Anammox bacteria disguised as denitrifiers: nitrate reduction to dinitrogen gas via nitrite and ammonium
2007
Kartal, Boran | Kuypers, Marcel M.M. | Lavik, Gaute | Schalk, Jos | Op den Camp, Huub J.M. | Jetten, Mike S.M. | Strous, Marc
Anaerobic ammonium-oxidizing (anammox) bacteria oxidize ammonium with nitrite and produce N₂. They reside in many natural ecosystems and contribute significantly to the cycling of marine nitrogen. Anammox bacteria generally live under ammonium limitation, and it was assumed that in nature anammox bacteria depend on other biochemical processes for ammonium. In this study we investigated the possibility of dissimilatory nitrate reduction to ammonium by anammox bacteria. Physically purified Kuenenia stuttgartiensis cells reduced ¹⁵NO₃⁻ to ¹⁵NH₄⁺ via ¹⁵NO₂⁻ as the intermediate. This was followed by the anaerobic oxidation of the produced ammonium and nitrite. The overall end-product of this metabolism of anammox bacteria was ¹⁵N¹⁵N dinitrogen gas. The nitrate reduction to nitrite proceeds at a rate of 0.3 ± 0.02 fmol cell⁻¹ day⁻¹ (10% of the 'normal' anammox rate). A calcium-dependent cytochrome c protein with a high (305 μmol min⁻¹ mg protein⁻¹) rate of nitrite reduction to ammonium was partially purified. We present evidence that dissimilatory nitrate reduction to ammonium occurs in Benguela upwelling system at the same site where anammox bacteria were previously detected. This indicates that anammox bacteria could be mediating dissimilatory nitrate reduction to ammonium in natural ecosystems.
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