Shifts in the dominant populations of ammonia-oxidizing b-subclass Proteobacteria along the eutrophic Schelde estuary

Abstract

The community structure of ammonia-oxidizing bacteria of the β-subclass Proteobacte- ria was investigated with respect to environmental gradients along the Schelde, a eutrophic estuary system. A dominance of Nitrosomonas-like sequences was detected using molecular techniques tar- geting the 16S rRNA gene on 3 separate sampling dates, and different Nitrosomonas-like populations were most dominant at different locations along the estuary. The most frequently detected ammonia oxidizer-like sequences in the freshwater part of the estuary were associated with a sequence cluster previously designated as Nitrosomonas Cluster 6a. This group, which is closely affiliated with the cul- tured species N. ureae, has previously been detected as the dominant ammonia-oxidizer group in various freshwater systems, and was also the dominant recovered sequence cluster from a contribu- tory, untreated sewage effluent sample. The 16S rDNA recovered from brackish locations further downstream was dominated by a group of novel Nitrosomonas-like sequences. Nitrosospira-like sequences represented only a small minority of those detected for all samples. The shift in dominant ammonia-oxidizer populations occurred in the estuarine region with the sharpest observed gradients in salinity, oxygen, and ammonia. These results provide evidence in support of the differential selec- tion of physiologically distinct Nitrosomonas-like groups according to the environmental gradients encountered along the estuary