities enriched from two sites maintained distinct microbial community traits inherited from their respective seed materials. Long-term operation (up to 128 days of fed-batch enrichment followed by up to 138 days of flowthrough experiments) of these two systems did not lead to the same, or even more similar, microbial communities. However, these bioreactors did oxidize Fe(II) and remove total iron [Fe(T)] at very similar rates. These results suggest that the performance of suspended-growth bioreactors for AMD treatment may not be strongly dependent on the inoculum used for reactor startup. This would be advantageous, because system performance should be well constrained and predictable for many different sites.

Thermodynamic controls on the kinetics of microbial low-pH Fe(II) oxidation.

Acid mine drainage (AMD) is a major worldwide environmental threat to surface and groundwater quality. Microbial low-pH Fe(II) oxidation could be exploited for cost-effective AMD treatment; however, its use is limited because of uncertainties associated with its rate and ability to remove Fe from solution. We developed a thermodynamic-based framework to evaluate the kinetics of low-pH Fe(II) oxidation. We measured the kinetics of low-pH Fe(II) oxidation at five sites in the Appalachian Coal Basin in the US and three sites in the Iberian Pyrite Belt in Spain and found that the fastest rates of Fe(II) oxidation occurred at the sites with the lowest pH values. Thermodynamic calculations showed that the Gibbs free energy of Fe(II) oxidation (ΔG(oxidation)) was also most negative at the sites with the lowest pH values. We then conducted two series of microbial Fe(II) oxidation experiments in laboratory-scale chemostatic bioreactors operated through a series of pH values (2.1-4.2) and found the same relationships between Fe(II) oxidation kinetics, ΔG(oxidation), and pH. Conditions that favored the fastest rates of Fe(II) oxidation coincided with higher Fe(III) solubility. The solubility of Fe(III) minerals, thus plays an important role on Fe(II) oxidation kinetics. Methods to incorporate microbial low-pH Fe(II) oxidation into active and passive AMD treatment systems are discussed in the context of these findings. This study presents a simplified model that describes the relationship between free energy and microbial kinetics and should be broadly applicable to many biogeochemical systems.

Spatiotemporal successions of N, S, C, Fe, and As cycling genes in groundwater of a wetland ecosystem: Enhanced heterogeneity in wet season.

Microorganisms in wetland groundwater play an essential role in driving global biogeochemical cycles. However, largely due to the dynamics of spatiotemporal surface water-groundwater interaction, the spatiotemporal successions of biogeochemical cycling in wetland groundwater remain poorly delineated. Herein, we investigated the seasonal coevolution of hydrogeochemical variables and microbial functional genes involved in nitrogen, carbon, sulfur, iron, and arsenic cycling in groundwater within a typical wetland, located in Poyang Lake Plain, China. During the dry season, the microbial potentials for dissimilatory nitrate reduction to ammonium and ammonification were dominant, whereas the higher potentials for nitrogen fixation, denitrification, methane metabolism, and carbon fixation were identified in the wet season. A likely biogeochemical hotspot was identified in the area located in the low permeable aquifer near the lake, characterized by reducing conditions and elevated levels of Fe2+ (6.65-17.1 mg/L), NH4+ (0.57-3.98 mg/L), total organic carbon (1.02-1.99 mg/L), and functional genes. In contrast to dry season, higher dissimilarities of functional gene distribution were observed in the wet season. Multivariable statistics further indicated that the connection between the functional gene compositions and hydrogeochemical variables becomes less pronounced as the seasons transition from dry to wet. Despite this transition, Fe2+ remained the dominant driving force on gene distribution during both seasons. Gene-based co-occurrence network displayed reduced interconnectivity among coupled C-N-Fe-S cycles from the dry to the wet season, underpinning a less complex and more destabilizing occurrence pattern. The rising groundwater level may have contributed to a reduction in the stability of functional microbial communities, consequently impacting ecological functions. Our findings shed light on microbial-driven seasonal biogeochemical cycling in wetland groundwater.

Bioavailability of mineral-associated trace metals as cofactors for nitrogen fixation by Azotobacter vinelandii.

