AME 40:67-76 (2005)  -  doi:10.3354/ame040067

Denitrification and anaerobic ammonium oxidation in sediments: effects of microphytobenthos and NO3

Nils Risgaard-Petersen1,*, Rikke Louise Meyer2,3, Niels Peter Revsbech2

1National Environmental Research Institute, Department of Marine Ecology, Vejlsøvej 25, 8600 Silkeborg, Denmark
2Department of Biological Sciences, University of Aarhus, bd 540, 8000 Århus C, Denmark
3Present address: University of Queensland, Advanced Wastewater Management Center, Research Road, Building 64A, St. Lucia, Brisbane, Queensland 4072, Australia

ABSTRACT: Microphytobenthos (MPB) regulate nitrogen fluxes across the sediment–water interface in shallow coastal and estuarine environments, where the water-phase concentrations exhibit pronounced variations in time and space. The impact of MPB and N-availability on anaerobic ammonium oxidation (anammox) and denitrification was studied in experimental microcosms using a combination of 15N isotope and microsensor techniques (NO3 + NO2 and O2). The presence of MPB at low water-column NO3 concentrations led to an 85% reduction in the capacity of the sediments for performing the anammox reaction within a 3 wk period, but did not affect the denitrification potential. The presence of MPB also had a significant impact on both O2 and NO3 + NO2 (NOx) concentrations in the sediment. At low NO3 concentrations, NOx was almost depleted in the alga-colonized sediments within 2 wk of incubation, due to assimilation and inhibition of nitrification. The depth distribution of O2 displayed significant variations during the light–dark cycles, leading to periodical O2 exposure of sediment strata that may harbor the anammox process. A sustained high anammox potential in sediments where MPB was allowed to colonize in the presence of 600 µM NO3 in the overlying water indicated that a steady supply of NOx and not protection from O2 exposure was the vital factor for maintenance of anammox capacity. In this case, NOx penetrated approximately 9 mm into the sediment. We therefore suggest that a continuous supply of NOx to the anoxic sediment layers is the key factor for the presence of anammox in marine sediments. On the basis of these results, we suggest that anammox is of very limited significance in environments that periodically experience N-limitation and that occurrence of high rates of anammox in coastal sediments is limited to estuaries with permanently high concentrations of NOx in the water column.

KEY WORDS: Anammox · Benthic microalgae · Biosensor

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