Spatial relation of organelle membranes in the coccolith forming marine alga Emiliania huxleyi
Authors: Ziegler Andreas , Yin Xiaofei , Griesshaber Erika , Miersch Lothar , Reusch Thorsten B. , Walther Paul , Schmahl Wolfgang W. 
Published: December 2016

The marine unicellular alga
Emiliania huxleyi
forms a shell of calcite scales called coccoliths. It can form large blooms and thus is the most important sink for marine CO
2
responsible for 33% of the marine CaCO
3
production [1]. The coccoliths are first formed intracellular within an organelle called the coccolith vesicle and then leave the cell and interlock with one another to form the shell. Calcium ions required for calcite formation need to be transported from the external medium to the coccolith vesicle, raising the question of compartmentation of calcium ions during its way through the cell to the growing coccolith. Current hypotheses suggest influx of calcium through plasma membrane calcium channels, uptake from the cytoplasm in peripheral cisternae of the smooth endoplasmic reticulum, and vesicular transport to Golgi cisternae. Further accumulation of calcium is thought to occur within Golgy cisterns and Golgi vesicles via a V‐type H
+
‐ATPase driven Ca
2+
/H
+
‐exchange mechanism before the vesicles fuse with the coccolith vesicle [2].

In order to contribute to the understanding in organelle interactions during coccolith formation, we used STEM tomography of high pressure frozen and freeze substituted
E. huxley
as well as chemically fixed material. The coccolith vesicle is confluent with membrane bound cisterns of an organelle called the reticular body. STEM tomography reveal seams of peculiar 7‐10 nm thick particles at the inner side of membranes of the reticular body, Golgi cisterns and Golgy vesicles (Figure 1) suggesting that the latter fuse with the reticular body. In early stages the coccolith vesicle is devoid of these particles, however, they can be found in later stages, suggesting that the reticular body contributes to the growth of the coccolith vesicle. The coccolith vesicles is always close to the nuclear envelope [3]. Only in late stages just before the extrusion of the coccolith into the external medium it is apart from the nucleus. In these cases we often find a second coccolith growing within a young coccolith vesicle. Cryofixation followed by freeze substitution revealed that the coccolith vesicle forms a close junction with the outer membrane of the nuclear envelope (Figure 2). Over a distance of 500 nm and more this junction maintains a constant gap of about 4 nm between the two membranes. This finding opens the possibility of another pathway of calcium ions required for coccolith formation. The nuclear envelope is continuous with endoplamic reticulum and is thus part of the large intracellular calcium store having a calcium concentration between 100 and 300 mM [4] and contains calcium release channels required for calcium signalling [5]. Therefore, we propose a model in which calcium ions from the endoplasmic reticulum are released at the site of the junction of the nuclear envelope with the coccolith vesicle causing a large local increase of calcium within the gap between the adjacent membranes, which facilitates quick uptake of calcium into the coccolith vesicle by a Ca
2+
/2H
+
exchange mechanism [6].