Abstract
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 Mg ∕ Ca 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.
Highlights
The calcification process of the foraminiferal test is the phase of growth in which elemental and isotopic compositions of the test is determined and is the key to generating their morphological diversity
An accurate overview and model of the chamber formation by pseudopodia and the calcification process in calcareous foraminifera is urgently needed to better our understanding of paleoceanographic proxies. To fill this knowledge gap, this study aims to elucidate the role of pseudopodial activities on the formation process of the foraminiferal chamber and its organic structures within the calcareous wall using the benthic foraminifera Ammonia beccarii, which has been used in a few relevant previous studies (e.g. Toyofuku et al, 2017), as a model system
A major problem was that such compositions differed greatly from inorganic calcium carbonate under the same environment
Summary
The calcification process of the foraminiferal test is the phase of growth in which elemental and isotopic compositions of the test is determined and is the key to generating their morphological diversity. Y. Nagai et al.: Biomineralization framework in foraminifera come one of the major tools in palaeoenvironmental reconstructions, even though the test morphology and chemical composition depend to a certain extent on the environment (De Nooijer et al, 2014; Schiebel et al, 2017). Nagai et al.: Biomineralization framework in foraminifera come one of the major tools in palaeoenvironmental reconstructions, even though the test morphology and chemical composition depend to a certain extent on the environment (De Nooijer et al, 2014; Schiebel et al, 2017) To this end, elucidating the detailed mechanisms of foraminiferal calcification has been treated with great interest in the field of geosciences. The biological processes of chamber formation is of great importance and interest
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