| Summary: | ACTIVITY, MICROENVIRONMENTS, AND COMMUNITY STRUCTURE OF AEROBIC AND ANAEROBIC AMMONIUM OXIDIZING PROKARYOTES IN ESTUARINE SEDIMENT (RANDERS FJORD, DK) A. Schramm 1 , N.P. Revsbech 1 , T. Dalsgaard 2 , E. Piña-Ochoa 3 , J. de la Torré 4 , D.A. Stahl 4 , N. Risgaard-Petersen 2 1 Department of Biological Sciences, Microbiology, University of Aarhus, Aarhus, Denmark; 2 National Environmental Research Institute, Silkeborg, Denmark; 3 Centro De Ciencias Medioambientales (CSIC), Madrid; 4 Department of Civil and Environmental Engineering, University of Washington, Seattle, WA, USA Anammox, the anaerobic conversion of ammonium with nitrite to N 2 , is increasingly recognized as link in the aquatic nitrogen cycle. However, factors regulating the occurrence and activity of anammox bacteria are still poorly understood. Besides the influence of abiotic factors, anammox might be controlled by either aerobic ammonia oxidizing bacteria and archaea (AOB and AOA) or nitrate-reducing/denitrifying bacteria via their supply of nitrite. Along the Randers Fjord estuary (Denmark), gradients of salinity, nutrients, and organic loading can be observed, and anammox has been detected previously at some sites. The aim of this study was to correlate the activity, abundance, and diversity of anammox bacteria, AOB and AOA in surface sediment along the fjord with the sediment microdistribution of nitrite. Using 15 N incubations, 16S rRNA gene sequencing, and fluorescence in situ hybridization, anammox was only detected at the two innermost stations where it accounted for 5-16% of the total N 2 production and 0.1-0.25% of the total prokaryotic population. These numbers result in cell-specific anammox rates of about 6 fmol N day-1, comparable to literature values. Anammox cells occurred mainly in clusters and were related to the candidate genus Scalindua. Based on ammonia monoxygenase (amoA) gene sequencing, AOB were affiliated with uncultured AOB commonly found in other estuarine sediments; in addition, AOA related to the recently described ...
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