Nitric Oxide Reductase Research Articles

One-ElectronNO to N2O Pathways via Heme Models and Lewis Acid: Metal Effectsand Differences from the Enzymatic Reaction.

Some pathogens use heme-containing nitric oxide reductases (NORs) to reduce NO to N2O as their defense mechanism to detoxify NO and reduce nitrosative stress. This reduction is also significant in the global N cycle. Our previous experimental work showed that Fe and Co porphyrin NO complexes can couple with external NO to form N2O when activated by the Lewis acid BF3. A key difference from conventional two-electron enzymatic reaction is that one electron is sufficient. However, a complete understanding of the entire reaction pathways and the more favorable reactivity for Fe remains unknown. Here, we present a quantum chemical study to provide such information. Our results confirmed Fe's higher experimental reactivity, showing advantages in all steps of the reaction pathway: easier metal oxidation for NO reduction and N-O cleavage as well as a larger size to expedite the N/O coordination mode transition. The Co system, with a similar product energy as the enzyme, shows potential for further development in catalytic NO coupling. This work also offers the first evidence that this new one-electron NO reduction is both kinetically competitive and thermodynamically more favorable than the native pathway, supporting future initiatives in optimizing NO reduction agents in biology, environment, and industry.

In Situ Enrichment of Anammox Bacteria from Pig Farm Anoxic Sludge Through Co-Cultivation with a Quorum-Sensing Functional Strain Pseudomonas aeruginosa

Anaerobic ammonium oxidation (anammox), as an efficient and low-carbon method for nitrogen removal from wastewater, faces the challenge of slow enrichment of functional bacteria. In this study, the enrichment of anammox bacteria Candidatus Brocadia was successfully accelerated by co-culturing with the quorum-sensing strain Pseudomonas aeruginosa and anoxic sludge from a pig farm. Experimental results showed that the R2, which had Pseudomonas aeruginosa added, exhibited chemical reaction ratios RS (NO2−-N consumption/NH4+-N consumption) and RP (NO3−-N production/NH4+-N consumption) closer to the theoretical values of the anammox reaction since Phase Ⅱ. Bacterial community analysis indicated that the abundance of Candidatus Brocadia in R2 reached 1.63% in cycle 20, significantly higher than the 0.45% in R1. More quorum-sensing signaling molecules, primarily C6-HSL, were detected in R2. C6-HSL was positively correlated with processes such as the secretion of anammox extracellular polymers (EPS) and the regulation of nitric oxide reductase (Nir), which may explain the reason behind the accelerated increase in the abundance of Candidatus Brocadia through co-culturing. Moreover, the metabolism of the dominant genus Paracoccus within the two groups of reactors also showed positive regulation by C6-HSL, with its abundance trend similar to that of Candidatus Brocadia, jointly completing the nitrogen removal process in the reactors. However, it is still unknown which genera secrete large amounts of C6-HSL after inoculation with Pseudomonas aeruginosa. This research provides a novel and low-cost method for the enrichment of anammox bacteria.

Biofilm Formation on Excavation Damaged Zone Fractures in Deep Neogene Sedimentary Rock.

Deep underground galleries are used to access the deep biosphere in addition to mining and other engineering applications, such as geological disposal of radioactive waste. Fracture networks developed in the excavation damaged zone (EDZ) are concerned with accelerating mass transport, where microbial colonization might be possible due to the availability of space and nutrients. In this study, microbial biofilms at EDZ fractures were investigated by drilling from a 350-m-deep gallery and subsequent borehole logging at the Horonobe Underground Research Laboratory (URL). By using microscopic and spectroscopic techniques, the dense colonization of microbial cells was demonstrated at the surfaces of the EDZ fractures with high hydraulic conductivity. 16S rRNA gene sequence analysis revealed the dominance of gammaproteobacterial lineages, the cultivated members of which are aerobic methanotrophs. The near-complete genomes from Horonobe groundwater, affiliated with the methanotrophic lineages, were fully equipped with genes involved in aerobic methanotrophy. Although the mediation of aerobic methanotrophy remains to be demonstrated, microbial O2 production was supported by the presence of genes in the near-complete genomes, such as catalase and superoxide dismutase that produce O2 from reactive oxygen species and a nitric oxide reductase gene with the substitutions of amino acids in motifs. It is concluded that the EDZ fractures provide energetically favorable subsurface habitats for microorganisms.

