Coupled abiotic-biotic cycling of nitrous oxide in tropical peatlands

Oxygen Availability Governs the Mitigating Effect of 3,4-Dimethylpyrazole Phosphate on Nitrous Oxide Emissions from Paddy Soils under Various Water Managements

Alternate wetting and drying (AWD) frequently triggers nitrous oxide (N2O) emissions from paddy fields, while the inhibitory effect of 3,4-dimethylpyrazole phosphate (DMPP) on N2O under various water managements remains uncertain. Here, we evaluated the effects of DMPP on N2O emissions and associated biological indicators under three water managements: continuous flooding (CF), mild AWD (Mi-AWD), and moderate AWD (Mo-AWD). The Mi-AWD and Mo-AWD practices increased N2O emissions by 2- and 0.9-fold compared to the CF treatment, respectively, due to enhanced oxygen availability, facilitating coupled nitrification-denitrification. DMPP application notably reduced N2O emissions in the AWD treatments, attributed to the reductions in nitrifier abundances, nitrification rates, and nitrate accumulation. Nevertheless, DMPP failed to suppress nitrification and, thereby, N2O emissions in the CF treatment. Overall, DMPP effectively mitigates N2O emissions under oxygen-rich AWD rather than anaerobic CF conditions, highlighting that the trade-offs between water-saving irrigation and N2O mitigation can be overcome via nitrification inhibitors application.

Production of Reactive Oxygen Species during Redox Manipulation and Its Potential Impacts on Activated Sludge Wastewater Treatment Processes

Reactive oxygen species (ROS) are ubiquitous in redox-fluctuating environments, exerting profound impacts on biogeochemical cycles. However, whether ROS can be generated during redox manipulation in activated sludge wastewater treatment processes (AS-WTPs) and the underlying impacts remain largely unknown. This study demonstrates that ROS production is ubiquitous in AS-WTPs due to redox manipulation and that the frequency and capacity of ROS production depend on the operating modes. The anaerobic/oxic continuous-flow reactor showed persistent ROS generation (0.8-2.1 μM of instantaneous H2O2), whereas the oxic/anoxic sequencing batch reactor (0.21-0.28 mM of H2O2 per cycle) and the anaerobic/anoxic digestion reactor (0.27-0.29 mM of H2O2 per cycle) exhibited periodic ROS production. Our results illustrated that ROS generated during redox manipulation can contribute to the removal of organic micropollutants. Due to their high activity, ROS can directly accelerate the abiotic oxidation of organic phenolics and Fe(II) minerals in sludges. ROS could also affect biotic nitrification by changing the microbial community composition and regulating the relative expression of functional genes, such as amoA, nrxA, and nrxB. This research demonstrates the ubiquitous production of ROS during redox manipulation in AS-WTPs, which provides new insights into pollutant removal and the abiotic and biotic elemental transformation in AS-WTPs.

Combined alkali-photocatalytic stimulation enables click microbial domestication for boosted ammonia nitrogen removal

Microbe-driven ammonia nitrogen removal plays a crucial role in the nitrogen cycle and wastewater treatment. However, the rational methods and mechanisms for boosting nitrogen conversion through microbial domestication are still limited. Herein, a combined alkali-photocatalytic stimulation strategy was developed to activate the Halomonas shizuishanensis DWK9 for efficient ammonia nitrogen removal. The strain DWK9 selected from saline-alkaline soil in Northwestern China possessed strong resistance to stress of saline-alkaline environment and free radicals, and was abundant in nitrogen conversion genes, thus is an ideal model for advanced microbial domestication. Bacterial in the combined alkali-photocatalytic stimulation group achieved a high ammonia nitrogen conversion rate of 67.5 %, 10 times outperforming the non-stimulated and single alkali/photocatalytic stimulation control groups. Morphology analysis revealed that the bacteria in the alkali-photocatalytic stimulated group formed a favorable structure for bioelectric transfer. Remarkably, the domesticated bacteria demonstrated improved electrochemical properties, including increased current capacity and lower overpotentials and impedance. Prokaryotic transcription genetic analysis together with qPCR analysis showed upregulation of denitrification-related metabolic pathway genes. A novel FAD dependent and NAD(P)H independent energy mode has been proposed. The universality and effectiveness of the as-developed combined alkali-photocatalytic microbial domestication strategy were further validated through indicator fish survival experiments. This work provides unprecedented degrees of freedom for the exploration of rational microbial engineering for optimized and controllable biogeochemical conversion.

