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The ammonia-oxidizing bacterial population of the tidal River Elbe was analysed. In the freshwater section of the river, abundance was generally at a magnitude of lo4 cells per g dry weight and lo3 cells per ml in sediments and in water samples, respectively. In the brackish water region, counts decreased drastically with the increase in salinity caused by the low level of abundance of ammonia-oxidizers in the seawater. The contribution of attached ammonia-oxidizers to the total number ranged between 50 and approximately lOO%, depending on the respective load with suspended particulate matter (SPM) of the water. The presence of seven distinct species of the genus Nitrosomonas was established by DNA hybridizations. Based on 16s rRNA gene partial sequence analyses, four groups, comprising phylogenetically closely related species, were defined. Dependence of ammonia oxidation on the NH&l concentration, tolerance against increasing salinity and the possession of urease activity were found to be useful ecophysiological characteristics, being in accordance with phylogenetic relationships among the species.

A lithotrophic ammonia-oxidizing bacterium of the Nitrosomonas type was isolated from the lower River Elbe. Enrichment was attained from suspended particulate matter (SPM) of a water sample. At its natural environment, this species almost exclusively occurred attached to flocs, as demonstrated with the immunofluorescence technique. On the species level, the isolate was not related to any of the described Nitrosomonas species. The strain was characterized by strong production of exopolymeric substances (EPS) and was observed to occur self-flocculating in pure cultures. Low ammonia concentrations stimulated EPS production. The EPS revealed an extensive capacity for binding particulate and dissolved materials, as well as cells of other bacterial species. This capacity was affected by changing pH values or salt concentrations of the medium. The EPS appeared to function as a buffer against toxic compounds and against changing environmental conditions. Another Nitrosomonas strain isolated from the Elbe estuary, but lacking recognizable EPS production, was used for comparison.

Nitrosomonas europaea, Nitrosomonas nitrosa and Nitrosococcus oceanus were successfully grown on hydroxylamine. Significant cell yields were obtained in media containing ammonia supplemented with successive small additions of hydroxylamine. The molar growth yield on hydroxylamine, measured as formation of cell protein per unit of substrate oxidized, was found to be approximately twice that on ammonia. In respiration experiments, the oxygen consumption was 1.5 mol O2 per mol ammonia and 1.0 mol O2 per mol hydroxylamine oxidized to nitrite.