Bacterial community driven nitrogen cycling in coastal sediments of intertidal transition zone

Microorganisms inhabiting in coastal sediments significantly affect the nitrogen cycling in coastal waters and ecosystems. However, the bacterial community that related to the key active nitrogen transformation processes in intertidal transition zone are still not understood. Across a long flat intertidal zone at depths from 0 to 3 m in Daya Bay, China, the bacterial communities in sediments and their driven nitrogen cycling potential were evaluated with environmental factors and 16S rRNA sequencing. The results showed that the intertidal zone is a divide for environmental factors as pH, salinity and C/N ratio, instead of an average shift from freshwater to salt water. At the same time, the environmental factors influenced the abundance of bacterial community related to nitrogen cycling. Across the intertidal zone, the dominant nitrogen transformation processes were different. At the high tide and middle tide sites, the primary nitrogen cycling process was nitrification that worked with Nitrosomonadaceae, Nitrospiraceae, 0319-6A21, and wb1-A12. At the low tide sites, nitrogen fixation was the dominant function conducted by Bradyrhizobiaceae. The reduction of nitrate was carried out with the help of Xanthomonadales but relatively weak in all sampling sites especially for low tide sites. This was mostly because the richness and evenness of bacterial community were the lowest at the low tide sites. Meanwhile, the pH, Cl-, salinity, NH4+, NO3 ? and C/N ratio were the important factors that shaped the composition of local bacterial community. Further, the nonmetric multidimensional scaling results indicated that there were significant statistical differences in the composition of bacterial community among samples at different layers. The dominant nitrogen cycling processes in coastal sediments at different tide levels were revealed in this study, which offered an extended concept of nitrogen transformation along the groundwater discharge path in the intertidal transition zone. The distributions and compositions of bacterial communities and predicted functions provided a new insight for coastal environment and ecosystem management.