Lake littoral zones are characterized by heterogeneity in the biogeochemistry of nutrient elements. This study aimed to explore the relationship between the nitrous oxide reductase gene （nosZ）-encoding denitrifier community composition/abundance and N2O reduction. Five samples （deep sediment, near-transition sediment, transition site, near-transition land and land soil） were collected along a littoral gradient of eutrophic Baiyangdian Lake, North China. To investigate the relationship between the nosZ-encoding denitrifier community structure and N20 reduction, the nosZ-encoding denitrifier community composition/abundance, potential denitrification rate （DNR） and potential N20 production rate （pN20） were investigated using molecular biological technologies and laboratory incubation experiments. The results showed that the average DNR of sediments was about 25 times higher than that of land soils, reaching 282.5 nmol N/（g dry weight （dw）.hr） and that the average pN20 of sediments was about 3.5 times higher than that of land soils, reaching 15.7 nmol N/（g dw-hr）. In the land area, the nosZ gene abundance showed a negative correlation with the N20/（N20＋N2） ratio, indicating that nosZ gene abundance dominated N20 reduction both in the surface soils of the land area and in the soil core of the transition site. Phylogenetic analysis showed that all the nosZ sequences recovered from sediment clustered closely with the isolates Azospirillum largimobile and Azospirillum irakense affiliated to Rhodospirillaceae in alpha-Proteobacteria, while about 92.3% （12/13） of the nosZ sequences recovered from land soil affiliated to Rhizobiaceae and Bradyrhizobiaceae in a-Proteobacteria. The community composition of nosZ gene-encoding denitrifiers appeared to be coupled with N20 reduction along the littoral gradient.
The diversity of Planctomycetes and related bacteria in 3 types of freshwater wetlands with different anthropogenic disturbances were investigated by cloning and sequencing PCR-amplified partial 16S rRNA genes. Three clone libraries were constructed using 16S rRNA-targeted forward PCR primer specific for Planctomycetales and general bacterial reverse primer. Phylogenetic analysis of the 16S rRNA gene sequences defined 95 operational taxonomic units (OTUs) with 163 sequences. The clone libraries covered a wide microbial diversity of Proteobacteria and the Planctomycetes-Verrucomicrobia-Chlamydiales (PVC) superphylum. The majority of the OTUs were related to the phylum of Planctomycetes (33 OTUs), Proteobacteria (22 OTUs) and Verrucomicrobia (22 OTUs). Four known genera from the Planctomycetes phylum were all detected. The genus Pirellula (18 OTUs) dominated the Planctomycetes community, but different patterns of distribution were observed in the wetlands. The littoral wetlands of Baiyangdian Lake with the least anthropogenic disturbances covered more species and showed the highest biodiversity. However, the Jiaxing paddy fields with the highest anthropogenic disturbances showed a higher biodiversity than that in the riparian wetlands of the North Canal. Bacteria distantly related to anammox bacteria were also detected with a small proportion (4 OTUs). It showed that wetlands hold a great biodiversity of phyla Planctomycetes and related bacteria; furthermore, there is ample opportunity to discover novel phylotypes of Planctomycetes in the wetland ecosystems.
Yu WangGuibing ZhuErwin van der BiezenMike S M JettenChengqing Yin
