Potential electron acceptors for ammonium oxidation in wastewater treatment system under anoxic condition: A review.

Anaerobic ammonium oxidation has been considered as an environmental-friendly and energy-efficient biological nitrogen removal (BNR) technology. Recently, new reaction pathway for ammonium oxidation under anaerobic condition had been discovered. In addition to nitrite, iron trivalent, sulfate, manganese and electrons from electrode might be potential electron acceptors for ammonium oxidation, which can be coupled to traditional BNR process for wastewater treatment. In this paper, the pathway and mechanism for ammonium oxidation with various electron acceptors under anaerobic condition is studied comprehensively, and the research progress of potentially functional microbes is summarized. The potential application of various electron acceptors for ammonium oxidation in wastewater is addressed, and the N2O emission during nitrogen removal is also discussed, which was important greenhouse gas for global climate change. The problems remained unclear for ammonium oxidation by multi-electron acceptors and potential interactions are also discussed in this review.

Long-Term Fertilization Alters Nitrous Oxide Cycling Dynamics in Salt Marsh Sediments.

Salt marsh sediments are known hotspots for nitrogen cycling, including the production and consumption of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting agent. Coastal eutrophication, particularly elevated nitrogen loading from the application of fertilizers, is accelerating nitrogen cycling processes in salt marsh sediments. Here, we examine the impact of long-term fertilization on nitrogen cycling processes with a focus on N2O dynamics in a New England salt marsh. By combining 15N-tracer experiments with numerical modeling, we found that both nitrification and denitrification contribute to net N2O production in fertilized sediments. Long-term fertilization increased the relative importance of nitrification to N2O production, likely a result of increased oxygen penetration from nutrient-induced increases in marsh elevation. Substrate utilization rates of key nitrogen cycling processes revealed links between functions and the corresponding microbial communities. Higher specific substrate utilization rates leading to N2O production from nitrification in fertilized sediments indicate a shift in the community composition of ammonia oxidizers, whereas the lack of change in specific substrate utilization of N2O production from denitrification under long-term fertilization suggests resilience of the denitrifying communities. Both are consistent with previous studies on the functional gene community composition in these experimental plots.

An automated, laser-based measurement system for nitrous oxide isotope and isotopomer ratios at nanomolar levels.

RATIONALE: Nitrous oxide (N2 O) is an atmospheric trace gas regulating Earth's climate, and is a key intermediate of many nitrogen cycling processes in aquatic ecosystems. Laser-based technology for N2 O concentration and isotopic/isotopomeric analyses has potential advantages, which include high analytical specificity, low sample size requirement and reduced cost. METHODS: An autosampler with a purge-and-trap module is coupled to a cavity ring-down spectrometer to achieve automated and high-throughput measurements of N2 O concentrations, N2 O isotope ratios (δ15 Nbulk and δ18 O values) and position-specific isotopomer ratios (δ15 Nα and δ15 Nß values). The system provides accuracy and precision similar to those for measurements made by traditional isotope ratio mass spectrometry (IRMS) techniques. RESULTS: The sample sizes required were 0.01-1.1 nmol-N2 O. Measurements of four N2 O isotopic/isotopomeric references were cross-calibrated with those obtained by IRMS. With a sample size of 0.50 nmol-N2 O, the measurement precision (1σ) for δ15 Nα , δ15 Nß , δ15 Nbulk and δ18 O values was 0.61, 0.33, 0.41 and 0.43‰, respectively. Correction schemes were developed for sample size-dependent isotopic/isotopomeric deviations. The instrumental system demonstrated consistent measurements of dissolved N2 O concentrations, isotope/isotopomer ratios and production rates in seawater. CONCLUSIONS: The coupling of an autosampler with a purge-and-trap module to a cavity ring-down spectrometer not only significantly reduces sample size requirements, but also offers comprehensive investigation of N2 O production pathways by the measurement of natural abundance and tracer level isotopes and isotopomers. Furthermore, the system can perform isotopic analyses of dissolved and solid nitrogen-containing samples using N2 O as the analytical proxy.

Nitric oxide isotopic analyzer based on a compact dual-modulation Faraday rotation spectrometer.