Enhancement of cold-adapted heterotrophic nitrification and denitrification in Pseudomonas sp. NY1 by cupric ions: Performance and mechanism

Cupric ions can restrain biological nitrogen removal processes, which comprise nitrite reductase and nitric oxide reductase. Here, Pseudomonas sp. NY1 can efficiently perform heterotrophic nitrification and aerobic denitrification with cupric ions at 15 °C. At optimal culturing conditions, low cupric ion levels accelerated nitrogen degradation, and ammonium and nitrite removal efficiencies increased by 2.33%–4.85% and 6.76%–12.30%, respectively. Moreover, the maximum elimination rates for ammonium and nitrite increased from 9.48 to 10.26 mg/L/h and 6.20 to 6.80 mg/L/h upon adding 0.05 mg/L cupric ions. Additionally, low cupric ion concentrations promoted electron transport system activity (ETSA), especially for nitrite reduction. However, high concentrations of cupric ions decreased the ETSA during nitrogen conversion processes. The crucial enzymes ammonia monooxygenase, nitrate reductase, and nitrite reductase possessed similarly trends as ETSA upon exposure to cupric ion. These findings deepen the understanding for the effect of cupric ions on nitrogen consumption and bioremediation in nitrogen-polluted waters.

Isotopic signature of N2O produced during sulfur‐ and thiosulfate‐driven chemoautotrophic denitrification in freshwaters

AbstractChemoautotrophic denitrification plays an important role in nitrogen removal and the formation of a greenhouse gas (N2O) in aquatic environments. Natural stable isotopes support the tracing of nitrogen sources and the identification of biogeochemical processes in field research. However, the isotopic characteristics of N2O produced during chemoautotrophic denitrification have not been investigated so far. In this study, we analyzed isotopic signatures of nitrate and N2O in sulfur‐ and thiosulfate‐dependent denitrifying enrichments obtained from freshwater lakes. Chemoautotrophic denitrification exhibited a nitrate isotope pattern similar to heterotrophic denitrification: the 18ε/15ε‐nitrate followed a ratio close to 1. However, chemoautotrophic denitrification produced N2O with lower δ15N, δ18O, and higher site preference (SP = δ15Nα‐δ15Nβ) values, compared to heterotrophic denitrification. The SP value, approximately 5.1‰, was characteristic in detecting chemoautotrophic denitrification driven by different sulfur forms. δ18O varied with specific electron donors, around 20‰ and 40‰ during sulfur‐ and thiosulfate‐dependent denitrification, respectively. The unique N2O isotope characteristics were likely regulated by nitric oxide reductases of Burkholderiaceae populations during sulfur‐dependent denitrification and Sulfurovum during thiosulfate‐dependent denitrification. These findings improve our understanding of N2O production processes and have important implications for predicting N2O emissions at a greater spatial and temporal resolution.

Methane-driven microbial nitrous oxide production and reduction in denitrifying anaerobic methane oxidation cultures

