Promoting the denitrification process by heavy metals in Liaohe Estuary sediment

The impact of heavy metal ions on the biodenitrification process remains unknown, which is the key to understand the nitrogen cycle in estuarine areas. Here, denitrification rate and the abundance of five denitrifying enzyme genes (narG, nirK, napA, norB and nosZ) in Liaohe Estuary sediments were examined, and the community structure of nirK denitrifying bacteria was also analyzed. The results demonstrate a significant positive correlation between heavy metal content (Cu²⁺, Zn²⁺, and Cr) and the denitrification rate, and the abundance of napA/norB (periplasmic nitrate reductase and nitric-oxide reductase) in sediments. The dominant narG denitrifiers were Pseudomonas, Hydrogenophaga, and Serratia known to be tolerant to heavy metal pollution. Sediment particle size, NO₃^-, NO₂^-, Zn²⁺, and Cd²⁺ were the key factors influencing the denitrifying community structure. These findings suggest that heavy metals may enhance the aerobic denitrification process in sediments and mitigate the adverse effects of high dissolved oxygen levels.