Effect of salinity shifts on microbial community composition in different nitrifying biofilms in continuous moving bed biofilm reactors.
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Salinity is considered a common stress factor when nitrification is applied in waste water treatment. Observations often show a sub-optimal nitrification performance as a response to variations in in-fluent salinity. Recently, researchers have coupled microbial community dynamics and changes in the community structure to process stability, and there is a need for interdisciplinary research at the borderline between microbial ecology and process engineering to understand these links.The aim of this study was to investigate and compare the community changes in two different nitrifying cultures adapted to different salinities as a response to a change in salinity. Two continuous moving bed bio-film reactors were first operated at salinities corresponding to the community origin. These were a 0 ppt salinity adapted culture, originating from low salinity municipal waste water, and a 33 ppt salinity adapted culture originating from a recirculated aquaculture filter. After a period of continuous operation at those salinities the salinity were switched: operating the 0 ppt salinity adapted culture with a 33 ppt salinity based cultivation medium, and the 33 ppt adapted culture with a 0 ppt salinity based cultivation medium. Changes in community structure and community dynamics were monitored over time with denaturing gradient gel electrophoresis (DGGE).Average Bray-Curtis similarities within each community showed that a static nitrifying community was not essential for complete nitrification, but rather an advantageous community trait that gave a higher resilience towards fluctuations in environmental factors such as pH, temperature and nitrogen loading.The results showed that the nitrifying culture adapted to 33 ppt salinity was more robust towards a change in salinity, and that the culture was halotolerant. Full nitrification was achieved from day 38 after the salinity change. The microbial adaptation strategy was not determined by either acclimation or by population shift, but rather a combination of the two determined by the community's inherent prerequisites. Further, it was demonstrated that during low salt adaptation a stable nitrification performance was not necessarily coupled to a stable community structure.Population shift was probably the main adaptation strategy for the 0 ppt adapted culture when adapting to 33 ppt salinity. Observations during continuous operation at this salinity showed only partial nitrification from day 54 after the salinity change with low ammonium oxidation rates. Up towards this point in time, and towards the end of the experiment, the community structure was constantly changing.