HR: 0800h
AN: B21B-0864    [Abstracts]
TI: Direct Microelectrodes Measurements at the Calcification Site of Foraminifera
AU: grinstein, m
EM: morgrin@pob.huji.ac.il
AF: The Institute of Earth Sciences, the Hebrew university, Givat Ram, Jerusalem, 91904 Israel
AU: Bentov, S
EM: bentov@vms.huji.ac.il
AF: The Institute of Earth Sciences, the Hebrew university, Givat Ram, Jerusalem, 91904 Israel
AU: Koller-Rink, S
EM: srink@mpi-bremen.de
AF: Max Planck Institute for Marine Microbiology, Celsiusstraáe 1, Bremen, D-28359 Germany
AU: De-Beer, D
EM: dbeer@mpi-bremen.de
AF: Max Planck Institute for Marine Microbiology, Celsiusstraáe 1, Bremen, D-28359 Germany
AU: * Erez, J
EM: erez@vms.huji.ac.il
AF: The Institute of Earth Sciences, the Hebrew university, Givat Ram, Jerusalem, 91904 Israel
AB: In addition to their importance in the global carbon cycle and for paleoceanograohic studies, foraminifera are excellent model organisms for studying biomineralization at the cellular level. We have shown previously that vacuolated seawater, which is modified intacellularly to pH of $\sim$9, is the solution from which foraminifera precipitate their CaCO$_{3}$ (Bentov and Erez. 2003). In the present study we measured the pH and [Ca$^{+2}$] at the extra-cellular microenviront adjacent to the outer membrane of calcifying foraminiferal preparations using microelectrodes. These preparations are amoeboids and recovering specimens after decalcification that are produced from the foraminifer {\it Amphistegina lobifera }. We observed clear spatial polarization of pH between the apical and the basal margins of these organisms. High pH (9.2-9.4) was measured on the basal side, between the organism and the glass substrate where calcification occurs. On the apical side (facing the bulk seawater) a lower pH (8.3-8.6) was measured. The pH on the apical side is light dependent and may be associated with photosynthesis of the symbiotic algae. The [Ca$^{+2}$] measurements showed a similar pattern of polarization on the basal and apical sides. The basal [Ca$^{+2}$] near the calcifying site, varied cyclically between 11.9mM and 11.7 with a ca.30 min period. At the apical side the [Ca$^{+2}$] is 11.7mM slightly lower than ambient seawater of Gulf of Eilat (11.8 mM), and doesn't show cyclic variation. These observations are in good agreement with our model of calcification in foraminifera based on seawater vacuolization (Bentov and Erez. 2003). We suggest that calcification involves the following sequential steps: Seawater is vacuolated in large vacuoles which are cycled in the cell. At this stage the seawater in the vacuoles is being modified to increase its pH (up to $\sim$9) and possibly increase their [Ca$^{+2}$] as well. These vacuoles are exocytosed into the calcification site where high pH and [Ca$^{+2}$] create supersaturation for calcite. CaCO$_{3}$ precipitation proceeds with strict biological control on the shape, size and crystal orientation, all probably mediated by the organic matrix. During this process the pH and [Ca$^{+2}$] drop and these residual depleted seawater are exocytosed at the periphery of the organism where lower [Ca$^{+2}$] is observed. This mechanism may cause the cyclic alternation between Ca-enriched vacuoles coming to the site of calcification and Ca-depleted seawater as a result of CaCO$_{3}$ precipitation. We believe that the amoeboids and the recovering individuals represent faithfully the process of calcification in foraminifera. However, the actual values of the Ca and pH dynamics in intact organisms may be different (probably with higher amplitude), because our preparations may behave as an open system relative to the intact foraminifera.
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting