Seminar Details

Biomineralization pathways in foraminifera: A cryo correlative approach




Gal Mor-Khalifa - Weizmann Institute of Science


Foraminifera are major contributors to calcium carbonate global production in the ocean. Their calcitic shells are also a major component of the ocean sediment. Understanding biomineralization processes in foraminifera is therefore important for both ecological studies and for the fields of paleo-oceanography and paleoclimate reconstruction from ocean sediment records. The specific precipitation mechanisms forming the foraminifera shell, however, are not yet understood and are the goal of my research. In this work we have used a novel cryo-correlative technique combining SEM, fluorescence and EDS data on the same sample and all under cryo conditions, to study the mineralization pathways in the benthic foraminifer Amphistegina. We confirm the presence of endocytosed sea water containing vesicles using cryo-SEM as reported by Bentov et al. (2009). Our correlative SEM-fluorescence-EDS observation on the foraminifera cytoplasm reveals that the major sea water reservoir in the cytoplasm is found in small (2-3µm) uniformly sized vesicles. These vesicles presumably contain an aqueous solution of sea water origin which undergoes some chemical modification post endocytosis. The role of these vesicles in the biomineralization process is yet unclear. We further characterized intracellular biominerals found in the foraminifer cytoplasm. We found each particle to reside within a sea water vacuole and to have high Mg low Ca content presumably in a mineral phase. These Mg biominerals may possibly be the product of a mechanism which removes Mg from the endocytosed sea water in order to promote calcite precipitation from the remaining solution. We also aimed at characterizing a previous hypothesis, namely that there are intracellular ACC reservoirs in the foraminifera cytoplasm. In our observation on both the juvenile and the adult life stage of A. lobifera and A. lessonii, we have found no evidence for intracellular Ca minerals or organelles with high Ca content that could account for the Ca pools expected in the foraminifer cytoplasm from earlier studies. We cannot rule out the possibility that ACC takes part as a transient mineral phase in the shell formation mechanism, but it does not constitute large reservoirs of Ca in the cytoplasm. This is the first cryo SEM characterization of the foraminifer cellular environment. The combination of the high resolution SEM data with fluorescence and EDS compositional analysis allows us to track specifically the distribution of incorporated calcium and endocytosed sea water in the foraminifer cytoplasm. Our results shed new light on the use of Ca reservoirs in the Amphistegina genus and on the role of endocytosed sea water in the mineralization process.

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