Potential Nitrate Reduction Research Articles

Unlocking electrocatalytic dynamics with anti-MXene borides monolayers for nitrate reduction

In this comprehensive study, we explore the electrocatalytic behavior of novel two-dimensional (2D) monolayers composed of anti-MXene borides (TMB) towards the nitrate reduction reaction (NO3RR) using density functional theory (DFT). The analysis reveals that these TMB monolayers emerge as promising candidates for electrochemical NO3RR, characterized by their exceptional stability, advantageous selectivity, and effective activation properties. Our study reveals that anti-MXene TMB monolayers, including MnB, RhB, CoB, IrB, OsB, and FeB, exhibit significant electrocatalytic potential for nitrate reduction, each with notable catalytic efficiencies demonstrated by their limiting potentials of −0.25 V, −0.29 V, −0.34 V, −0.36 V, −0.66 V, and −0.70 V, respectively. MnB shows a distinct preference for the NO3--to-N2 conversion pathway, whereas CoB, FeB, and OsB effectively facilitate both the NO3--to-NH3 and NO3--to-N2 pathways. Intriguingly, IrB and RhB primarily favor the NO3--to-NH3 pathway, highlighting their potential in ammonia synthesis applications. These variations in pathway preference not only underscore the diverse catalytic capabilities of these monolayers but also open new avenues for tailored catalytic applications and deepen our understanding of the mechanisms of nitrate reduction.

Geographical factor dominates spatial patterns of potential nitrate reduction rates in coastal wetland sediments in Fujian Province, China

Nitrate (NO3−) reduction is a key process governing the nitrogen (N) dynamics of coastal wetland sediments. Although the effects of environmental factors on the NO3− reduction mechanism in coastal wetland sediments have been examined in various studies, the effects of spatial variation in potential NO3− reduction processes in coastal wetland sediments and the factors driving geographical variation in these processes have not been widely examined. Here, we conducted research on surface sediment samples from four different vegetation types at six coastal wetland sites across two regions. We characterized potential rates of NO3− reduction processes (including denitrification (DF), anammox (ANA), and dissimilatory nitrate reduction to ammonium (DNRA)) using a15N tracer method. Additionally, we assessed the abundances of functional genes, and microbial community structure using high-throughput sequencing, and metagenomic sequencing. In six wetland sites, the contribution ranges of DF, ANA, and DNRA to NO3− reduction were 38.43%–55.69%, 31.33%–45.65%, and 5.26%–17.11%, respectively, and potential NO3− reduction was mainly driven by N removal via gaseous N (DF+ANA). Significant spatial differences were observed in the structure of bacterial and fungal microbial communities, suggesting that geographical distance has a major effect on microbial community structure. Environmental factors and Functional gene abundances were significantly related to potential NO3− reduction processes, and physicochemical properties had a stronger effect on potential NO3− reduction processes than gene abundances. Factors showing significant differences across regions were the main drivers of variation in potential NO3− reduction processes. Overall, our study showed that sediment substrates and geographical environmental factors rather than the abundance of functional genes and vegetation types were the main indicators of potential NO3− reduction activities in coastal wetlands.

Impact of different sulfur sources on the structure and function of sulfur autotrophic denitrification bacteria

Nitrate pollution in surface water has become a significant environmental concern. Sulfur autotrophic denitrification (SAD) technology is gaining attention for its cost-effectiveness and efficiency in nitrate removal. This study aimed to investigate the structure and function of sulfur autotrophic denitrification microbial communities in systems using sodium thiosulfate (Group A) and elemental sulfur (Group B) as the sole electron donors. Metagenomic amplicon sequencing and physicochemical analysis were performed to examine the microbial communities. The results revealed that on day 13, the nitrate nitrogen removal rate in Group A was significantly higher (89.2%) compared to Group B (74.4%). The dominant genus in both Groups was Thiobacillus, with average abundances of 34.15% and 16.34% in Groups A and B, respectively. β-diversity analysis based on species level showed significant differences in bacterial community structure between the two Groups (P < 0.001). Group A exhibited a greater potential for nitrate reduction and utilized both thiosulfate and elemental sulfur (P < 0.01) compared to Group B. This study provides a sufficient experimental basis for improving the start-up time and operating cost of SAD system through sulfur source switching and offers new prospects for in-depth mechanistic analysis.