We have developed a transportable spectroscopic nitrogen isotopic analyzer. The spectrometer is based on dual-modulation Faraday rotation spectroscopy of nitric oxide isotopologues with near shot-noise limited performance and baseline-free operation. Noise analysis indicates minor isotope ((15)NO) detection sensitivity of 0.36 ppbv·Hz(-1/2), corresponding to noise-equivalent Faraday rotation angle (NEA) of 1.31 × 10(-8) rad·Hz(-1/2) and noise-equivalent absorbance (αL)min of 6.27 × 10(-8) Hz(-1/2). White-noise limited performance at 2.8× the shot-noise limit is observed up to ~1000 s, allowing reliable calibration and sample measurement within the drift-free interval of the spectrometer. Integration with wet-chemistry based on acidic vanadium(III) enables conversion of aqueous nitrate/nitrite samples to gaseous NO for total nitrogen isotope analysis. Isotopic ratiometry is accomplished via time-multiplexed measurements of two NO isotope transitions. For 5 µmol potassium nitrate samples, the instrument consistently yields ratiometric precision below 0.3‰, thus demonstrating potential as an in situ diagnostic tool for environmental nitrogen cycle studies.

Aquacultural source of nitrous oxide revealed by nitrogen isotopes.

The rapid expansion of coastal aquaculture has led to an increase in the coverage of aquaculture ponds, where intense feed-derived nitrogen is causing significant emissions of nitrous oxide (N2O). Multiple N2O production pathways and the relative importance of water column vs. sedimentary production in aquaculture ponds remain uncertain. Clarifying these pathways is vital for sustainable aquaculture development. Using 15N-labeled dissolved inorganic nitrogen, the pathways and rates of N2O production in subtropical aquaculture ponds located in south China, cultivating whiteleg shrimp, Japanese seabass, and giant river prawn, were successfully characterized. Total N2O production rates ranged from 6 to 70 µmol-N m-2 d-1, with the shrimp pond exhibiting the highest total N2O production rates, followed by ponds for seabass and prawn. These differences are primarily due to varying feed amounts causing differences in dissolved nutrients in water column and sediment. Particularly, nutrient and organic matter accumulation at the surface sediment stimulated N2O production. The oxygenated sediment on a centimeter scale could produce substantially more N2O compared to the water column above on a meter scale. Partial denitrification, i.e., nitrate and nitrite reduction to N2O, was more important (> 60 %) for N2O production in aquaculture ponds. The availability of nitrite is likely a major factor driving partial denitrification for both sedimentary and water column N2O production.

Seasonal isotopic and isotopomeric signatures of nitrous oxide produced microbially in a eutrophic estuary.

Anthropogenic input of excess nutrients stimulates massive nitrous oxide (N2O) production in estuaries with distinct seasonal variations. Here, nitrogen isotopic and isotopomeric signatures were utilized to investigate the seasonal dynamics of N2O production and nitrification at the middle reach of the eutrophic Pearl River Estuary in the south of China. Elevated N2O production primarily via ammonia oxidation (> 1 nM-N d-1) occurred from April to November, along with increased temperature and decreased dissolved oxygen concentration. This consistently oxygenated water column showed active denitrification, contributing 20-40 % to N2O production. The water column microbial N2O production generally constituted a minor fraction (10-15 %) of the estuarine water-air interface efflux, suggesting that upstream transport and tidal dilution regulated the dissolved N2O inventory in the middle reach of the estuary. Nitrification (up to 3000 nM-N d-1) played a critical role in bioavailable nitrogen conversion and N2O production, albeit with N2O yields below 0.05 %.

Ferric and sulfate coupled ammonium oxidation enhanced nitrogen removal in two-stage partial nitrification - Anammox/denitrification process for food waste liquid digestate treatment.