Life on Earth depends on N2 -fixing microbes to make ammonia from atmospheric N2 gas by the nitrogenase enzyme. Most nitrogenases use Mo as a cofactor; however, V and Fe are also possible. N2 fixation was once believed to have evolved during the Archean-Proterozoic times using Fe as a cofactor. However, δ15 N values of paleo-ocean sediments suggest Mo and V cofactors despite their low concentrations in the paleo-oceans. This apparent paradox is based on an untested assumption that only soluble metals are bioavailable. In this study, laboratory experiments were performed to test the bioavailability of mineral-associated trace metals to a model N2 -fixing bacterium Azotobacter vinelandii. N2 fixation was observed when Mo in molybdenite, V in cavansite, and Fe in ferrihydrite were used as the sole sources of cofactors, but the rate of N2 fixation was greatly reduced. A physical separation between minerals and cells further reduced the rate of N2 fixation. Biochemical assays detected five siderophores, including aminochelin, azotochelin, azotobactin, protochelin, and vibrioferrin, as possible chelators to extract metals from minerals. The results of this study demonstrate that mineral-associated trace metals are bioavailable as cofactors of nitrogenases to support N2 fixation in those environments that lack soluble trace metals and may offer a partial answer to the paradox.

Using Compound Specific Isotope Analysis to decipher the 1,2,3-trichloropropane-to-Allyl chloride transformation by groundwater microbial communities.

1,2,3-trichloropropane (TCP), a refractory contaminant, can be reductive dehalogenated to allyl chloride (AC) by microorganisms, which has been shown a potential in situ bioremediation (ISB) strategy for TCP remediation in groundwater. In practice, however, it is hard to monitor the bioreduction extent because the TCP concentrations may also be decreased by non-biodegradation processes. Compound specific isotope analysis (CSIA) can be promising in determining the extent of degradation by quantifying the isotope enrichment factors (ε) of relevant degradation mechanisms. To date, no CSIA study has been reported on TCP degradation. In this study, a novel TCP-to-AC transformation enrichment culture (dominated by Azotobacter, Parabacteroides, Fusibacter, Hydrogenophaga, Trichococcus Desulfovibrio, etc) in the absence of the already identified TCP anaerobic reductive dechlorinating microorganisms (e.g., Dehalogenimonas) was derived from a chlorinated hydrocarbon-contaminated aquifer. A TCP degradation experiment was carried out by adding yeast extract to produce hydrogen as an electron donor. The TCP-to-AC transformation was found to conform to zero-order conversion kinetics with the rate constant 11 ± 0.34 µmol L-1 d-1 during the main biodegradation stage. The bulk carbon isotope enrichment factor (εbulk) of the TCP-to-AC transformation was firstly evaluated as -5.2 ± 0.1‰. This study for the first time characterized the carbon isotope fractionations during TCP biodegradation using a novel enrichment culture, which would provide a promising tool for the incorporation of ISB for TCP removal in the future.

Effects of Temperature and Humidity on Soil Gross Nitrogen Transformation in a Typical Shrub Ecosystem in Yanshan Mountain and Hilly Region.

Shrubland is a pivotal terrestrial ecosystem in China. Soil nitrogen transformations play a crucial role in maintaining the productivity of this ecosystem, yet the driving forces underlying it have not been sufficiently addressed, particularly under ongoing climate changes. Herein, by incorporating 15N isotope pool dilution method in laboratory incubation, the rates of gross N ammonification, nitrification, and inorganic N consumption in soils in response to varying temperature and humidity conditions were determined at different depths (SL10: 0-10 cm, and SL20: 10-20 cm) in a typical shrub ecosystem in the Yanshan mountain and hilly region, North China. The gross rates of ammonification and nitrification of soils in SL10 were higher than those in SL20, which was likely affected by the higher soil organic matter and total N contents at a shallower depth. Both temperature and humidity significantly affected the N transformations. The gross ammonification and nitrification were significantly stimulated as the incubation temperature increased from 5 to 35 °C. The gross ammonification increased exponentially, while the gross nitrification increased differently in different temperature ranges. The increment of soil water contents (from 30% WHC to 60% and 100% WHC) promoted the gross nitrification rate more significantly than the gross ammonification rate. The gross nitrification ceased until soil water content reached 60%WHC, indicating that soil water availability between 60% and 100% WHC was not a limiting factor in the nitrification process for the shrubland soils in this study. The ammonium (NH4+) immobilization was significantly lower than nitrification irrespective of varying environmental conditions, even though the NH4+ consumption rate might be overestimated, uncovering two putative processes: (1) heterotrophic nitrification process; (2) and more competitive nitrifying bacteria than NH4+-immobilizing microorganisms. Our study is indispensable for assessing the stability and sustainability of soil N cycling in the shrub ecosystem under climate changes.