Benthic nitrate removal capacity in marine mangroves of Guadeloupe, Lesser Antilles.

Mangrove sediments are known to be potentially active reducing zones for nitrogen removal. The goal of this work was to investigate the potential for nitrate reduction in marine mangrove sediments along a canal impacted by anthropogenic activity (Guadeloupe, Lesser Antilles). To this end, the effect of nitrate concentration, organic carbon load, and hydraulic retention time was assessed as factors affecting these nitrate reduction rates. Nitrate reduction potential was determined using flow-through reactors in marine mangrove sediments collected along "The Canal des Rotours" in Guadeloupe. Potential nitrate reduction rates, in the presence of indigenous organic carbon, generally increased upon increasing nitrate supply from around 120nmol cm-3 h-1 (low nitrate) up to 378nmol cm-3 h-1 (high nitrate). The potential for nitrate reduction increased significantly with the addition of mangrove leaves, whereas the addition of simple, easily degradable carbon (acetate) resulted in an almost fivefold increase in nitrate reduction rates (up to 748nmol cm-3 h-1 ). The hydraulic retention time also had an impact on the nitrate reducing capacity due to an increased contact time between nitrate and the benthic microbial community. Marine mangrove sediments have a high potential to mitigate nitrogen pollution, mainly governed by the presence of large amounts of degradable carbon in the form of litter. The mangrove sediments from this Caribbean island, currently exposed to a small tidal effect, could increase their nitrate elimination capacities due to prolonged water retention via engineering.

Effectiveness of Saturated Buffers on Water Pollutant Reduction from Agricultural Drainage

Highlights Saturated buffers redistribute tile drainage water below riparian buffers to reduce nitrate-nitrogen loading. NRCS Conservation Practice Standard 604 guides saturated buffer design in the USA. Annual edge-of-field nitrate-nitrogen loss reductions averaged 46 ± 24% and 9.4 ± 5.9 kg N/ha/y. Further research on design, siting, and mechanisms will enhance performance. Abstract. It is a pivotal time in the development of saturated buffers as a conservation drainage practice. Field data have demonstrated that this practice can effectively reduce nitrate loads in subsurface drainage. The compilation and assessment of current knowledge for this relatively new practice is timely to help identify future opportunities. This review summarizes the state of the science for saturated buffers in the US within the context of this special collection’s emphasis on performance and cost. Suggested research areas are identified to improve understanding of saturated buffer function and performance and to refine design processes and criteria to accelerate adoption. As currently designed, saturated buffers removed an average of 46 ± 24% (mean ± sd) of the N load that would have otherwise entered receiving waters (9.4 ± 5.9 kg N removed/ha-y; n = 30 site-years). Cost efficiencies, which generally trended around $3 to $5/kg N removed per year, were considered relatively efficient compared to similar nitrate removal practices (range: $1.20 to $9.20/kg N/y), with planning level costs between $25 and $66/ha treated/y. As adoption is scaled, engineering design costs need to be considered unless the design model can be simplified. Future research should refine design processes, management, and siting criteria to facilitate scaled adoption for water quality goals. Additional studies on nutrient cycling within saturated buffers are needed to fill gaps about nitrogen and phosphorus dynamics and the role of buffer vegetation. Saturated buffers have significant nitrate reduction potential for tile-drained landscapes, but design adaptations may be needed to facilitate adoption in varied landscapes. Keywords: Denitrification, Edge-of-Field, Nitrate, Nonpoint-source pollution, Saturated riparian buffer, Subsurface drainage, Tile drainage, Water quality.

Coupling effects of nitrate reduction and sulfur oxidation in a subtropical marine mangrove ecosystem with Spartina alterniflora invasion

