Factors controlling anaerobic ammonium oxidation with nitrite in marine sediments

Abstract

Factors controlling the anaerobic oxidation of ammonium with nitrate and nitrite were explored in a marine sediment from the Skagerrak in the Baltic-North Sea transition. In anoxic incubations with the addition of nitrite, approximately 65% of the nitrogen gas formation was due to anaerobic ammonium oxidation with nitrite, with the remainder being produced by denitrification. Anaerobic ammonium oxidation with nitrite exhibited a biological temperature response, with a rate optimum at 15 degrees C and a maximum temperature of 37 degrees C. The biological nature of the process and a 1:1 stoichiometry for the reaction between nitrite and ammonium indicated that the transformations might be attributed to the anammox process. Attempts to find other anaerobic ammonium-oxidizing processes in this sediment failed. The apparent K(m) of nitrite consumption was less than 3 microM, and the relative importance of ammonium oxidation with nitrite and denitrification for the production of nitrogen gas was independent of nitrite concentration. Thus, the quantitative importance of ammonium oxidation with nitrite in the jar incubations at elevated nitrite concentrations probably represents the in situ situation. With the addition of nitrate, the production of nitrite from nitrate was four times faster than its consumption and therefore did not limit the rate of ammonium oxidation. Accordingly, the rate of this process was the same whether nitrate or nitrite was added as electron acceptor. The addition of organic matter did not stimulate denitrification, possibly because it was outcompeted by manganese reduction or because transport limitation was removed due to homogenization of the sediment.

FIG. 1.

Concentrations of ²⁹N₂, ³⁰N₂, NO₃⁻, NO₂⁻, and NH₄⁺ as functions of time in anoxic sediment incubations with addition of ¹⁵NO₃⁻ plus ¹⁴NH₄⁺ (A and B), ¹⁵NO₂⁻ plus ¹⁴NH₄⁺ (C and D), ¹⁵NH₄⁺ plus ¹⁴NO₃⁻ (E and F), or ¹⁵NH₄⁺ (G and H). In panels A and B, ¹⁵NO₃⁻ accounted for 98% of the NO₃⁻ pool; in panels C and D, ¹⁵NO₂⁻ accounted for 98% of the NO₂⁻ pool; in panels E and F, ¹⁵NH₄⁺ initially accounted for 53% of the NH₄⁺ pool, decreasing to 16% at the end of the experiment; and in panels G and H, the ¹⁵N content of the NH₄⁺ pool decreased from 77 to 39% between the start and the end of the experiment. Error bars indicate standard errors.

FIG. 2.

Concentrations of NO₂⁻ as functions of time in two experiments with either NO₃⁻ added (A) or NO₂⁻ added (B). The solid lines indicate the theoretical decrease in NO₂⁻ concentration assuming Michaelis-Menten kinetics and a K_m value of 0.1 μM, a maximum NO₂⁻ reduction rate calculated as the slope of the linear portion of the curve, and an initial concentration calculated as the intersection of this linear regression with the y axis. The other lines depict the decrease in NO₂⁻ concentration assuming higher K_m values. Error bars indicate standard errors.

FIG. 3.

Rates of dinitrogen production by anaerobic oxidation of NH₄⁺ with NO₂⁻ and by denitrification as a function of temperature (A) and the production of dinitrogen by the oxidation of NH₄⁺ with NO₂⁻ relative to the total dinitrogen production in the sediment (B). Error bars indicate standard errors.