Influence of oxygen partial pressure and salinity on the community composition of ammonia-oxidizing bacteria in the Schelde estuary

The influence of environmental factors on the community structure of ammonia-oxidizing bacteria (AOB) was investigated in the Schelde estuary. Simultaneously with the increase of oxygen and salinity, a shift of the dominant AOB was observed. Molecular analysis based on 16S rRNA genes showed that the freshwater area was dominated by Nitrosomonas-like sequences which were closely related to N. ureae and N. oligotropha. In the brackish area, sequences of a novel group of Nitrosomonas-like bacteria and relatives of N. marina and N. aestuarii were found. Batch and continuous culture experiments were used to investigate the reason behind the community shift. Batch incubations showed a positive influence of the natural water from the Schelde estuary on ammonia oxidation, compared to mineral medium. The ammonia oxidation rates of the AOB originating from the brackish water were higher in the absence of salt. Water from the freshwater part was incubated in continuous cultures under 3 different conditions of salinity and oxygen partial pressure. Under all conditions, ammonia consumption in the continuous cultures started at once. Molecular analysis resulted in a clear phylogenetic difference between the absence and presence of salt, indicating that salt is the factor which causes the community shift of the AOB in the Schelde estuary.