Nitrogen species and microbial community coevolution along groundwater flowpath in the southwest of Poyang Lake area, China.

Nitrate and ammonia overload in groundwater can lead to eutrophication of surface water in areas where surface water is recharged by groundwater. However, this process remained elusive due to the complicated groundwater N cycling, which is governed by the co-evolution of hydrogeochemical conditions and N-cycling microbial communities. Herein, this process was studied along a generalized groundwater flowpath in Ganjing Delta, Poyang Lake area, China. From groundwater recharge to the discharge area near the lake, oxidation-reduction potential (ORP), NO3-N, and NO2-N decreased progressively, while NH3-N, total organic carbon (TOC), Fe2+, sulfide, and TOC/NO3- ratio accumulated in the lakeside samples. The anthropogenic influences such as sewage and agricultural activities drove the nitrate distribution, as observed by Cl- vs. NO3-/Cl- ratio and isotopic composition of nitrate. The hydrogeochemical evolution was intimately coupled with the changes in microbial communities. Variations in microbial community structures was significantly explained by Fe2+, NH3-N, and sulfide, while TOC/NO3- controlled the distribution of predicted N cycling gene. The absence of NH3-N in groundwater upstream was accompanied by the enrichment in Acinetobacter capable of nitrification and aerobic denitrification. These two processes were also supported by Ca2+ + Mg2+ vs. HCO3- ratio and isotopic composition of NO3-. The DNRA process downstream was revealed by both the presence of DNRA-capable microbes such as Arthrobacter and the isotopic composition of NH4+ in environments with high concentrations of NH3-N, TOC/NO3-, Fe2+, and sulfide. This coupled evolution of N cycling and microbial community sheds new light on the N biogeochemical cycle in areas where surface water is recharged by groundwater.

Response and contribution of bacterial and archaeal communities to eutrophication in urban river sediments.

Excessive loading of nitrogen (N) and phosphorus (P) that leads to eutrophication mutually interacts with sediment microbial community. To unravel the microbial community structures and interaction networks in the urban river sediments with the disturbance of N and P loadings, we used high-throughput sequencing analysis and ecological co-occurrence network methods to investigate the responses of diversity and community composition of bacteria and archaea and identify the keystone species in river sediments. The alpha-diversity of archaea significantly decreased with the increased total nitrogen (TN), whereas the operational taxonomic unit (OTU) number of bacteria increased with the increase of available phosphorus (AP). The beta-diversity of archaea and bacteria was more sensitive to N content than P content. The relative abundance of predominant bacterial and archaeal taxa varied differently in terms of different N and P contents. Complexity and connectivity of bacteria and archaea interaction networks showed significant variations with eutrophication, and competition between bacteria became more significant with the increase of N content. The sensitive and the highest connective species (keystone species) were identified for different N and P loadings. Total carbon (TC), water content (WC), microbial alpha-diversity and interaction networks played pivotal roles in the N and P transformation in urban river sediments.

Groundwater quality assessment and hydrogeochemical processes in typical watersheds in Zhangjiakou region, northern China.

It is of significance to elucidate the groundwater quality and hydrogeochemical processes for sustainable utilization of groundwater resources in water shortage regions. A total of 256 groundwater samples were collected in typical watersheds in Zhangjiakou, northern China. The hydrochemical parameters, conventional ions, and trace elements were measured, and δD and δ18O data were collected to delineate the groundwater quality and hydrogeochemical processes. The results showed that 32.91% of the groundwater could be directly used for drinking water sources in the Bashang Plateau, north of the study area. The F- and NO3--N were the main parameters above the standard threshold for drinking water. In contrast, the groundwater quality in the Baxia River Basins, south of the study area, was of a better scenario. Nonetheless, high concentrations of F-, total hardness, and SO42- were still observed. Most samples in the Bashang Plateau had relatively higher salinity than the Baxia River Basins. Both surface water and groundwater in the study area originated from local meteoric water with considerable hydraulic connections. The high-fluoride groundwater was primarily formed by dissolution of fluoride-rich minerals under conditions of high pH and Na+, low Ca2+, and rich in HCO3-. The dissolution of carbonate and silicate minerals accompanied by strong cation exchange and weak evaporation was the dominant water-rock interaction affecting the hydrochemical composition of groundwater, and anthropogenic NO3- input had an extra influence on hydrochemical process. This study provides a scientific guideline for the protection and allocation of local groundwater resources.

