The community structure of ammonia-oxidizing bacteria of the β-subclass Proteobacteria 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 targeting 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 cultured 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 contributory, 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 selection of physiologically distinct Nitrosomonas-like groups according to the environmental gradients encountered along the estuary KEY WORDS: Ammonia-oxidizing bacteria · Estuaries · DGGE · Nitrosomonas Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 23: [225][226][227][228][229][230][231][232][233][234][235][236] 2001 ents of salinity, ammonia concentration, and dissolved oxygen levels. Thus, as bacteria travel with the residual seaward current, they encounter changing environmental conditions. The residence time of water in the total estuary is about 60 d (Soetaert & Herman 1995b), although this may be extended by attachment to particles or by (temporary) sedimentation (Owens 1986). The mean residence time of particles in one compartment of the estuary (see Fig. 1) is comparable with the generation time of many cultured ammonia-oxidizing bacteria (Helder & de Vries 1983). Thus, competition and selection may occur between distinct ammonia-oxidizer populations as they travel through the Schelde estuary. Alternatively, ammonia-oxidizing bacteria may possess the ability to adapt to the environmental gradients encountered. Clues as to which of these processes most affect ammonia-oxidizer populations might therefore be gained by examining their community structure along the estuarine region where these key environmental gradients are observed.Ecological studies of ammonia-oxidizing bacteria have been hampered by the difficulties and biases associated with the isolation and manipulation of these organisms in pure culture (Koops & Harms 1985, Prosser 1989. The monophyletic nature of the β-subclass ammonia-oxidizing bacteria has h...

No abstract

This paper examines the effect of oxygen on nitrous oxide (N 2 O) concentrations in estuarine waters. N 2 O has been measured year-round in the Schelde estuary, a high-nitrogen, lowoxygen macrotidal system. N 2 O concentrations were above atmospheric equilibrium levels indicating that this estuary represents a source to the atmosphere. The distribution of N 2 O showed consistent and systematic relationships with distribution patterns of ammonium, oxygen, nitrite and nitrification activities. A controlled laboratory experiment with a natural bacterial community from the Schelde estuary revealed maximum N 2 O production to occur at oxygen concentrations of about 5 µM. This production was inhibited by acetylene, a nitrification inhibitor. Maximum N 2 O concentration in the field occurred at oxygen concentrations below 35 µM. The difference in the oxygen concentration that results in maximum N 2 O may have arisen because low-oxygen environments present in the estuary were destroyed by stirring in our laboratory experiment. It appears that low oxygen concentrations in estuarine water trigger enhanced N 2 O production if ammonium is present in sufficient amounts. This conclusion is further illustrated by data from the Thames, Loire and Gironde estuaries.

We present a 15-month dataset on nitrification measurements in the Schelde estuary (Belgium and The Netherlands). Nitrification was estimated using the N-serve sensitive dark (14)C-bicarbonate incorporation technique. A peak of nitrification activity was observed in the freshwater part of the estuary. Downstream from this peak, nitrification declined, probably because of ammonium limitation. A range of nitrification inhibitors was tested on both a Nitrosomonas europaea culture and estuarine samples. It was found that methyl fluoride and acetylene stimulated dark (14)C-bicarbonate incorporation and those inhibitors were therefore considered inappropriate nitrification inhibitors in combination with this technique. The effect of the inhibitor N-serve was studied on the dark incorporation of (13)C-bicarbonate into polar lipid derived fatty acids to further identify the dominant chemoautotrophic processes. Inhibition of polar lipid derived fatty acid labelling in the presence of N-serve was complete, suggesting that nitrifying bacteria dominated the chemoautotrophic community.