title: Nitrogen and Oxygen Isotope Effects of Ammonia Oxidation by Thermophilic Thaumarchaeota from a Geothermal Water Stream.
abstract: Ammonia oxidation regulates the balance of reduced and oxidized nitrogen pools in nature. Although ammonia-oxidizing archaea have been recently recognized to often outnumber ammonia-oxidizing bacteria in various environments, the contribution of ammonia-oxidizing archaea is still uncertain due to difficulties in the in situ quantification of ammonia oxidation activity. Nitrogen and oxygen isotope ratios of nitrite (δ(15)NNO2- and δ(18)ONO2-, respectively) are geochemical tracers for evaluating the sources and the in situ rate of nitrite turnover determined from the activities of nitrification and denitrification; however, the isotope ratios of nitrite from archaeal ammonia oxidation have been characterized only for a few marine species. We first report the isotope effects of ammonia oxidation at 70°C by thermophilic Thaumarchaeota populations composed almost entirely of "Candidatus Nitrosocaldus." The nitrogen isotope effect of ammonia oxidation varied with ambient pH (25‰ to 32‰) and strongly suggests the oxidation of ammonia, not ammonium. The δ(18)O value of nitrite produced from ammonia oxidation varied with the δ(18)O value of water in the medium but was lower than the isotopic equilibrium value in water. Because experiments have shown that the half-life of abiotic oxygen isotope exchange between nitrite and water is longer than 33 h at 70°C and pH ≥6.6, the rate of ammonia oxidation by thermophilic Thaumarchaeota could be estimated using δ(18)ONO2- in geothermal environments, where the biological nitrite turnover is likely faster than 33 h. This study extended the range of application of nitrite isotopes as a geochemical clock of the ammonia oxidation activity to high-temperature environments.
  
  
  
  Because ammonia oxidation is generally the rate-limiting step in nitrification that regulates the balance of reduced and oxidized nitrogen pools in nature, it is important to understand the biological and environmental factors underlying the regulation of the rate of ammonia oxidation. The discovery of ammonia-oxidizing archaea (AOA) in marine and terrestrial environments has transformed the concept that ammonia oxidation is operated only by bacterial species, suggesting that AOA play a significant role in the global nitrogen cycle. However, the archaeal contribution to ammonia oxidation in the global biosphere is not yet completely understood. This study successfully identified key factors controlling nitrogen and oxygen isotopic ratios of nitrite produced from thermophilic Thaumarchaeota and elucidated the applicability and its limit of nitrite isotopes as a geochemical clock of ammonia oxidation rate in nature. Oxygen isotope analysis in this study also provided new biochemical information on archaeal ammonia oxidation.
authors.affiliation: Laboratory of Ocean-Earth Life Evolution Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan m_nishizawa@jamstec.go.jp.
authors.forename: Manabu
authors.initials: M
authors.lastname: Nishizawa
authors.affiliation: Department of Subsurface Geobiological Analysis and Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan.
authors.forename: Sanae
authors.initials: S
authors.lastname: Sakai
authors.affiliation: Department of Subsurface Geobiological Analysis and Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan.
authors.forename: Uta
authors.initials: U
authors.lastname: Konno
authors.affiliation: Department of Subsurface Geobiological Analysis and Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan Department of Civil and Environmental Engineering, Nagaoka University of Technology, Nagaoka, Japan.
authors.forename: Nozomi
authors.initials: N
authors.lastname: Nakahara
authors.affiliation: Department of Subsurface Geobiological Analysis and Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan.
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Takaki
authors.affiliation: Department of Subsurface Geobiological Analysis and Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan.
authors.forename: Yumi
authors.initials: Y
authors.lastname: Saito
authors.affiliation: Department of Subsurface Geobiological Analysis and Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Imachi
authors.affiliation: Department of Subsurface Geobiological Analysis and Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan.
authors.forename: Eiji
authors.initials: E
authors.lastname: Tasumi
authors.affiliation: Graduate School of Agriculture, Tokyo University of Agriculture and Technology, Tokyo, Japan.
authors.forename: Akiko
authors.initials: A
authors.lastname: Makabe
authors.affiliation: Graduate School of Agriculture, Tokyo University of Agriculture and Technology, Tokyo, Japan Center for Ecological Research, Kyoto University, Otsu, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Koba
authors.affiliation: Laboratory of Ocean-Earth Life Evolution Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan Department of Subsurface Geobiological Analysis and Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan.
authors.forename: Ken
authors.initials: K
authors.lastname: Takai
mesh_terms: D064751:Ammonium Compounds / Q000378:metabolism*; D001105:Archaea / Q000378:metabolism*; D058440:Denitrification; D045482:Hot Springs / Q000382:microbiology*; D058465:Nitrification; D009573:Nitrites / Q000378:metabolism; D009587:Nitrogen Isotopes / Q000378:metabolism; D010084:Oxidation-Reduction; D010103:Oxygen Isotopes / Q000378:metabolism; D045483:Rivers / Q000737:chemistry / Q000382:microbiology*
chemical_list: D064751:Ammonium Compounds; D009573:Nitrites; D009587:Nitrogen Isotopes; D010103:Oxygen Isotopes