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

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^– (NO_x^–) concentrations in the sediment. At low NO₃^– concentrations, NO_x^– 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 NO_x^– and not protection from O₂ exposure was the vital factor for maintenance of anammox capacity. In this case, NO_x^– penetrated approximately 9 mm into the sediment. We therefore suggest that a continuous supply of NO_x^– 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 NO_x^– in the water column.

KEY WORDS: Anammox · Benthic microalgae · Biosensor