Magnetite-mediated shifts in denitrifying consortia in bioelectrochemical system: Insights into species selection and metabolic dynamics

Conductive materials, such as magnetite, are recognized for their ability to enhance electron transfer and stimulate microbial metabolic activities. This study aimed to elucidate the metabolic potential and species interactions of dominant microbial species within complex communities influenced by magnetite. It indicated that the optimal dosage of magnetite at 4.5 mg/cm², would significantly improve denitrification efficiency and then reduce the time for removing 50 mg/L nitrate by 24.33 %. This enhancement was attributed to the reduced charge transfer resistance and the promoted formation of extracellular polymeric substances (EPS) facilitated by magnetite. Metagenomic analysis revealed that magnetite addition mitigated the competition among truncated denitrifiers for downstream nitrogen species, diminished the contribution of bacteria with complete nitrogen metabolism pathways to denitrification, and fostered a transition towards co-denitrification through interspecies cooperation, consequently leading to decreased nitrite accumulation and increased tolerance to nitrate shock loads. Furthermore, an in-depth study on a key species, Geobacter anodireducens JN93 within the bioelectrochemical system revealed that while magnetite with varying Fe(II) and Fe(III) ratios improved denitrification performance, the metabolic potential of Geobacter sp. varied for different nitrogen metabolism pathways. Collectively, this research provides insights into the microecological effects of magnetite on denitrifying consortia by shifting interspecific interactions via enhanced electron transfer.

Nitrous oxide respiration in acidophilic methanotrophs

Aerobic methanotrophic bacteria are considered strict aerobes but are often highly abundant in hypoxic and even anoxic environments. Despite possessing denitrification genes, it remains to be verified whether denitrification contributes to their growth. Here, we show that acidophilic methanotrophs can respire nitrous oxide (N2O) and grow anaerobically on diverse non-methane substrates, including methanol, C-C substrates, and hydrogen. We study two strains that possess N2O reductase genes: Methylocella tundrae T4 and Methylacidiphilum caldifontis IT6. We show that N2O respiration supports growth of Methylacidiphilum caldifontis at an extremely acidic pH of 2.0, exceeding the known physiological pH limits for microbial N2O consumption. Methylocella tundrae simultaneously consumes N2O and CH4 in suboxic conditions, indicating robustness of its N2O reductase activity in the presence of O2. Furthermore, in O2-limiting conditions, the amount of CH4 oxidized per O2 reduced increases when N2O is added, indicating that Methylocella tundrae can direct more O2 towards methane monooxygenase. Thus, our results demonstrate that some methanotrophs can respire N2O independently or simultaneously with O2, which may facilitate their growth and survival in dynamic environments. Such metabolic capability enables these bacteria to simultaneously reduce the release of the key greenhouse gases CO2, CH4, and N2O.

Appropriate stoichiometric ratios of dissolved organic carbon and nitrate can trigger a transition in nitrate removal in groundwater around plateau lakes, Southwest China

Increasing dissolved organic carbon (DOC) in groundwater as a carbon source for microorganisms that stimulate nitrate attenuation is considered a sustainable strategy to mitigate nitrate pollution in groundwater. However, little is known on the stoichiometric ratio of DOC and nitrate required in groundwater nitrate reduction processes, which has become an obstacle for evaluating the current status of DOC limitations on nitrate reduction. Here, the NO3--N and DOC concentrations in groundwater around 8 plateau lakes were investigated, and a microcosm experiment was performed to elucidate the effects of different DOC:NO3--N levels in groundwater on NO3--N reduction, and the current status of DOC limitations on groundwater NO3--N reduction around 8 lakes was further evaluated. The results indicated that nearly 41 % of the groundwater NO3--N concentrations exceeded the WHO threshold for drinking water (11.3 mg L-1) and 79 % of the groundwater DOC concentrations exceeded 5 mg L-1. The differences in groundwater NO3--N and DOC concentrations among the 8 lakes were controlled by the intensity of agricultural and human activities and hydrogeological background. The stoichiometric ratio of DOC:NO3--N regulated the NO3--N reduction process, and groundwater NO3--N accumulation rate appeared to become limited and sharply decreased when the DOC concentration was approximately 10 mg L-1 or when the DOC:NO3--N ratio was close to 1:1, and the DOC:NO3--N ratio threshold for limiting the NO3--N reduction process was approximately 2.25. Based on this threshold, >33 %-86 % of the groundwater samples around the 8 plateau lakes were strongly limited in the reduction of groundwater NO3--N due to a lack of sufficient DOC provides energy for heterotrophic microorganisms. Additionally, we highlight that the sustainable strategy of increasing DOC to stimulate groundwater NO3- attenuation should be combined with short-term strategies to jointly coordinate and control groundwater NO3- pollution.

Bacterial denitrification drives elevated N2O emissions in arid southern California drylands

Soils are the largest source of atmospheric nitrous oxide (N2O), a powerful greenhouse gas. Dry soils rarely harbor anoxic conditions to favor denitrification, the predominant N2O-producing process, yet, among the largest N2O emissions have been measured after wetting summer-dry desert soils, raising the question: Can denitrifiers endure extreme drought and produce N2O immediately after rainfall? Using isotopic and molecular approaches in a California desert, we found that denitrifiers produced N2O within 15 minutes of wetting dry soils (site preference = 12.8 ± 3.92 per mil, δ15Nbulk = 18.6 ± 11.1 per mil). Consistent with this finding, we detected nitrate-reducing transcripts in dry soils and found that inhibiting microbial activity decreased N2O emissions by 59%. Our results suggest that despite extreme environmental conditions-months without precipitation, soil temperatures of ≥40°C, and gravimetric soil water content of <1%-bacterial denitrifiers can account for most of the N2O emitted when dry soils are wetted.