Biphasic behavior of anammox regulated by nitrite and nitrate in an estuarine sediment

Abstract

The production of N2 gas via anammox was investigated in sediment slurries at in situ NO2- concentrations in the presence and absence of NO3-. With single enrichment above 10 microM 14NO2- or 14NO3- and 15NH4+, anammox activity was always linear (P < 0.05), in agreement with previous findings. In contrast, anammox exhibited a range of activity below 10 microM NO2- or NO3-, including an elevated response at lower concentrations. With 100 microM NO3-, no significant transient accumulation of NO2- could be measured, and the starting concentration of NO2- could therefore be regulated. With dual enrichment (1 to 20 microM NO2- plus 100 microM NO3-), there was a pronounced nonlinear response in anammox activity. Maximal activity occurred between 2 and 5 microM NO2-, but the amplitude of this peak varied across the study (November 2003 to June 2004). Anammox accounted for as much as 82% of the NO2- added at 1 microM in November 2003 but only for 15% in May 2004 and for 26 and 5% of the NO2- added at 5 microM for these two months, respectively. Decreasing the concentration of NO3- but holding NO2- at 5 microM decreased the significance of anammox as a sink for NO2-. The behavior of anammox was explored by use of a simple anammox-denitrification model, and the concept of a biphasic system for anammox in estuarine sediments is proposed. Overall, anammox is likely to be regulated by the availability of NO3- and NO2- and the relative size or activity of the anammox population.

FIG. 1.

(a and c) Production of ²⁹N₂ from labeled ¹⁵NH₄⁺ oxidation in the direct presence of ¹⁴NO₂⁻ or ¹⁴NO₃⁻ in November and December 2003. (b and d) Total anammox N₂ production (calculated from ²⁹N₂ production) as a percentage of the respective NO₂⁻ or NO₃⁻ spike. (e and f) Composite of the above NO₂⁻ data plus additional NO₂⁻ data for October 2003. Values are means ± 1 standard error of the means (SEM) (n = 4).

FIG. 2.

(a) Reduction of NO₃⁻ over time in anaerobic sediment slurries at four NO₃⁻ enrichment concentrations. (b) Subsequent accumulation and reduction of NO₂⁻ over time for each respective NO₃⁻ enrichment. Values are means ± 1 SEM (n = 4). The same symbols are used in each panel.

FIG. 3.

(a) Total anammox (AAO) N₂ production in the direct presence of 1 to 10 μM ¹⁴NO₂⁻, 100 μM ¹⁴NO₃⁻, and 500 μM ¹⁵NH₄⁺. (b) AAO as a percentage of each NO₂⁻ spike. (c and d) Representative ranges of patterns for AAO with NO₂⁻ from 1 to 20 μM and 100 μM ¹⁴NO₃⁻. (e and f) AAO with 5 μM ¹⁴NO₂⁻ and decreasing concentrations of ¹⁴NO₃⁻. Note that a measurement with 0 μM ¹⁴NO₃⁻ was not determined in November 2003. Values are means ± 1 SEM (n = 4). The same symbols are used for each respective left and right panel.

FIG. 4.

Model runs for anammox, with θ varying between 0.003 and 0.045 in each case. (a) Anammox as non-Michaelis f_p (equation 7); (c) anammox as Michaelis f_m (equation 8); (e) anammox as a function of the combined components f_p + f_m (equation 6). (b, d, and f) NO₂⁻ accounted for by anammox for each respective scenario (a, c, and e).