Liquid digestate of food waste is an ammonium-, ferric- and sulfate-laden leachate produced during digestate dewatering, where the carbon source is insufficient for nitrogen removal. A two-stage partial nitrification-anammox/denitrification process was established for nitrogen removal of liquid digestate without pre-treatment (>300 d), through which nitrogen (95 %), biodegradable organics (100 %), sulfate (78 %) and iron (100 %) were efficiently removed. Additional ammonium conversion (20 %N) might be coupled with ferric and sulfate reduction, while produced nitrite could be further converted to di-nitrogen gas through anammox (75 %) and denitrification (25 %). Notably, since increasingly contribution of hydroxylamine producing nitrous oxide, and up-regulated expression of electron transfer and cytochrome c protein, the enhanced ammonium oxidation was probably conducted through extracellular polymeric substances-mediated electron transfer between sulfate/ferric-reducers and aerobic ammonium oxidizers. Thus, the established partial nitrification-anammox/denitrification process might be a cost-efficient nitrogen removal technology for liquid digestate, benefitting to domestic waste recycling and carbon neutralization.

Network analysis of 16S rRNA sequences suggests microbial keystone taxa contribute to marine N2O cycling.

The mechanisms by which large-scale microbial community function emerges from complex ecological interactions between individual taxa and functional groups remain obscure. We leveraged network analyses of 16S rRNA amplicon sequences obtained over a seven-month timeseries in seasonally anoxic Saanich Inlet (Vancouver Island, Canada) to investigate relationships between microbial community structure and water column N2O cycling. Taxa separately broadly into three discrete subnetworks with contrasting environmental distributions. Oxycline subnetworks were structured around keystone aerobic heterotrophs that correlated with nitrification rates and N2O supersaturations, linking N2O production and accumulation to taxa involved in organic matter remineralization. Keystone taxa implicated in anaerobic carbon, nitrogen, and sulfur cycling in anoxic environments clustered together in a low-oxygen subnetwork that correlated positively with nitrification N2O yields and N2O production from denitrification. Close coupling between N2O producers and consumers in the anoxic basin is indicated by strong correlations between the low-oxygen subnetwork, PICRUSt2-predicted nitrous oxide reductase (nosZ) gene abundances, and N2O undersaturation. This study implicates keystone taxa affiliated with common ODZ groups as a potential control on water column N2O cycling and provides a theoretical basis for further investigations into marine microbial interaction networks.

Community Composition of Nitrous Oxide-Related Genes in Salt Marsh Sediments Exposed to Nitrogen Enrichment.

Salt marshes provide many key ecosystem services that have tremendous ecological and economic value. One critical service is the removal of fixed nitrogen from coastal waters, which limits the negative effects of eutrophication resulting from increased nutrient supply. Nutrient enrichment of salt marsh sediments results in higher rates of nitrogen cycling and, commonly, a concurrent increase in the flux of nitrous oxide, an important greenhouse gas. Little is known, however, regarding controls on the microbial communities that contribute to nitrous oxide fluxes in marsh sediments. To address this disconnect, we generated profiles of microbial communities and communities of micro-organisms containing specific nitrogen cycling genes that encode several enzymes (amoA, norB, nosZ) related to nitrous oxide flux from salt marsh sediments. We hypothesized that communities of microbes responsible for nitrogen transformations will be structured by nitrogen availability. Taxa that respond positively to high nitrogen inputs may be responsible for the elevated rates of nitrogen cycling processes measured in fertilized sediments. Our data show that, with the exception of ammonia-oxidizing archaea, the community composition of organisms involved in the production and consumption of nitrous oxide was altered under nutrient enrichment. These results suggest that previously measured rates of nitrous oxide production and consumption are likely the result of changes in community structure, not simply changes in microbial activity.

Multiple organ injury at early stage of intestinal and hepatic ischemia-reperfusion in rats.

OBJECTIVE: To investigate the early-stage multiple organ injuries in rats subjected to intestinal and hepatic ischemia-reperfusion. METHODS: Seventy-five normal male Wistar rats were randomized equally into hepatic ischemia, intestinal ischemia and intestinal-hepatic ischemia groups. Before and at the end of occlusion (45 min), as well as at the time points of 0.5, 2.0 and 6.0 h during the reperfusion, respectively, 5 rats from each group were killed to obtain samples for determination of the contents of malondialdehyde (MDA), superoxide dismutase (SOD) in the blood, lung, kidney, pancreas and heart tissues, along with blood urea nitrogen (BUN), amylase (AMY), and creatine kinase MB (CK-MB). RESULTS: The activity of SOD was decreased (P<0.05) and MDA, BUN, AMY and CK-MB levels increased significantly (P<0.05) after ischemia-reperfusion as compared with those before ischemia. CONCLUSIONS: Intestinal and hepatic ischemia-reperfusion can induce injury of multiple organs at early stage. With the same duration of ischemia-reperfusion, intestinal ischemia may induce severer injury than hepatic ischemia.