The mangrove ecosystem has a high nitrate reduction capacity, which significantly alleviates severe nitrogen pollution. However, current research on nitrate reduction mechanisms in the mangrove ecosystem is limited. Furthermore, Spartina alterniflora invasion has disrupted the balance of the mangrove ecosystem and the effect of S. alterniflora on nitrate reduction has not yet been fully elucidated. Nitrate reduction was comprehensively investigated in a subtropical mangrove ecosystem in this study, which has been invaded by S. alterniflora for 40 years. Results showed that S. alterniflora significantly increased the relative and absolute abundance of nitrate reduction genes, especially nirS (nitrite reductase), in the mangrove ecosystem. Dissimilatory nitrate reduction to ammonium was the main pathway of nitrate reduction in the mangrove ecosystem. Nitrate reduction was mainly performed by Desulfobacterales and occurred in the shallow layers (0–10 cm) of mangrove sediments. A strong positive correlation was found between nitrate reduction and sulfur oxidation (especially sulfide oxidation), and the sulfide content was significantly positively correlated with the relative abundance of nitrate reduction genes. Moreover, 207 metagenomic assembled genomes (MAGs) were constructed, including 50 MAGs with high numbers (≥ 10) of nitrate reduction genes. This finding indicates that the dominant microbes had strong nitrate reduction potential in mangrove sediments. Our findings highlight the impact of S. alterniflora invasion on nitrate reduction in a subtropical marine mangrove ecosystem. This study provides new insights into our understanding of nitrogen pollution control and contributes to the exploration of new nitrogen-degrading microbes in mangrove ecosystems.

Managing nitrate-nitrogen in the intensively drained upper Mississippi River Basin, USA under uncertainty: a perennial path forward.

The upper Mississippi River basin has been identified as the most significant contributor of excessive nutrients to the hypoxic zone in the Gulf of Mexico. The land-use changes from an internally drained prairie-wetland complex to an intensively managed corn-soybean production system drained by subsurface tile drainage system in the north-central Iowa and south-central Minnesota are the primary cause of nutrient loads into the Mississippi River and many other environmental stresses. The present study summarizes the water-quality degradation from land-use change and offers a fuzzy logic-based decision support for assessing degree of suitability of the four recommended perennial plant options for managing water and nitrate-nitrogen export. These options are designed based on landscape position that currently fails to produce high yielding row crops and scale: (1) marginal upland depressions for water storage by planting deep-rooted perennial grasses and fast-growing woody poplar, willow, and alder in poorly drained swales; (2) saturated buffers and/or subtle changes in landscape slope for draining high nitrate-nitrogen subsurface (through multi-species phytoremediation treatment buffers or strips of perennial vegetation); (3) two-stage ditches with linear floodplains planted with perennial grasses; and (4) riparian and in-channel ecologically engineered trees, shrubs, and grasses to better connect meander belt width to frequent peak stream flows at larger scales. When applied throughout a typical (Des Moines Lobe Till) DMLT watershed, each option can have positive cumulative environmental effects. Fuzzy logic enhanced the precision in watershed decision-making by incorporating the uncertainty associated with factors like cost effectiveness, nitrate reduction potential, water quality improvement, and level of acceptance.

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.

Dissimilatory nitrate reduction in urban lake ecosystems: A comparison study between closed and open lakes in Chengdu, China

Urban lake ecosystems play important roles in nitrogen cycling, yet the occurrence, contribution and mechanism of nitrate reduction in urban closed and open lakes (UCL and UOL) remain unclear. On November – December of 2020, the potential rates of denitrification (DEN), anammox (ANA), and dissimilatory nitrate reduction to ammonium (DNRA) were quantified using slurries incubations in six urban lakes of Chengdu, China. The environmental variables, genes abundance (nirS, hzsB and nrfA), bacterial 16S rRNA gene were also measured. UOL had higher water ammonium (NH4+), nitrate (NO3−) and nitrite (NO2−), and sediment NH4+, NO3−, total organic carbon (TOC) and ferrous iron (Fe2+) content than UCL. The potential rates of DEN and anammox in UOL were 2.16- and 3.45-times more than in UCL, respectively. Conversely, the DNRA rate in UCL was 1.20-fold higher than UOL. Higher nirS and hzsB abundance were found in UOL, while higher nrfA abundance occurred in UCL. High-throughput sequencing analysis showed that the relative abundance of DEN bacteria was higher in UOL (2.59–12.30%) than in UCL (1.96–6.70%) at the genus level, while the relative abundance of DNRA bacteria was higher in UCL (2.02–4.19%) than in UOL (1.14–2.31%). The difference in the relative abundance of anammox bacteria at the genus level was not significant. Multiple linear regression showed that the physicochemical properties and nitrate reduction bacteria together control the potential nitrate reduction rates. Since a higher nitrogen retention capability appears in UCL, according to the nitrogen retention index (NRI), further management should be focused on urban closed lakes to avoid the potential for eutrophication.