Methane (CH4) and nitrous oxide (N2O) are two important greenhouse gases (GHGs), and it is very important to achieve the reduction of both gases in the case of increasingly serious climate change. The study confirms the simultaneous mitigation of potent GHGs N2O and CH4 in denitrifying anaerobic methane oxidation (DAMO) enrichment, supplementing the understanding that DAMO microorganisms do not produce and reduce N2O. We observed rapid accumulation and consumption of N2O in DAMO cultures subjected to high nitrate and nitrite loadings. Moreover, when N2O served as the sole electron acceptor, a significant correlation was found between the flux of N2O reduction and CH4 oxidation. Inhibition of the particulate methane monooxygenase (pMMO) led to a notable decrease of 95% in N2O reduction and 50% in CH4 oxidation. Metagenomic analysis revealed that Candidatus Methanoperedens and Candidatus Methylomirabilis were the dominant genera, with all functional genes related to denitrification and methane oxidation identified, including NO reductase (nor) of Ca. Methylomirabilis. Transcriptomic analysis further confirmed that the transcripts of nor were one order of magnitude higher than other functional genes. These results indicate that when the substrate NO2- surpasses the dismutation capacity of NO dismutase (nod), DAMO bacteria can alternatively induce nor to reduce NO to N2O. However, considering that N2O reductase (nosZ) is still missing in Ca. Methylomirabilis, the partner Methyloversatilis may be responsible for N2O reduction. Our results provide new insights into the link between GHG emissions in DAMO systems under anaerobic conditions.

The Oxylipin Dependent Quorum Sensing System enhances Pseudomonas aeruginosa dissemination during burn-associated infection.

Following severe burn injury, Pseudomonas aeruginosa is the leading cause of life-threatening infection. Herein, we unveil how P. aeruginosa strategically employs host-derived oleic acid, released as consequence of burn-injury, to induce a hypervirulent phenotype via its Oxylipin Dependent Quorum Sensing system (ODS). ODS activation enhanced P. aeruginosa invasion of burned skin and promoted its dissemination to distant organs in vivo . ODS regulation of P. aeruginosa virulence involved the control of nitic oxide levels, a key signaling molecule in bacteria, through upregulation of the nitric oxide reductases NorCB. Immunization with OdsA, one of the enzymes involved in oxylipin generation, or treatment with a pharmacological inhibitor of OdsA, protected mice against lethal P. aeruginosa infection following burn-injury. Our findings reveal a new mechanism underlying P. aeruginosa hypervirulence in burn wounds and identifies OdsA as a promising target for preventing disseminated infections following burns.

3D-printed controllable bio-accelerators with sustained release property to boost chromium (VI) inhibited denitrification recovery

Although soluble bio-accelerators have proven effective in mitigating Cr(VI) inhibition within denitrification system, issues persist in immobilizing bio-accelerators and making them slow-release for sustained regulation. In this study, a novel strategy was proposed to fabricate immobilized bio-accelerators with controlled structure, sustained release property by 3D printing technology. Notably, the sustained release of bio-accelerators from 3D-printed bio-accelerators (3DP-B) lasted for at least 144 h. Compared to control group, 3DP-B with basic components (3DP-BB) shortened the recovery time by 1.4 folds, and the COD and NO3--N removal efficiency was 36.5 % and 38.0 % higher than that of natural recovery. Correspondingly, the activity of key enzymes (nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase), electron transfer system activity and extracellular polymer substances of denitrification biofilm maintained at relatively high levels. Furthermore, introducing 60 mg·L-1 anthraquinone-2,6-disulfonate (AQDS) into the ink showed noticeable superiority on the bio-inhibition release over 1000 mg·L-1 AQDS. The released AQDS facilitated the electron transport capacity by 1.25 times compared with control group. The groundbreaking findings of this study could advance the development of 3D printing technology and utilization of bio-accelerators in the field of wastewater treatment.

Diversity and Activity of Soil N2O-Reducing Bacteria Shaped by Urbanization.

