Community composition of nirS-type denitrifier in a shallow eutrophic lake.

Denitrification is a major biological process to reduce nitrate to molecular nitrogen (N2). In shallow eutrophic lakes, this process can remove the largest portion of fixed nitrogen and plays an important role in self-purification of this ecosystem. To understand the structure of denitrifying communities in a shallow eutrophic lake, denitrifier communities in four sub-lakes of East Lake in Wuhan, China, were explored by restriction fragment length polymorphisms (RFLP) analysis and sequencing of nirS gene clone libraries. nirS is a functional marker gene for denitrification encoding cytochrome cd 1-containing nitrite reductase, which catalyzes the reduction of nitrite to nitric oxide. Both RFLP fingerprints clustering analysis and phylogeny analysis based on the amino acid sequences of NirS revealed that NirS-type communities in East Lake sediment could be roughly divided into three clusters. Cluster I accounted for 74-82 % of clones from the moderately eutrophic sub-lakes Tuan, Tang Ling, and Guo Zheng. Cluster II accounted for 76 % of the communities in hypertrophic sub-lake Miao Lake and cluster III as a minor group (7 % of the total), mainly presented in Miao Lake. Phylogenetic analysis revealed that cluster I was related to the reference clones from a broad range of ecological environments, and clusters II and III were more phylogenetically related to the reference clones from entrophic environments. Canonical correspondence analysis indicated that total nitrogen, total phosphate, total organic carbon, and NH4-N and NO2-N were important environmental factors affecting the dispersion of NirS-type denitrifier in the sediments. Cluster I showed a weak relationship with the nutrient content, while cluster II and III were positively related with the nutrient content. Principal coordinates analysis indicated that NirS-type communities from Tuan Lake, Tang Ling Lake, and Guo Zheng Lake sediments were divergent from those found in river, estuary sediment, and forest soil but similar to communities in constructed wetland sediment despite large geographic distances. The communities from the hypertrophic sub-lake Miao Lake deviated from other sub-lakes and the reference communities and clustered independently. Our results support the argument that environmental factors regulate the composition and distribution of the functional bacterial groups.