Simultaneous removal of organic carbon and nitrogen pollutants in the Yangtze estuarine sediment: The role of heterotrophic nitrifiers

The Yangtze Estuary is one of the most eutrophic coastal areas in the world. The engagement of heterotrophic nitrification bacteria in the simultaneous removal of organic carbon and ammonium in the Yangtze estuarine sediment was investigated. The specific nitrification rate in the selective autotrophic nitrification inhibition treatment was about 25% of that in the control without autotrophic nitrification inhibition, suggesting that heterotrophic nitrification, in addition to autotrophic nitrification, was an important nitrification process in the sediment. The increase of heterotrophic nitrification can offset the decrease in autotrophic nitrification, which subsequently leads to the high tolerance of nitrification to the organic carbon. The number of heterotrophic nitrification bacteria was 7.1 × 107 MPN g−1 in sediment collected from Site 1 while that of autotrophic nitrification bacteria was 4.2 × 108 MPN g−1. The isolation of heterotrophic nitrification bacteria provides direct evidence of the engagement of heterotrophs in the nitrification of the Yangtze estuarine sediment. The results show that nitrification is catalyzed by both the autotrophs and the heterotrophs, indicating functional redundancy of nitrification in sediment. Since organic carbon usually coexists with ammonium, these findings indicate an alternative bioprocess for the simultaneous removal of organic carbon and ammonium in Yangtze estuarine sediment.