Nitrous oxide (N2O) is a potent greenhouse gas with various production pathways. N2O reductase (N2OR) is the primary N2O sink, but the distribution of its gene clades, typically nosZI and atypically nosZII, along urbanization gradients remains poorly understood. Here we sampled soils from forests, parks, and farmland across eight provinces in eastern China, using high-throughput sequencing to distinguish between two N2O-reducing bacteria clades. A deterministic process mainly determined assemblies of the nosZI communities. Homogeneous selection drove nosZI deterministic processes, and both homogeneous and heterogeneous selection influenced nosZII. This suggests nosZII is more sensitive to environmental changes than nosZI, with significant changes in community structure over time or space. Ecosystems with stronger anthropogenic disturbance, such as urban areas, provide diverse ecological niches for N2O-reducing bacteria (especially nosZII) to adapt to environmental fluctuations. Structural equation modeling (SEM) and correlation analyses revealed that pH significantly influences the community composition of both N2O-reducing bacteria clades. This study underscores urbanization's impact on N2O-reducing bacteria in urban soils, highlighting the importance of nosZII and survival strategies. It offers novel insights into the role of atypical denitrifiers among N2O-reducing bacteria, underscoring their potential ecological importance in mitigating N2O emissions from urban soils.

Unraveling the genetic potential of nitrous oxide reduction in wastewater treatment: insights from metagenome-assembled genomes.

This study provides critical insights into the genetic diversity of nitrous oxide reductase (nosZ) genes and the microorganisms harboring them in wastewater treatment plants (WWTPs) by exploring 1,083 high-quality metagenome-assembled genomes (MAGs) from 23 Danish full-scale WWTPs. Despite the pivotal role of nosZ-containing organisms, their diversity remains largely unexplored in WWTPs. Our custom pipeline for detecting nosZ provides near-full-length genes with detailed information on secretory pathways and accessory nos genes. Using these genes as templates, we developed taxonomically diverse clade-specific primers that generate nosZ amplicons for phylogenetic annotation and gene-to-MAG linkage. This approach improves detection and expands the discovery of novel sequences, highlighting the prevalence of non-denitrifying N2O reducers and their potential as N2O sinks. These findings have the potential to optimize nitrogen removal processes and mitigate greenhouse gas emissions from WWTPs by fully harnessing the capabilities of the microbial communities.

Temperature differentially regulates estuarine microbial N2O production along a salinity gradient

Nitrous oxide (N2O) is atmospheric trace gas that contributes to climate change and affects stratospheric and ground-level ozone concentrations. Ammonia oxidizers and denitrifiers contribute to N2O emissions in estuarine waters. However, as an important climate factor, how temperature regulates microbial N2O production in estuarine water remains unclear. Here, we have employed stable isotope labeling techniques to demonstrate that the N2O production in estuarine waters exhibited differential thermal response patterns between nearshore and offshore regions. The optimal temperatures (Topt) for N2O production rates (N2OR) were higher at nearshore than offshore sites. 15N-labeled nitrite (15NO2-) experiments revealed that at the nearshore sites dominated by ammonia-oxidizing bacteria (AOB), the thermal tolerance of 15N-N2OR increases with increasing salinity, suggesting that N2O production by AOB-driven nitrifier denitrification may be co-regulated by temperature and salinity. Metatranscriptomic and metagenomic analyses of enriched water samples revealed that the denitrification pathway of AOB is the primary source of N2O, while clade II N2O-reducers dominated N2O consumption. Temperature regulated the expression patterns of nitrite reductase (nirK) and nitrous oxide reductase (nosZ) genes from different sources, thereby influencing N2O emissions in the system. Our findings contribute to understanding the sources of N2O in estuarine waters and their response to global warming.

Synthesis and Structural Characterization of a Non-Heme Iron Hyponitrite Complex.

