Community Assembly Mechanisms of nirK- and nirS-type Denitrifying Bacteria in Sediments of Eutrophic Lake Taihu, China.

Denitrifying bacteria, particularly nirK- and nirS-type, are functionally equivalent and could occupy different niches, but their community assembly mechanisms and responses to environmental heterogeneity are poorly understood in eutrophic lakes. In this study, we investigated the community assembly mechanisms of nirK- and nirS-type denitrifying bacteria and clarified their responses to sediments environmental factors in Lake Taihu, China. The quantitative real-time PCR and Illumina HiSeq-based sequencing revealed that the abundance and composition of two types of denitrifying bacterial communities varied among different sites in the sediments of Lake Taihu. The functions of these two types of denitrifying bacteria were assigned to mainly nitrogen cycling along with carbon, oxygen, and sulfur cycling, indicating their diverse ecosystems functions. Neutral community model showed that majority of nirK- and nirS-type denitrifying bacteria were neutrally distributed, while dispersal and selection were the dominant drivers in shaping community assembly of nirK-type bacteria. The community assembly of nirS-type was mainly driven by homogeneous selection. We found complex network interactions between nirK- and nirS-type denitrifying bacteria with other bacterial communities, indicating the importance of other bacterial coexistence for ecosystem functions by denitrifying bacteria in lake sediments. Keystone taxa of other bacteria showed the highest interactions with denitrifying bacteria; further, a strong significant correlation between keystone taxa with environmental factors and sediment enzyme content revealed by Mantel tests. Specially, total phosphorous was the key environmental factor determining the composition and diversity of nirK and nirS-type denitrifying bacteria in lake sediments, whereas NAR, AmoA, and NIR were the key reductase enzymes directly or indirectly affected to them. Our results provide significant insights into understanding the effects of changing nirK- and nirS-type denitrifying bacterial diversities and underlying community assembly mechanisms under changing environmental conditions in eutrophic lake ecosystems.