Enhanced electron transfer for the improvement of nitrogen removal efficiency and N₂O reduction at low temperatures

Low temperature generally restricts biological activity, slowing down electron transfer in biogeochemical cycles and causing a series of environmental problems such as nitrogen pollution. We present a strategy to boost electron transfer in microbial cell at low temperatures via stimulation with low current. It is demonstrated by establishing a constructed wetland system coupled with solar powered microbial electrolysis cell, which enhances microbial activity through external micro currents (18.9 ± 5.5 μA) for removing nitrogen pollution in winter (average temperature from -6.6 to 4.5 °C). We investigated the efficiency of pollutants removal, microbial activity, N₂O production and its mechanisms using complexes activity detection, RT-qPCR, incubation, and ¹⁵N-¹⁸O dual-isotope labeling techniques. The activity of complexes I, II, III, and IV collectively represent the microbial electron transfer rate. Results indicated that the microcurrents increased the activity of complexes II, III and IV by 96 %, 172 %, and 313 %, respectively. The transcription abundance of amoA genes in ammonia oxidation and nirS/K genes in denitrification by 263 % and 51 %, respectively. Consequently, NH₄⁺-N removal efficiency improved from 23 % to 35 %, and NO₃^--N removal efficiency from 21 % to 31 %. Moreover, microcurrents reduced N₂O emission by 44 %. However, external microcurrent stimulation did not alter the microbial production pathway of N₂O as determined by the ¹⁵N-¹⁸O dual isotope labeling technique. The relative abundance of the nitrifying bacteria Nitrosomonas, Nitrosospira, and Nitrospira, as well as the denitrifying bacteria Methylotenera, significantly increased due to microcurrent stimulation. Specifically, Nitrospira exhibited the highest increase of 156 %. Our findings provide a novel way to enhance N removal efficiency and simultaneously reduce N₂O emission of biological system like constructed wetlands in winter conditions.