Biogeochemical controls and isotopic signatures of nitrous oxide production by a marine ammonia-oxidizing bacterium
Name
Frame-2010-Biogeochemical controls and isotopic signatures of nitrous oxide.pdf
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Author(s)
Frame, Caitlin H.
Casciotti, K. L.
Date Issued
September 2010
Journal
Biogeosciences
Publisher
European Geosciences Union / Copernicus
Citation
Frame, C. H., and K. L. Casciotti. “Biogeochemical Controls and Isotopic Signatures of Nitrous Oxide Production by a Marine Ammonia-oxidizing Bacterium.” Biogeosciences 7.9 (2010) : 2695-2709. © Author(s) 2010
Version
Final published version
Abstract
Nitrous oxide (N2O)[N subscript 2 O] is a trace gas that contributes to the greenhouse effect and stratospheric ozone depletion. The N2O [N subscript 2 O] yield from nitrification (moles N2O-N [N subscript 2 O - N] produced per mole ammonium-N consumed) has been used to estimate marine N2O [N subscript 2 O] production rates from measured nitrification rates and global estimates of oceanic export production. However, the N2O [N subscript 2 O] yield from nitrification is not constant. Previous culture-based measurements indicate that N2O [N subscript 2 O] yield increases as oxygen (O2) [O subscript 2] concentration decreases and as nitrite (NO2−) [NO subscript 2 overscore] concentration increases. Here, we have measured yields of N2O [N subscript 2 O] from cultures of the marine β-proteobacterium [beta-proteobacterium] Nitrosomonas marina C-113a as they grew on low-ammonium (50 μM)[50 mu M] media. These yields, which were typically between 4 × 10−4 [10 superscript -4] and 7 × 10−4 [10 superscript -4] for cultures with cell densities between 2 × 102 [10 super script 2] and 2.1 × 104 [10 superscript 4] cells ml−1 [ml superscript -1], were lower than previous reports for ammonia-oxidizing bacteria. The observed impact of O2 [O subscript 2] concentration on yield was also smaller than previously reported under all conditions except at high starting cell densities (1.5 × 106 cells ml−1) [1.5 x 10 superscript 6 cells ml superscript -1], where 160-fold higher yields were observed at 0.5% O2 [O subscript 2](5.1 μM [mu M] dissolved O2 [O subscript 2]) compared with 20% O2 [O subscript 2] (203 μM [mu M] dissolved O2 O subscript 2]). At lower cell densities (2 × 102 [10 superscript 2] and 2.1 × 104 [10 superscript 4] cells ml−1 [ml superscript -1]), cultures grown under 0.5% O2 [O subscript 2] had yields that were only 1.25- to 1.73-fold higher than cultures grown under 20% O2 [O subscript 2]. Thus, previously reported many-fold increases in N2O [N subscript 2 O] yield with dropping O2 [O subscript 2] could be reproduced only at cell densities that far exceeded those of ammonia oxidizers in the ocean. The presence of excess NO2− [NO subscript 2 overscore] (up to 1 mM) in the growth medium also increased N2O [N subscript 2 O] yields by an average of 70% to 87% depending on O2 [O subscript 2] concentration. We made stable isotopic measurements on N2O [N subscript 2 O] from these cultures to identify the biochemical mechanisms behind variations in N2O [N subscript 2 O] yield. Based on measurements of δ15Nbulk [delta superscript 15 N superscript bulk], site preference (SP = δ15Nα−δ15Nβ [delta superscript 15 N superscript alpha - delta superscript 15 N superscript beta]), and δ18O [delta superscript 18 O] of N2O [N subscript 2 O] (δ18O-N2O [delta superscript 18 O - N subscript 2 O]), we estimate that nitrifier-denitrification produced between 11% and 26% of N2O [N subscript 2 O] from cultures grown under 20% O2 [O subscript 2] and 43% to 87% under 0.5% O2 [O subscript 2]. We also demonstrate that a positive correlation between SP and δ18O-N2O [delta superscript 18 O - N subscript 2 O] is expected when nitrifying bacteria produce N2O [N subscript 2 O]. A positive relationship between SP and δ18O-N2O [delta superscript 18 O - N subscript 2 O] has been observed in environmental N2O [N subscript 2 O] datasets, but until now, explanations for the observation invoked only denitrification. Such interpretations may overestimate the role of heterotrophic denitrification and underestimate the role of ammonia oxidation in environmental N2O [N subscript 2 O] production.
MIT Department
Joint Program in Chemical Oceanography
Massachusetts Institute of Technology. Sea Grant College Program
Woods Hole Oceanographic Institution
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Creative Commons Attribution 3.0
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DOI of Published Version
https://doi.org/10.5194/bg-7-2695-2010