Nitrate reduction potential of a fractured Middle Triassic carbonate aquifer in Southwest Germany

Nitrate reduction constitutes an important natural mechanism to mitigate the widespread and persistent nitrate contamination of groundwater resources. In fractured aquifers, however, the abundance and accessibility of electron donors and their spatial correlation with groundwater flow paths are often poorly understood. In this study, the nitrate reduction potential of a fractured carbonate aquifer in the Upper Muschelkalk of SW Germany was investigated, where denitrification is due to the oxidation of ferrous iron and reduced sulfur. Petrographical analyses of rock samples revealed concentrations of syn-sedimentary and diagenetically formed pyrite ranging from 1 to 4 wt.% with only small differences between different facies types. Additional ferrous iron is available in saddle dolomites (up to 2.6 wt.%), which probably were formed by tectonically induced percolation of low-temperature hydrothermal fluids. Borehole logging at groundwater wells (flowmeter, video, gamma) indicates that most groundwater flow occurs along karstified bedding planes partly located within dolomites of the shoal and backshoal facies. The high porosity (15–30%) of these facies facilitates molecular diffusive exchange of solutes between flow paths in the fractures and the reactive minerals in the pore matrix. The high-porosity facies together with hydraulically active fractures featuring pyrite or saddle dolomite precipitates constitute the zones of highest nitrate reduction potential within the aquifer. Model-based estimates of electron acceptor/donor balances indicate that the nitrate reduction potential protecting water supply wells increases with increasing porosity of the rock matrix and decreases with increasing hydraulic conductivity (or effective fracture aperture) and spacing of the fracture network.

MinION sequencing from sea ice cryoconites leads to de novo genome reconstruction from metagenomes

Genome reconstruction from metagenomes enables detailed study of individual community members, their metabolisms, and their survival strategies. Obtaining high quality metagenome-assembled genomes (MAGs) is particularly valuable in extreme environments like sea ice cryoconites, where the native consortia are recalcitrant to culture and strong astrobiology analogues. We evaluated three separate approaches for MAG generation from Allen Bay, Nunavut sea ice cryoconites—HiSeq-only, MinION-only, and hybrid (HiSeq + MinION)—where field MinION sequencing yielded a reliable metagenome. The hybrid assembly produced longer contigs, more coding sequences, and more total MAGs, revealing a microbial community dominated by Bacteroidetes. The hybrid MAGs also had the highest completeness, lowest contamination, and highest N50. A putatively novel species of Octadecabacter is among the hybrid MAGs produced, containing the genus’s only known instances of genomic potential for nitrate reduction, denitrification, sulfate reduction, and fermentation. This study shows that the inclusion of MinION reads in traditional short read datasets leads to higher quality metagenomes and MAGs for more accurate descriptions of novel microorganisms in this extreme, transient habitat and has produced the first hybrid MAGs from an extreme environment.

Hydroxylamine metabolism in mainstream denitrifying ammonium oxidation (DEAMOX) process: Achieving fast start-up and robust operation with bio-augmentation assistance under ambient temperature

Nitrogen removal from mainstream wastewater via DEnitrifying AMmonium OXidation (DEAMOX) is often challenged by undulated actual temperature and high loading rate. Here, we discovered NH2OH addition (HA) and bio-augmentation (BA) tactics on start-up and operation performance of DEAMOXs (R1 and R2) under ambient temperature (11.3–31.7 °C). Over 340-day operation suggested that R2 received 10 mg/L HA and 1:25 BA ratio (v/v, anammox/partial denitrification sludge) achieved desirable nitrogen removal efficiency (NRE) of 97.22% after 145-day, while R1 under higher BA ratio of 1:12.5 without HA obtained lower NRE (90.86%) after 184-day. Batch tests revealed that nitrate-nitrite transformation ratio reached 98.64% at low COD/NO3-–N of 2.6 with HA. Significantly, compared with R2, R1 recovered quickly with satisfactory effluent total nitrogen of 4.21 mg/L despite nitrogen loading rate greater than 0.15 kg N/m3/d and temperature decreased to 14.6 °C. The abundant narG represented high nitrate reduction potential, hzsA and hdh were extensively detected as the symbolisation of anammox metabolism. Thauera, Denitratisoma and unclassified f Comamonadaceae dominated nitrite accumulation. Ca. Brocadia as the dominant anammox bacteria, and its population maintained stable against low temperature and load shocks by NH2OH intensification. Overall, this study offers an opportunity for the wide-applications of DEAMOX treating mainstream wastewater.