Incorporating the Soil Gas Gradient Method and Functional Genes to Assess the Natural Source Zone Depletion at a Petroleum-Hydrocarbon-Contaminated Site of a Purification Plant in Northwest China.

An increasing number of studies have demonstrated that natural source zone depletion (NSZD) in the vadose zone accounts for the majority (90%~99%) of the natural attenuation of light non-aqueous phase liquid (LNAPL). Until now, 0.05 to 12 kg/a.m2 NSZD rates at tens of petroleum LNAPL source zones have been determined in the middle or late evolution stage of LNAPL release, in which limited volatile organic compounds (VOCs) and methane (CH4) were detected. NSZD rates are normally estimated by the gradient method, yet the associated functional microbial activity remains poorly investigated. Herein, the NSZD at an LNAPL-releasing site was studied using both soil gas gradient methods quantifying the O2, CO2, CH4, and VOCs concentrations and molecular biology methods quantifying the abundance of the pmoA and mcrA genes. The results showed that the methanogenesis rates were around 4 to 40 kg/a.m2. The values were greater than the rates calculated by the sum of CH4 escaping (0.3~1.2 kg/a.m2) and O2 consuming (3~13 kg/a.m2) or CO2 generating rates (2~4 kg/a.m2), suggesting that the generated CH4 was oxidized but not thoroughly to CO2. The functional gene quantification also supported the indication of this process. Therefore, the NSZD rates at the site roughly equaled the methanogenesis rates (4~40 kg/a.m2), which were greater than most of the previously studied sites with a 90th percentile value of 4 kg/a.m2. The study extended the current knowledge of the NSZD and has significant implications for LNAPL remediation management.

Microbial compositional and functional traits of BTEX and salinity co-contaminated shallow groundwater by produced water.

Intrusion of salinity and petroleum hydrocarbons (e.g., benzene, toluene, ethylbenzene, and xylenes, BTEX) into shallow groundwater by so-called 'produced water' (the water associated with oil and gas production) has recently drawn much attention. However, how this co-contamination affects the groundwater microbial community remains unknown. Herein, geochemical methods (e.g., ion ratios) and high-throughput sequencing (amplicon and shotgun metagenomic) were used to study the contaminant source, hydrogeochemical conditions, microbial community and function in salinity and BTEX co-contaminated shallow groundwater in an oil field, northwest China. The desulfurization coefficient (100rSO42-/rCl-), coefficient of sodium and chloride (rNa+/rCl-), and coefficient of magnesium and chloride (rMg2+/rCl-) revealed an intrusion of produced water into groundwater, resulting in elevated levels of salinity and BTEX. The consumption of terminal electron acceptors (e.g., NO3-, Fe3+, and SO42-) was likely coupled with BTEX degradation. Relative to the bacteria, decreased archaeal diversity and enriched community in produced water-contaminated groundwater suggested that archaea were more susceptible to elevated BTEX and salinity. Relative to the nitrate and sulfate reduction genes, the abundance of marker genes encoding fermentation (acetate and hydrogen production) and methanogenesis (aceticlastic and methylotrophic) was more proportional to BTEX concentration. The produced water intrusion significantly enriched the salt-tolerant anaerobic fermentative heterotroph Woesearchaeia in shallow groundwater, and its co-occurrence with BTEX-degrading bacteria and methanogen Methanomicrobia suggested mutualistic interactions among the archaeal and bacterial communities to couple BTEX degradation with fermentation and methanogenesis. This study offers a first insight into the microbial community and function in groundwater contaminated by produced water.