| Anaerobic ammonium oxidation?anmamlox?,which reloves Litrogen by NH4+ oxidation coupled with NO2- reduction,has been considered as a significant process for nitrogen removal in aquatic ecosystems of the estuarine and coastal area.Freshwater and seawater mix intensively in estuarine and coastal areas,which generally creates a remarkable salinity gradient.Salinity performs significant effects on the community structure and activity of anammox bacteria in estuarine regions.In general,the salinity of estuarine and coastal areas varies dramatically with season shift,as the river flow is much higher in wet seasons than dry seasons.Moreover,sea level rise and seawater intrusion,caused by global climate change,can lead to an increase in the salinity of estuaries,potentially affecting the nitrogen removal of estuarine and coastal wetlands.So far,there have been several studies exploring the effects of salinity changes on anammox bacterial community integrated with activity,which were based only on the field investigations.However,in these field studies,other environmental variables vary dynamically as well as salinity,which may co-affect the anammox bacterial community structure,abundance,and activity to some extent.Therefore,to explore specific response of anammox to changing salinity and associated controlling mechanisms in the estuarine and coastal wetlands,we selected the Yangtze Estuarine and Coastal area as the study area,and implemented field investigations and salinity-treated incubation experiments.Followings are the main findings:?1?Shift in the community structure of anammox bacteria along the salinity gradient was explored based on anammox bacterial 16S rRNA gene.Four anammox bacterial genera were detected in the sediments of the Yangtze Estuarine and Coastal area,including Brcadia?Kuenenia?Scalindua and Anammoxoglbus.A significant shift in anammox community structure was shown along the salinity gradient from the freshwater region to the open ocean,with the dominant genus changing from Brocadia to Scalindua.In the salinity-treated incubation experiments,Kuenenia and Scalindua were detected.After the short-term incubation,the anammox community structures changed slightly.When the incubation lasted for a long term,the proportion of Kuenenia in the anammox community decreased along the salinity gradient,whereas the proportion of Scalindua increased,with the dominant genus changing from Kuenenia to Scalindua,?2?The abundances of anammox bacterial 16S rRNA gene in the sediments of the Yangtze Estuarine and Coastal area ranged from 3.67×105 to 8.22×107 copies g-1,and reached the highest values at the salinity of 27.6-28.3.As for the salinity incubation experiments,the abundances increased along the salinity gradient but decreased at the salinity of 40 during the short-term incubation,while the highest abundance occurred at the salinity of 5 and 10 after the long-term incubation.?3?The anammox rates measured in the sediments of the Yangtze Estuarine and Coastal area were between 0.08 and 6.46 nmol N g-1 h-1,and the highest anammox rates were shown in the mid-salinity region.During the salinity-treated incubation,the anammox rates increased as the salinity varied from 0 to 20 but decreased sharply when the salinity reached 30 and 40 at the first 10 days.However,the peak of anammox rates during the long-term incubation occurred at the salinity of 5,which was similar to the in situ sediment salinity?4.7?.Moreover,a significant linear relationship was observed between the anammox activity and associated bacterial abundance.?4?The anammox process was estimated to contribute 5-77%to the total nitrogen loss along the salinity gradient in the Yangtze Estuarine and Coastal area.The relative contribution of anammox was negatively related to the salinity.As for the salinity-treated experiments,the anammox relative contributions to the total nitrogen loss varied from 10.5 to 15.3%,which increased along the salinity gradient,showing a different relationship with salinity to the field investigation. |
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