Estuarine benthic nitrate reduction rates: Potential role of microalgae?

The ecological functioning of the Seine estuary is strongly affected by the input of nitrogen, especially in the form of nitrate, which contributes to the eutrophication of the Seine Bight (France). Elimination of nitrate by benthic denitrification in riparian zones or adjacent wetlands could significantly improve the water quality of the Seine estuary. The goal of this study was to investigate the potential for denitrification and the factors affecting these rates. To this end, we measured nitrate reduction and ammonium production rates using flow-through reactors in contrasted sediments collected along the Seine Estuary. Sediment and organic carbon characteristics (organic C, Corg:N ratio, bioavailable carbon, extracellular polymeric substances (EPS), chlorophyll a and phaeopigments and abundance of nitrogen transforming microorganisms were determined and related to the potential nitrate reduction rates. Nitrate reduction rates showed a large spatial and seasonal variation and showed a significant correlation with sediment phaeopigments, whereas overall microbial activity (ammonium production rates) were highly correlated to chlorophyll a and EPS fractions. Surprisingly, bacterial abundance was not correlated to nitrate reduction nor to ammonium production rates. The presence of microalgae appears to be an important driver for nitrate reduction rates in these riparian sediments and seems to have fueled the benthic nitrate reducing activity.

Phylogenetic diversity of culturable marine bacteria from sediments underlying the oxygen minimum zone of the Arabian Sea and their role in nitrate reduction

AbstractThe eastern Arabian Sea has a unique and permanent oxygen minimum zone (OMZ) extending along the western continental margin of India. The Arabian Sea OMZ is a site of intense denitrification, contributing to 20% of the global oceanic denitrification, playing a significant role in the nitrogen cycle. This study reveals the members of the culturable marine bacterial communities thriving and functioning in the sediment underlying the Arabian Sea OMZ, through 16S rDNA phylogenetic analysis and elucidating their nitrate reduction potential. Phylogenetic analyses revealed that these cultured bacteria distributed into five phyla, viz. Firmicutes (45.7%) Gammaproteobacteria (36.84%), Actinobacteria (8.7%), Bacteroidetes (5.26%), and Alphaproteobacteria (3.5%). The presence of denitrification genes such as narG, nirS, and nosZ in the cultured bacteria (39%) explains their importance in this process. Present study on the diversity and distribution of bacterial communities in the OMZ surface sediment and their probable role in the nitrogen cycle will provide insight into the microbial biogeochemical processes occurring in this region.

Nitrogen dynamic in vitro using sludge of a sewage stabilization pond from Patagonia (Argentina).

A relevant and current aspect of wastewater treatment systems is related to the processes of the nitrogen cycle that results in its elimination in gaseous forms. In the present study, we report the first measurements of nitrate-reducing rate (NRR) at lab-scale, using the flow-through reactor technique with sludge of a sewage stabilization pond system located in Patagonia (Argentina). Sludge was collected from Inlet and Outlet areas, in winter and summer. The sludge was characterized by having high moisture content (>94%) and organic matter concentration greater than 37%. The nitrate reduction experimental dates fitted significantly to the Michaelis-Menten model, allowing the estimation of the parameters that regulate the NR kinetics. The maximum potential nitrate reduction rate (Rmax) showed great variability, registering a maximum of 131.6 μmol-N·gdw-1·h-1 (Outlet-Summer) and a minimum of 4.1 μmol-N·gdw-1·h-1 (Inlet-Winter). The lowest half saturation constant (Km) was recorded in the Inlet sludge during the winter (6.1 mg N-NO3-·L-1), which indicates a greater affinity for nitrate of this bacterial consortium. An unusually high activity of NR was registered, being higher with sludge from the Outlet zone and with summer temperature. In full-scale ponds, the NR activity could explain a relevant part of the nitrogen removal that involves the escape of gaseous forms.

