Evidence for iron-dependent anaerobic ammonium oxidation to nitrate (FEAMMOX) in deep-sea sediments

Abstract

The coupling of iron and manganese with the biogeochemical cycling of nitrogen is an emerging topic in geomicrobiology. Iron oxidation coupled to nitrate reduction is common in a variety of sediments and several organisms have been isolated. The anaerobic reduction of manganese oxides coupled to the oxidation of ammonium to nitrate has been observed in marine sediments and appears sensitive to the reactivity of the manganese oxides. A comparable process proposed to couple iron reduction to the anaerobic production of nitrate has mostly been observed in riparian soils and anaerobic waste reactors. In this study, geochemical profiles of the main redox species involved in carbon transformation in marine sediments from the Congo Lobe demonstrated a peak in nitrate and/or nitrite approximately 1 to 2 cm below the maximum oxygen penetration depth at different stations along the lobes. Batch reactor incubations with the same sediment amended with increasing concentrations of ammonium revealed that nitrate can be produced anaerobically and that its formation is coupled to the reduction of iron oxides. These incubations also demonstrated that the formation of nitrate in anaerobic sediments promotes its removal in a process that is tentatively coupled to the oxidation of reduced iron. A one dimensional reactive transport model that includes all the main diagenetic reactions could not reproduce the formation of the subsurface peak in nitrate unless the anaerobic oxidation of ammonium coupled to iron reduction was added to the set of reactions. These findings indicate that nitrite or nitrate are not only a product of aerobic nitrification in marine sediments and suggest that the importance of denitrification or anaerobic ammonium oxidation to nitrogen gas (anammox) in marine sediments may be controlled by the presence of reactive iron oxides.