HR: 1330h
AN: B12C-0788    [PDF]
TI: New Observations on Seawater Vacuoles, pH Distribution, and Their Role in the Biomineralization Process in Foraminifera
AU: * Bentov, S
EM: bentov@vms.huji.ac.il
AF: Institute of Earth Sciences The Hebrew University, givat ram, Jerusalem, 91904 Israel
AU: Erez, J
EM: erez@vms.huji.ac.il
AF: Institute of Earth Sciences The Hebrew University, givat ram, Jerusalem, 91904 Israel
AB: We suggest that biomineralization in radial foraminifera is based mainly on seawater vacuolation. During calcification the benthic foraminifer Amphistegina lobifera showed large, static, intra-shell seawater vacuoles, with residence time of an hour in their last chamber. Mobile, intra-shell, smaller vacuoles were initially observed in the periphery of the last chamber and later within the endoplasm of internal chambers. The residence time of water in the mobile vacuoles was up to 12 hr in calcifying individuals and more then 2 days in non-calcifying ones. Experiments with fluorescent dyes showed that labeled seawater entered the large vacuoles first and later appeared in the smaller mobile ones. Fluorescence pH imaging showed that the pH in these vacuoles increased with time by about 0.5 pH unit relative to seawater reaching 8.7. Prior to secondary calcification, the organism generated a thin pseudopodial sheet, which delineates the biomineralization zone. It contained many extra-shell seawater vacuoles with short residence time of about 10 min that were highly abundant during the calcification process. The pH in these vacuoles was 8.5 to 8.7. In the vicinity of the growing crystals, we observed intensive cytoplasmic streams, which are loaded with dark granules that are also alkaline (8.2-8.5), well above the cytosol pH (7.2-7.5). Use of specific probe, showed that about 50% of these alkaline granules are mitochondria while the other ones were vesicles containing fibrillar material (based on TEM). Both were more abundant in the pseudopodial sheet then in the endoplasm. We propose that during secondary calcification, the organism separates itself from the environment using the pseudopodial sheet. The alkaline seawater vacuoles (of both intra and extra-shell origin) are transferred via the pseudopodial sheet and are exocytosed into the biomineralization space between the inner membrane of the sheet and the existing skeleton. Inorganic carbon is concentrated in the vacuoles relative to seawater by diffusion of CO$_{2}$ from the mitochondria and from the cytosol into the alkaline vacuoles. Therefore the seawater vacuoles provide both Calcium and carbonate to the biomineralization site.
DE: 0400 Biogeosciences
DE: 1050 Marine geochemistry (4835, 4850)
DE: 3030 Micropaleontology
DE: 3099 General or miscellaneous
DE: 4267 Paleoceanography
SC: Biogeosciences [B]
MN: 2003 Fall Meeting