Effects of Ferrous Iron and Hydrogen Sulfide on Nitrate Reduction in the Sediments of an Estuary Experiencing Hypoxia

Hypoxia is common feature of eutrophic estuaries and semi-enclosed seas globally. One of the key factors driving hypoxia is nitrogen pollution. To gain more insight into the effects of hypoxia on estuarine nitrogen cycling, we measured potential nitrate reduction rates at different salinities and levels of hypoxia in a eutrophic temperate microtidal estuary, the Neuse River Estuary, North Carolina, USA. We also tested the effect of hydrogen sulfide and ferrous iron additions on the nitrate reduction pathways. Overall, DNRA dominated over denitrification in this periodically hypoxic estuary and there was no correlation between the potential nitrate reduction rates, salinity, or dissolved oxygen. However, when hypoxia lasted several months, denitrification capacity was almost completely lost, and nearly all nitrate added to the sediment was reduced via DNRA. Additions of hydrogen sulfide stimulated DNRA over denitrification. Additions of ferrous iron stimulated nitrate consumption; however, the end product of nitrate consumption was not clear. Interestingly, substantial nitrous oxide formation occurred in sediments that had experienced prolonged hypoxia and were amended with nitrate. Given expanding hypoxia predicted with climate change scenarios and the increasing nitrate loads to coastal systems, coastal sediments may lose their capability to mitigate nitrogen pollution due to DNRA dominating over denitrification during extended hypoxic periods.

Microbial Diversity and Metabolic Potential in the Stratified Sansha Yongle Blue Hole in the South China Sea

The Sansha Yongle Blue Hole is the world’s deepest (301 m) underwater cave and has a sharp redox gradient, with oligotrophic, anoxic, and sulfidic bottom seawater. In order to discover the microbial communities and their special biogeochemical pathways in the blue hole, we analyzed the 16S ribosomal RNA amplicons and metagenomes of microbials from seawater depths with prominent physical, chemical, and biological features. Redundancy analysis showed that dissolved oxygen was the most important factor affecting the microbial assemblages of the blue hole and surrounding open sea waters, and significantly explained 44.7% of the total variation, followed by silicate, temperature, sulfide, ammonium, methane, nitrous oxide, nitrate, dissolved organic carbon, salinity, particulate organic carbon, and chlorophyll a. We identified a bloom of Alteromonas (34.9%) at the primary nitrite maximum occurring in close proximity to the chlorophyll a peak in the blue hole. Genomic potential for nitrate reduction of Alteromonas might contribute to this maximum under oxygen decrease. Genes that would allow for aerobic ammonium oxidation, complete denitrification, and sulfur-oxidization were enriched at nitrate/nitrite-sulfide transition zone (90 and 100 m) of the blue hole, but not anammox pathways. Moreover, γ-Proteobacterial clade SUP05, ε-Proteobacterial genera Sulfurimonas and Arcobacter, and Chlorobi harbored genes for sulfur-driven denitrification process that mediated nitrogen loss and sulfide removal. In the anoxic bottom seawater (100-300 m), high levels of sulfate reducers and dissimilatory sulfite reductase gene (dsrA) potentially created a sulfidic zone of ~200 m thickness. Our findings suggest that in the oligotrophic Sansha Yongle Blue Hole, O2 deficiency promotes nitrogen- and sulfur-cycling processes mediated by metabolically versatile microbials.

Methane-based denitrification kinetics and syntrophy in a membrane biofilm reactor at low methane pressure

A methane-based membrane biofilm reactor (MBfR) was assessed for a tertiary nitrogen removal process in domestic wastewater treatment. To mitigate effluent dissolved methane concentrations, the MBfR was operated with a 20% methane mixing ratio and a low pressure of 0.003 atm. The nitrate concentration was reduced from 20 to 4 mg/L with a low methane concentration of 3.3 mg/L in the effluent at 4 h hydraulic retention time (HRT). An in situ dissolved oxygen sensor showed a concentration of 0.045 mg/L in the MBfR, demonstrating methane oxidation under hypoxic conditions. Both 16S rRNA gene sequencing and metagenomic analysis identified bacteria capable of oxidation of methane coupled to denitrification (Methylocystis), whereas other bacteria were implicated in either methane oxidation (Methylococcus) or nitrate reduction (Escherichia). Reduced genetic potential for nitrate reduction to nitrite at a shorter HRT coincided with a decreased efficiency of denitrification, suggesting rate limitation by this initial step of denitrification. Genes encoding nitrite reduction to dinitrogen were at similar relative abundance under both HRT conditions. Our results provide mechanistic evidence for microbial syntrophy between aerobic methanotrophs and denitrifiers in methane-fed MBfRs operated under varying HRTs, with important implications for novel biological nitrogen removal to dilute wastewater.

