Distinctive Microbial Processes and Controlling Factors Related to Indirect N2O Emission from Agricultural and Urban Rivers in Taihu Watershed.

Inland rivers are hotspots of anthropogenic indirect nitrous oxide (N2O) emissions, but the underlying microbial processes remain poorly understood. This study measured N2O fluxes from agricultural and urban rivers in Taihu watershed and investigated the microbial processes driving N2O production and consumption. The N2O fluxes were significantly higher in agricultural rivers (140.1 ± 89.1 µmol m-2 d-1) than in urban rivers (25.1 ± 27.0 µmol m-2 d-1) (p < 0.001). All wind-based models significantly underestimated N2O flux in urban rivers (p < 0.05) when using the Intergovernmental Panel on Climate Change method because they underestimated the N2O emission factor (EF5r). Wind speed and nitrate were the key factors affecting N2O fluxes in agricultural and urban rivers, respectively. NirK-type denitrifiers produced N2O in urban river water, while nirS-type denitrifiers consumed N2O in the sediments of all rivers. Co-occurrence network analysis indicated organics from Microcystis served as electron donors for denitrifiers (dominated by Flavobacterium) in water, while direct interspecies electron transfer between Thiobacillus and methanogens and between Dechloromonas and sulfate-reducing bacteria enhanced N2O reduction in sediments. This study advances our knowledge on the distinctive microbial processes that determine N2O emissions in inland rivers and illustrates the need to revise EF5r for N2O estimation in urban rivers.

Nitrogen removal by heterotrophic nitrifying and aerobic denitrifying bacterium Pseudomonas sp. DM02: Removal performance, mechanism and immobilized application for real aquaculture wastewater treatment.

A bacterial strain was isolated and identified as Pseudomonas sp. DM02 from an aquaculture system. Strain DM02 showed efficient heterotrophic nitrification-aerobic denitrification capability. Total ammonia nitrogen (TAN, 10 mg/L) could be completely removed by strain DM02 within 12 h under low nutrient condition. Nitrogen mass balance indicated that 70.8% of the initial TAN was translated into gaseous nitrogen and 28.1% was converted into intracellular nitrogen. Various carbon sources can be used for nitrate removal (>95% within 28 h). The optimal conditions for TAN, nitrate and nitrite removal were pH 7 with carbon/nitrogen (C/N) ratios of 8, 12 and 12, respectively. The napA, nirK, and nosZ functional genes were successful amplified from strain DM02. Both bioaugmentation and immobilized technology of strain DM02 present ability (>88%) for continuous treatment of real aquaculture wastewater. This research indicated a great potential for practical application of Pseudomonas sp. DM02 in aquaculture wastewater treatment.

Aerobic Denitrification Microbial Community and Function in Zero-Discharge Recirculating Aquaculture System Using a Single Biofloc-Based Suspended Growth Reactor: Influence of the Carbon-to-Nitrogen Ratio.

In this study, the effect of aerobic denitrification on nitrogen removal was investigated using two zero-discharge biofloc-based recirculating aquaculture systems with representative carbon-to-nitrogen (C/N) ratios of 15 (CN15) and 20 (CN20). Aquaculture wastewater, residual feed, and fish feces were treated in an aerated suspended growth reactor (SGR, dissolved oxygen > 5.0 mg L-1). Low toxic NH3 (<0.1 mg L-1) and NO2 --N (<0.5 mg L-1) concentrations and high NO3 --N (83.3%) and NO2 --N (100%) removal efficiencies were achieved in the fish tank and SGR of CN20, respectively. The nitrogen mass balances indicated that the gaseous nitrogen loss accounted for 72-75% of the nitrogen input. Illumina sequencing and quantitative polymerase chain reaction revealed that increasing the C/N ratio significantly increased the amount of aerobic denitrifying bacteria (Dechloromonas, Rhodobacter, Flavobacterium, and Zoogloea) and aerobic denitrifying functional genes (napA, nirK, and nosZ). Autotrophic Nitrosomonas was the dominant nitrifying bacteria in the CN15 system, and autotrophic (Nitrosomonas) and heterotrophic nitrifiers coexisted in the CN20 system. Moreover, the functional prediction analysis showed that the carbohydrate, energy, and amino acid metabolisms in the SGR of the latter increased. In conclusion, aerobic denitrification should widely exist in biofloc systems.