title: Intracellular Isotope Localization in Ammonia sp. (Foraminifera) of Oxygen-Depleted Environments: Results of Nitrate and Sulfate Labeling Experiments.
abstract: Some benthic foraminiferal species are reportedly capable of nitrate storage and denitrification, however, little is known about nitrate incorporation and subsequent utilization of nitrate within their cell. In this study, we investigated where and how much (15)N or (34)S were assimilated into foraminiferal cells or possible endobionts after incubation with isotopically labeled nitrate and sulfate in dysoxic or anoxic conditions. After 2 weeks of incubation, foraminiferal specimens were fixed and prepared for Transmission Electron Microscopy (TEM) and correlative nanometer-scale secondary ion mass spectrometry (NanoSIMS) analyses. TEM observations revealed that there were characteristic ultrastructural features typically near the cell periphery in the youngest two or three chambers of the foraminifera exposed to anoxic conditions. These structures, which are electron dense and ~200-500 nm in diameter and co-occurred with possible endobionts, were labeled with (15)N originated from (15)N-labeled nitrate under anoxia and were labeled with both (15)N and (34)S under dysoxia. The labeling with (15)N was more apparent in specimens from the dysoxic incubation, suggesting higher foraminiferal activity or increased availability of the label during exposure to oxygen depletion than to anoxia. Our results suggest that the electron dense bodies in Ammonia sp. play a significant role in nitrate incorporation and/or subsequent nitrogen assimilation during exposure to dysoxic to anoxic conditions.
authors.affiliation: Department of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology Yokosuka, Japan.
authors.forename: Hidetaka
authors.initials: H
authors.lastname: Nomaki
authors.affiliation: Geology and Geophysics Department, Woods Hole Oceanographic Institution Woods Hole, MA, USA.
authors.forename: Joan M
authors.initials: JM
authors.lastname: Bernhard
authors.affiliation: Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo Kashiwa, Japan.
authors.forename: Akizumi
authors.initials: A
authors.lastname: Ishida
authors.affiliation: Department of Marine Biodiversity, Japan Agency for Marine-Earth Science and Technology Yokosuka, Japan.
authors.forename: Masashi
authors.initials: M
authors.lastname: Tsuchiya
authors.affiliation: Marine Works Yokosuka, Japan.
authors.forename: Katsuyuki
authors.initials: K
authors.lastname: Uematsu
authors.affiliation: Marine Works Yokosuka, Japan.
authors.forename: Akihiro
authors.initials: A
authors.lastname: Tame
authors.affiliation: Project Team for Research and Development of Next-generation Technology for Ocean Resources Exploration, Japan Agency for Marine-Earth Science and Technology Yokosuka, Japan.
authors.forename: Tomo
authors.initials: T
authors.lastname: Kitahashi
authors.affiliation: Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo Kashiwa, Japan.
authors.forename: Naoto
authors.initials: N
authors.lastname: Takahata
authors.affiliation: Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo Kashiwa, Japan.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Sano
authors.affiliation: Department of Marine Biodiversity, Japan Agency for Marine-Earth Science and Technology Yokosuka, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Toyofuku
keywords: NanoSIMS; denitrification; electron dense body; endobionts; foraminifer; nitrate; ultrastructure