Spatial stratification of cathodic biofilm on nitrogen removal in the microbial fuel cell under different carbon/nitrogen ratios

The aim of this study was to investigate nitrogen removal in the microbial fuel cell (MFC) related to spatial stratification of bacterial community in cathodic biofilm under different carbon/nitrogen ratios. Single-chamber air-cathode MFC was tested under different ratio of COD/N (i.e., 40, 16, and 10), respectively. With the freezing microtome method, matured cathodic biofilms after 13 operation cycles were sliced into 5 layers (layer 1 close to anolyte and layer 5 close to cathode) with a thickness of 20-30 & mu;m each layer. Results showed that high removals of ammonia and total nitrogen were achieved with stratification of bacterial communities in the cathodic biofilms of MFC. The relative abundance of SM1A02 was in a range of 14.7 % - 26.5 % in all the layers under COD/ N of 40, but only 0.5 % in layers of 1 and 2 under COD/N of 10. Paracoccus had the relative abundance higher than 30 % and lower than 1.5 % in all the layers under COD/N of 10 and 40, respectively. The functional genes including AMO, nxrA, nxrB, narG, nirS and norC were upregulated under low COD/N ratio, indicating that the nitrification and denitrification were enhanced in the cathodic biofilms. The freezing microtome combined with high throughput sequencing of biofilms provided a useful way to characterize the spatial stratification of bacterial community in the nitrogen removal.