Flavodiiron NO reductases (FNORs) are important enzymes in microbial pathogenesis, as they equip microbes with resistance to the human immune defense agent nitric oxide (NO). Despite much efforts, intermediates that would provide insight into how the non-heme diiron active sites of FNORs reduce NO to N2O could not be identified. Computations predict that iron-hyponitrite complexes are the key species, leading from NO to N2O. However, the coordination chemistry of non-heme iron centers with hyponitrite is largely unknown. In this study, we report the reactivity of two non-heme iron complexes with preformed hyponitrite. In the case of [Fe(TPA)(CH3CN)2](OTf)2, cleavage of hyponitrite and formation of an Fe2(NO)2 diamond core is observed. With less Lewis-acidic [Fe2(BMPA-PhO)2(OTf)2] (2), reaction with Na2N2O2 in polar aprotic solvent leads to the formation of a red complex, 3. X-ray crystallography shows that 3 is a tetranuclear iron-hyponitrite complex, [{Fe2(BMPA-PhO)2}2(μ-N2O2)](OTf)2, with a unique hyponitrite binding mode. This species provided the unique opportunity to us to study the interaction of hyponitrite with non-heme iron centers and the reactivity of the bound hyponitrite ligand. Here, either protonation or oxidation of 3 is found to induce N2O formation, supporting the hypothesis that hyponitrite is a viable intermediate in NO reduction.

Synthesis and Structural Characterization of a Non‐Heme Iron Hyponitrite Complex

Flavodiiron NO reductases (FNORs) are important enzymes in microbial pathogenesis, as they equip microbes with resistance to the human immune defense agent nitric oxide (NO). Despite much efforts, intermediates that would provide insight into how the non‐heme diiron active sites of FNORs reduce NO to N2O could not be identified. Computations predict that iron‐hyponitrite complexes are the key species, leading from NO to N2O. However, the coordination chemistry of non‐heme iron centers with hyponitrite is largely unknown. In this study, we report the reactivity of two non‐heme iron complexes with preformed hyponitrite. In the case of [Fe(TPA)(CH3CN)2](OTf)2, cleavage of hyponitrite and formation of an Fe2(NO)2 diamond core is observed. With less Lewis‐acidic [Fe2(BMPA‐PhO)2(OTf)2] (2), reaction with Na2N2O2 in polar aprotic solvent leads to the formation of a red complex, 3. X‐ray crystallography shows that 3 is a tetranuclear iron‐hyponitrite complex, [{Fe2(BMPA‐PhO)2}2(μ‐N2O2)](OTf)2, with a unique hyponitrite binding mode. This species provided the unique opportunity to us to study the interaction of hyponitrite with non‐heme iron centers and the reactivity of the bound hyponitrite ligand. Here, either protonation or oxidation of 3 is found to induce N2O formation, supporting the hypothesis that hyponitrite is a viable intermediate in NO reduction.

Comparative analysis of sulfur-driven autotrophic denitrification for pilot-scale application: Pollutant removal performance and metagenomic function

Two parallel pilot-scale reactors were operated to investigate pollutant removal performance and metabolic pathways in elemental sulfur-driven autotrophic denitrification (SDAD) process under low temperature and after addition of external electron donors. The results showed that low temperature slightly inhibited SDAD (average total nitrogen removal of ∼4.7 mg L−1) while supplement of sodium thiosulfate (stage 2) and sodium acetate (stage 3) enhanced denitrification and secretion of extracellular polymeric substances (EPS), leading to the average removal rate of 0.75 and 1.01 kg N m−3 d−1, respectively with over twice higher total EPS. Correspondingly, nitrogen and sulfur related microbial metabolisms especially nitrite reductase and nitric oxide reductase encoding were promoted by genera including Thermomonas and Thiobacillus. The variations revealed that extra sodium acetate improved denitrification and enriched more SDAD-related microorganisms compared with sodium thiosulfate, which potentially catalyzed the refinement of practical strategies for optimizing denitrification in low carbon to nitrogen ratio wastewater treatment.

