Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
Journal topic
Volume 15, issue 22
Biogeosciences, 15, 6773-6789, 2018
https://doi.org/10.5194/bg-15-6773-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Biogeosciences, 15, 6773-6789, 2018
https://doi.org/10.5194/bg-15-6773-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 Nov 2018

Research article | 14 Nov 2018

Weaving of biomineralization framework in rotaliid foraminifera: implications for paleoceanographic proxies

Yukiko Nagai1,2,3, Katsuyuki Uematsu4, Chong Chen3, Ryoji Wani5, Jarosław Tyszka6, and Takashi Toyofuku3,7 Yukiko Nagai et al.
  • 1Graduate School of Environment and Information Sciences, Yokohama National University, 79-7, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan
  • 2National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba-shi, Ibaraki 305-0005, Japan
  • 3Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Natsushima-cho 2-15, Yokosuka, 237-0061, Japan
  • 4Marine Works Japan Ltd., 3-54-1 Oppama-higashi, Yokosuka 237-0063, Japan
  • 5Faculty of Environment and Information Sciences, Yokohama National University, 79-7, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan
  • 6ING PAN – Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Cracow, Senacka 1, 31-002 Kraków, Poland
  • 7Tokyo University of Marine Science and Technology (TUMSAT), 4-5-7, Konan Minato-ku, Tokyo 108-8477, Japan

Abstract. Elemental and/or isotopic signatures of calcareous tests of foraminifera are commonly used to reconstruct paleoenvironmental conditions. A major problem, often referred to as the vital effect, is that such geochemical signatures stored in inorganic calcium carbonates differ greatly under the same environmental conditions, as well as between taxa, species, individuals, etc. This effect was previously explained by relative contributions between passive vs. active ion transport patterns, but their details are still under investigation. In this study, the functional role of pseudopodial structures during chamber formation is elucidated by detailed observation of Ammonia beccarii (Linnaeus, 1758) using a time-lapse optical imaging system and high-resolution electron microscopy. We document triple organic layers sandwiching carbonate precipitation sites for the first time. The three major organic layers (outer organic layer, primary organic sheet, and inner organic layer) are formed by an initial framework of pseudopodia overlaid with further layer-like pseudopodia. The primary organic sheet seems to facilitate early calcium carbonate nucleation, then entrapped by double precipitation sites. We further show that calcification starts when outer or inner organic layers still exhibit tiny gaps (holes within the framework) that may serve as pathways for passive ion exchange (e.g. Mg2+) between seawater and the confined precipitation space. Nevertheless, the majority of wall thickening occurs when the precipitation site is completely isolated from seawater, which implies active ion exchange. This may explain the differences in MgCa ratios in early and later stages of calcification observed in previous studies. Our study provides insight into resolving a key missing piece in understanding foraminiferal calcification through culture experiments and in-depth observations of living animals. Our findings contribute to interpreting and understanding biogeochemical proxies by showing that the vital effect, specifically elemental and isotopic ratios along chamber walls, is directly linked to spatio-temporal organization of the biomineralization sandwich controlled by the three major organic layers.

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We interpret detailed SEM and time-lapse observations of the calcification process in living foraminifera, which we reveal to be directly linked to the construction mechanism of organic membranes where the calcium carbonate precipitation takes place. We show that these membranes are a highly perforated outline is first woven by skeletal pseudopodia and then later overlaid by a layer of membranous pseudopodia to close the gaps. The chemical composition is related to these structures.
We interpret detailed SEM and time-lapse observations of the calcification process in living...
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