Coupling of bacterial nitrification with denitrification and anammox supports N removal in intertidal sediments (Arcachon Bay, France)

Abstract

Microbial activity measurements and molecular approaches were combined to study temporal variation in nitrogen (N) removal mechanisms from muddy and sandy intertidal sediments (Arcachon Bay, France). Stable isotopes (¹⁵NH₄+/¹⁵NO₃-) were used to measure oxidation of NH₄+ to NO₃-/NO₂- and its subsequent reduction to N₂ via denitrification and/or anammox. We found that denitrification mainly fuelled N loss in both sediment types. However in sandy sediments, anammox accounted for relatively higher N₂ production (45%). Nitrification–denitrification (D_n) and nitrification–anammox coupling was observed particularly in muddy sediments. Temporal variations in prokaryotic abundance (Bacteria, Archaea) and of functional groups responsible for nitrification and denitrification were studied by qPCR and for anammoxifiers by cloning. Bacterial density in both sediment types varied between 10^7-8 equivalent cell per g^-1 dry weight sediment and were overall more abundant in muddy sediments. Archaeal equivalent cell varied between 0.5 and 6.5% of the total prokaryotes, with extreme values observed in muddy sediment during October and January respectively. Denitrifiers were 1–2 orders higher than ammonium oxidizers and archeal nitrifiers appeared insignificant compared to betaproteobacterial counterpart. Furthermore, nitrifiers exhibited greater fluctuations in sandy sediments compared to muddy ones. Anammoxifiers were mainly restricted to the “Candidatus Scalindua” group, and were detected in both sediment types and all sampling periods providing an indirect proof on the occurrence of anammox. In muddy sediments, N₂ production presented overall higher rate over sandy sediments and exhibited the same trend as nitrifier abundance. This finding suggests a central role of bacterially-mediated benthic nitrification in supporting dissimilatory processes through generation of NO₃/NO₂ and preventing eutrophication in this mesotidal lagoon.