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

The community structure of ammonia-oxidizing bacteria of the beta -subclass Proteobacteria was investigated with respect to environmental gradients al ong the Schelde, a eutrophic estuary system. A dominance of Nitrosomonas-Li ke sequences was detected using molecular techniques targeting the 16S rRNA gene on 3 separate sampling dates, and different Nitrosomonas-like populat ions 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 designate d as Nitrosomonas Cluster 6a. This group, which is closely affiliated with the cultured species N, ureae, has previously been detected as the dominant ammonia-oxidizer group in various freshwater systems, and was also the dom inant recovered sequence cluster from a contributory, untreated sewage effl uent sample. The 16S rDNA recovered from brackish locations further downstr eam was dominated by a group of novel Nitrosomonas-like sequences. Nitrosos pira-like sequences represented only a small minority of those detected for all samples. The shift in dominant ammonia-oxidizer populations occurred i n the estuarine region with the sharpest observed gradients in salinity, ox ygen, and ammonia. These results provide evidence in support of the differe ntial selection of physiologically distinct Nitrosomonas-like groups accord ing to the environmental gradients encountered along the estuary.