“Candidatus Accumulibacter delftensis”: A clade IC novel polyphosphate-accumulating organism without denitrifying activity on nitrate

Populations of “Candidatus Accumulibacter”, a known polyphosphate-accumulating organism, within clade IC have been proposed to perform anoxic P-uptake activity in enhanced biological phosphorus removal (EBPR) systems using nitrate as electron acceptor. However, no consensus has been reached on the ability of “Ca. Accumulibacter” members of clade IC to reduce nitrate to nitrite. Discrepancies might relate to the diverse operational conditions which could trigger the expression of the Nap and/or Nar enzyme and/or to the accuracy in clade classification. This study aimed to assess whether and how certain operational conditions could lead to the enrichment and enhance the denitrification capacity of “Ca. Accumulibacter” within clade IC. To study the potential induction of the denitrifying enzyme, an EBPR culture was enriched under anaerobic–anoxic–oxic (A2O) conditions that, based on fluorescence in situ hybridization and ppk gene sequencing, was composed of around 97% (on a biovolume basis) of affiliates of “Ca. Accumulibacter” clade IC. The influence of the medium composition, sludge retention time (SRT), polyphosphate content of the biomass (poly-P), nitrate dosing approach, and minimal aerobic SRT on potential nitrate reduction were studied. Despite the different studied conditions applied, only a negligible anoxic P-uptake rate was observed, equivalent to maximum 13% of the aerobic P-uptake rate. An increase in the anoxic SRT at the expenses of the aerobic SRT resulted in deterioration of P-removal with limited aerobic P-uptake and insufficient acetate uptake in the anaerobic phase. A near-complete genome (completeness = 100%, contamination = 0.187%) was extracted from the metagenome of the EBPR biomass for the here-proposed “Ca. Accumulibacter delftensis” clade IC. According to full-genome-based phylogenetic analysis, this lineage was distant from the canonical “Ca. Accumulibacter phosphatis”, with closest neighbor “Ca. Accumulibacter sp. UW-LDO-IC” within clade IC. This was cross-validated with taxonomic classification of the ppk1 gene sequences. The genome-centric metagenomic analysis highlighted the presence of genes for assimilatory nitrate reductase (nas) and periplasmic nitrate reductase (nap) but no gene for respiratory nitrate reductases (nar). This suggests that “Ca. Accumulibacter delftensis” clade IC was not capable to generate the required energy (ATP) from nitrate under strict anaerobic-anoxic conditions to support an anoxic EBPR metabolism. Definitely, this study stresses the incongruence in denitrification abilities of “Ca. Accumulibacter” clades and reflects the true intra-clade diversity, which requires a thorough investigation within this lineage.

The Role of Pore Structure on Nitrate Reduction in Peat Soil: A Physical Characterization of Pore Distribution and Solute Transport

Soil type is an important factor defining terrestrial ecosystems and plays a major role for the movement of solutes and cycling of nutrients and carbon. This paper focuses on the effect of peat complex dual-porosity structure on nitrate reduction, with the main objective to show how this process is controlled by pore-scale mass transfer and exchange of nitrate between mobile and immobile pore fractions. A mesocosm experiment was conducted where input solutions of bromide (Br−) and nitrate (NO3 −) were continuously supplied downward into 40 cm depth of peat. Br− and NO3 − breakthrough curves were used to constrain transport parameters and nitrate reduction rates in the peat depth profile. The vertical distribution of potential nitrate reduction rates were compared with depth distributions of partitioning mobile-immobile pores and the exchange coefficient between the pores. The results showed that an increase of immobile pore fractions with depth increases the common interface surface area between mobile and immobile pores which constitutes to a more pronounced exchange between the two transport domains and enhances the nitrate reduction. Hence, the pore structure with mobile-immobile pore fractions and exchange rate of solutes between mobile and immobile phases play a major role in nitrate reduction in peat soils.