Cell Biology, Biochemistry and Genomics of Coccolithophore Biomineralization
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Cell Biology, Biochemistry and Genomics of Coccolithophore Biomineralization. Elma L. González1 and Betsy A. Read2 1 Dept. Ecology and Evolutionary Biol., Univ. California, Los Angeles, 90095-1606; U.S.A. 2 Dept. of Biology, California State University, San Marcos, 91093-0016, U.S.A. ABSTRACT The coccoliths that form the outer cell covering of the unicellular coccolithophores are assembled in a coccolith vesicle. The coccolith vesicle maintains the conditions and protein carriers sufficient to import the raw materials (Ca2+ and CO32-) for calcite formation and, furthermore, to initiate and sustain mineralization. We have isolated the coccolith vesicle membrane on which a proton-pumping, vacuolar ATPase has been localized. The V-ATPase has been characterized and its membrane-spanning subunit c has been cloned and sequenced. We used the highly conserved sequence for designing and constructing probes and primers that allowed us to analyze expression patterns to show a direct relationship between expression of the gene and calcification. Our collaborative work has now turned to a genomics approach that will lead us to additional calcification-specific genes.
INTRODUCTION The coccolithophores are members of an ancient group of unicellular, photosynthetic organisms with a global distribution in the world’s oceans. Between 300 to 500 species in the class Haptophyceae are known. The coccolithophores are characterized by their ability to form coccosphere composed of overlapping coccoliths (Fig. 1). Coccoliths have an organic baseplate composed of polysaccharide fibers assembled in radial and spiral configuration and a mineral component consisting of calcite laid onto the baseplate [1-2]. The entire coccolith is fabricated within the Golgi apparatus, first in the cis- and mid-cisternae, where the organic polymers are synthesized and assembled, and later in the coccolith vesicle (cv), derived from the trans-Golgi, where mineralization occurs [2].
Figure 1. Scanning electron micrograph of Emiliania huxleyi.
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Among all mineralizing taxa the coccolithophores stand out for having uniquely co-opted the cell’s secretory membrane system to accomplish a highly regulated production of coccoliths. The regulatory elements responsible for the mineralization process are reliable to such a degree that much of the taxonomy of the coccolithophores is reliant upon the morphological stability of their coccoliths underscoring the genetic control of coccolithogenesis. Considering that fossilized coccoliths have been examined in sediments dating from the Jurassic period and provided useful tools for stratigraphic analyses because of the gradual evolutionary modification of their morphologies, the genetic modifications that permitted controlled mineralization have been around a long time. Ancient coccoliths make up the massive chalk cliffs seen around the world. The coccolithophores exhibit surprising ecological diversity among their members. Although the bloom-forming coccolithophore species such as Emiliania huxleyi and Gephyra
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