The morphological and settling properties of ANAMMOX granular sludge in high-rate reactors.

Digital macro photography and settling tests were carried out to investigate the morphological and settling characteristics of ANAMMOX granules in a high-rate reactor. The ANAMMOX granules could be divided into settling and floating granules. The settling granules with an average diameter of 2.96±0.99 mm were smaller than the floating granules with an average diameter of 4.58±1.22 mm. A settling model was established and validated to correlate the settling velocity with the density (ρG), mass shape factor (ψmass), shape-correction factor (characterized by sphericity (Φ(')) or roundness (ξ)) and projected area equivalent sphere diameter (dP) of ANAMMOX granules. The sphericity was more suitable than the roundness for describing the settling behavior. The sensitivity of four parameters was in the order of ρG,ψmass, dP and Φ('). Based on the settling model, ANAMMOX granules with diameter of 1.75-4.00 mm were supposed to be optimal for the ANAMMOX process.

Performance of autotrophic nitrogen removal in the granular sludge bed reactor.

The autotrophic nitrogen removal process in the granular sludge bed reactor (GSB-ANR process) is a new and promising biotechnology for nitrogen removal from wastewater, which requires single reactor, simple operation and inorganic carbon. The results showed that the GSB-ANR process could be successfully started up with nitrifying granular sludge as inoculum. The volumetric nitrogen loading rate and the volumetric nitrogen removal rate reached 5.44 and 2.57kgNm(-3)day(-1), respectively, which were significantly higher than the level reported for the autotrophic nitrogen removal processes in single reactor. The predominant functional microorganisms were from Planctomycetes and Nitrosomonas. The excellent performance of GSB-ANR process was ascribed to: (a) The high activities of aerobic ammonia-oxidizing bacteria (AOB) and anaerobic ammonium oxidation (ANAMMOX) bacteria; (b) the good settlability of the granular sludge; (c) the suitable DO concentration that satisfied the oxygen requirement of AOB and prevented ANAMMOX bacteria from oxygen inhibition.

The structure, density and settlability of anammox granular sludge in high-rate reactors.

Microscopic observation and settling test were carried out to investigate the structure, density and settlability of anammox granules taken from a high-rate upflow anaerobic sludge blanket (UASB) reactor. The results showed that the anammox granules were irregular in shape and uneven on surface, and their structure included granule, subunit, microbial cell cluster and single cell. The gas pockets were often observed in the anammox granules, and they originated from the obstruction of gas tunnel by extracellular polymer substances (EPSs) and the inflation of produced dinitrogen gas. The volume of gas pockets became larger with the increasing diameter of anammox granules, which led to the decreasing density and the floatation of anammox granules. The diameter of anammox granules should be controlled at less than 2.20mm to avoid the granule floatation. A hypothesized mechanism for the granulation and floatation of anammox biomass was proposed.

Floatation and control of granular sludge in a high-rate anammox reactor.

The granule floatation is a serious issue of the anammox (anaerobic ammonium oxidation) process when high loading rates are applied that results in instability or even system collapse. The present study reports the granule floatation in an anammox reactor when high loading rates were applied. The comparison of enlarged photos taken for the settling and floating granules showed that the two kinds of granules both contained macroscopic gas pockets accounting for 11 +/- 14% of total volume. The settling granules had gas tunnels that could release the gas bubbles, while the floating granules did not. The presence of gas bubbles enclosed in the gas pockets led to the small density of 979.2 +/- 15.8 mg L(-1) and flotation of anammox granules. Consequently, the flotation caused washout of anammox granules and the deterioration of anammox process (volumetric removal rate decreased from 4.00 to 2.46 kg N m(-3) d(-1)). The collection of floating granules, breaking them into small pieces and then returning to the anammox reactor proved an effective control strategy. The volumetric removal rate was finally up to 16.5 kg N m(-3) d(-1) after the control strategy was put into use.