Denitrifying communities enriched with mixed nitrogen oxides preferentially reduce N2O under conditions of electron competition in wastewater

The reduction rate of nitrous oxide (N2O) is affected by the electron competition among the four denitrifying steps, limiting the mitigation of N2O emissions during wastewater treatment. We foresee essential to understand how combinations of electron acceptors (EAs) affect the microbial composition and reduction mechanisms of denitrifying communities. We enriched three denitrifying communities from activated sludge biomass with equivalent loads of different EAs: NO3– (R1), N2O (R2), and NO3– + N2O (R3). The resulting enrichments were compared in terms of (1) reduction of nitrogen oxides in absence/presence of other EAs (NO3–, NO2–, N2O), (2) their denitrification gene composition and (3) their microbial community composition. Batch results showed the presence of NO3– and NO2– suppressed N2O reduction rates in all three reactors. The effect was lower in the mixed-substrate feed community than in the single-substrate feed under infinite sludge retention time and chemostat operation modes. N2O-reducers of type nosZ II were enriched when N2O serves as the sole EA, whereas nosZ I type N2O-reducers were more prone to enrichment with NO3– as EA. The EA composition rather than the sludge retention mode differentiated the microbial communities. The genus Flavobacterium seems to play a significant role in alleviating the suppression of the N2O reduction rate caused by electron competition. Limited conditions of electron supply are the norm independently of high C/N levels, and a community co-enriched with NO3– and N2O alleviates more the competition for electrons in the nitrous oxide reductase enzyme than communities enriched with NO3– or N2O individually.

Insertion of non-heme iron in cytochrome c-dependent nitric oxide reductase

Insertion of non-heme iron in cytochrome c-dependent nitric oxide reductase

Composted maize straw under fungi inoculation reduces soil N2O emissions and mitigates the microbial N limitation in a wheat upland

Enhancement of microbial assimilation of inorganic nitrogen (N) by straw addition is believed to be an effective pathway to improve farmland N cycling. However, the effectiveness of differently pretreated straws on soil N2O emissions and soil N-acquiring enzyme activities remains unclear. In this study, a pot experiment with four treatments (I, no addition, CK; II, respective addition of maize straw, S; III, composted maize straw under no fungi inoculation, SC; and IV, composted maize straw under fungi inoculation, SCPA) at the same quantity of carbon (C) input was conducted under the same amount of inorganic N fertilization. Results showed that the seasonal cumulative N2O emissions following the SCPA treatment were the lowest at 4.03 kg N ha−1, representing a significant reduction of 19 % compared with the CK treatment. The S and SC treatments had no significant effects on N2O emissions. The decrease of soil N2O emissions following the SCPA treatment was mainly attributed to the increase of microbial N assimilation and the increased abundance of functional genes related to N2O reductase. The SCPA treatment significantly decreased soil alkaline phosphatase (ALP) activity and increased leucine aminopeptidase (LAP) activity at the basal fertilization, while increased soil ALP and LAP activity, decreased soil N-Acetyl-β-D-Glucosidase (NAG) activity at harvest. Compared with the CK treatment, the S, SC, and SCPA treatment significantly increased soil β-glucosidase (β-GC) activity at harvest. The decrease in the (NAG+LAP)/ALP ratio following the SCPA treatment indicated that the composted maize straw under fungi inoculation alleviated microbial N limitation at harvest. Moreover, PICRUSt analysis also suggested that the SCPA treatment increased the abundance of bacterial genes associated with N assimilation and N2O reduction, whereas the S and SC treatment did not significantly affect the abundance of N2O reduction genes compared with the CK treatment. Our results suggest that the composted maize straw under fungi inoculation would reduce the risk of N2O emissions and effectively mitigate the microbial N limitation in dryland wheat system.

Discovery of Candidatus Nitrosomaritimum as a New Genus of Ammonia-Oxidizing Archaea Widespread in Anoxic Saltmarsh Intertidal Aquifers.

