海洋の窒素循環を解明する新たな研究～アナモックスの酸素同位体分別測定に初めて成功～

2025-06-20 北海道大学,海洋研究開発機構,総合地球環境学研究所,国立環境研究所,京都大学,福島大学

図 1．海洋の窒素循環における窒素（15ε）、酸素（18ε）同位体分別。「？」は最後の欠けているピース（アナモックス反応の 18ε）を示す

＜関連情報＞

• https://www.hokudai.ac.jp/news/2025/06/post-1933.html
• https://www.hokudai.ac.jp/news/pdf/250620_pr2.pdf
• https://academic.oup.com/ismej/advance-article/doi/10.1093/ismejo/wraf115/8154966

海洋性アナモックス細菌 Candidatus Scalindua sp. による嫌気的アンモニア酸化に伴う酸素同位体分別 Oxygen isotope fractionation during anaerobic ammonium oxidation by the marine representative Candidatus Scalindua sp.

Kanae Kobayashi , Kazuya Nishina , Keitaro Fukushima , Yuji Onishi , Akiko Makabe , Mamoru Oshiki , Keisuke Koba , Satoshi Okabe
The ISME Journal  Published:02 June 2025
DOI:https://doi.org/10.1093/ismejo/wraf115

Abstract

Analyzing the nitrogen (¹⁵ε) and oxygen (¹⁸ε) isotope effects of anaerobic ammonium oxidation (anammox) is essential for accurately assessing its potential contribution to fixed-N losses in the ocean, yet the ¹⁸ε of anammox remains unexplored. Here, we determined the previously unexplored ¹⁸ε of anammox using a highly enriched culture of the marine anammox species “Ca. Scalindua sp”. Because Scalindua significantly accelerated oxygen isotope exchange between NO₂^– and H₂O, we introduced a new rate constant for anammox-mediated oxygen isotope exchange (k_{eq, AMX} = 8.44 ~ 13.56 ×10^-2 h^-1), which is substantially faster than abiotic oxygen isotope exchange (k_{eq, abio} = 1.13 ×10^-2 h^-1), into a numerical model to estimate the ¹⁸ε during anammox. Based on our experimental results, we successfully determined the ¹⁸ε associated with: (1) conversion of NO₂^– to N₂ (¹⁸ε_NO2-→N2 = 10.6 ~ 16.1‰), (2) NO₂^– oxidation to NO₃^– (¹⁸ε_NO2-→NO3- = -2.9 ~ -11.0‰, inverse fractionation), (3) incorporation of oxygen from water during NO₂^– oxidation to NO₃^– (¹⁸ε_H2O = 16.4 ~ 19.2‰). Our study underscores the possibility that unique anammox oxygen isotope signals may be masked due to substantial anammox-mediated oxygen isotope exchange between NO₂^– and H₂O. Therefore, careful consideration is required when utilizing δ¹⁸O_NO3- and δ¹⁸O_NO2- as geochemical markers to assess the potential contribution of anammox to fixed-N losses in the ocean.