Ammonia-oxidizing archaea (AOA) are widely distributed in marine and terrestrial habitats, contributing significantly to global nitrogen and carbon cycles. However, their genomic diversity, ecological niches, and metabolic potentials in the anoxic intertidal aquifers remain poorly understood. Here, we discovered and named a novel AOA genus, Candidatus Nitrosomaritimum, from the intertidal aquifers of Yancheng Wetland, showing close metagenomic abundance to the previously acknowledged dominant Nitrosopumilus AOA. Further construction of ammonia monooxygenase-based phylogeny demonstrated the widespread distribution of Nitrosomaritimum AOA in global estuarine-coastal niches and marine sediment. Niche differentiation among sublineages of this new genus in anoxic intertidal aquifers is driven by salinity and dissolved oxygen gradients. Comparative genomics revealed that Candidatus Nitrosomaritimum has the genetic capacity to utilize urea and possesses high-affinity phosphate transporter systems (phnCDE) for surviving phosphorus-limited conditions. Additionally, it contains putative nosZ genes encoding nitrous-oxide (N2O) reductase for reducing N2O to nitrogen gas. Furthermore, we gained first genomic insights into the archaeal phylum Hydrothermarchaeota populations residing in intertidal aquifers and revealed their potential hydroxylamine-detoxification mutualism with AOA through utilizing the AOA-released extracellular hydroxylamine using hydroxylamine oxidoreductase. Together, this study unravels the overlooked role of priorly unknown but abundant AOA lineages of the newly discovered genus Candidatus Nitrosomaritimum in biological nitrogen transformation and their potential for nitrogen pollution mitigation in coastal environments.

Inorganic carbon metabolism enhanced hydrogen-driven denitrification: Evaluation of carbon fixation pathways and microbial traits

H2-driven denitrification offers a promising alternative for treating low C/N wastewater amidst global nitrogen pollution, due to its advantages of high removal efficiency, low energy consumption, and independence from exogenous organic carbon source. While inorganic carbon assimilation is crucial for the growth and reproduction of autotrophic microorganisms, it remains uncertain whether the fixation of inorganic carbon could impact H2-driven denitrification. This work demonstrated that the supplement of inorganic carbon efficiently improved the efficiency of nitrate removal to 90.3%, compared to 78.0% in the control. Further analysis indicated that inorganic carbon supplementation shifted the microbial structure from autotrophic-dominated (e.g., Hydrogenophaga and Rhodococcus with abundance of 2.3% and 2.6%) to mixotrophic-dominated (e.g., Thauera and Microscillaceae sp. with abundance of 5.6% and 32.8%). Additionally, although H2 boosted the carbon fixation via the reductive TCA cycle and Wood-Ljungdahl pathway, the Calvin-Benson cycle emerged as the predominant pathway, significantly boosted under inorganic carbon supplementation, along with the upregulation of critical enzyme expression including RuBisCo (EC 4.1.1.39), PGK (EC 2.7.2.3) and GAPDH (EC 1.2.1.12). Moreover, the expressions and activities of functional enzymes involved in nitrogen transformation, including nitrate reductase (NAR), nitrite reductase (NIR) and nitric oxide reductase (NOR), as well as microbial electron transfer ability, were remarkably enhanced under sufficient inorganic carbon conditions. This work contributed to understanding the carbon metabolism mechanism in H2-driven denitrification, thereby facilitating the promotion of the efficient and low-carbon approach for nitrogen removal from wastewater.

Effects of exogenous thermophilic bacteria and ripening agent on greenhouse gas emissions, enzyme activity and microbial community during straw composting

This research examined the impact of exogenous thermophilic bacteria and ripening agents on greenhouse gas (GHG) emission, enzyme activity, and microbial community during composting. The use of ripening agents alone resulted in a 30.9 % reduction in CO2 emissions, while the use of ripening agents and thermophilic bacteria resulted in a 50.8 % reduction in N2O emissions. Pearson’s analysis showed that organic matter and nitrate nitrogen were the key parameters affecting GHG emissions. There was an inverse correlation between CO2 and CH4 releases and methane monooxygenase α subunit and N2O reductase activity (P<0.05). Additionally, N2O emissions were positively related to β-1, 4-N-acetylglucosaminidase, and ammonia monooxygenase activity (P<0.05). Deinococcota, Chloroflexi, and Bacteroidota are closely related to CO2 and N2O emissions. Overall, adding thermophilic bacteria represents an effective strategy to mitigate GHG emissions during composting.