抄録
Coccolithophorids are photosynthetic unicellular calcifying algae that form calcium carbonate crystals in the cell. The crystal with a fine structure, named coccolith, is synthesized in the coccolith vesicle that was derived from Golgi body, then transported onto the cell surface. The fundamental feature of coccolith growth, termed as the V/ R model, involves the assembly of a ring of single crystals with altering orientations, radial (R) and vertical (V). A living cell of a cosmopolitan species of coccolithophorid, Emilania huxleyi, seemed anomalous because only R crystals had been observed. However, relict V crystals are found in a proto-coccolith ring and the crystals were overgrown by preferential development of R units in a complete coccolith. Such composition and structure of coccolith crystals have varied during biological evolution. The calcification is mediated by organic base plate composed of acid polysaccharides. Active species of dissolved inorganic carbon utilized for intracellular calcification is a bicarbonate ion, whereas that for photosynthetic fixation is CO_2 Calcification is known as a CO_2 producing process when it proceeds chemically. However, the evolved CO_2 can immediately absorbed by photosynthesis when the activity is higher than that of intracellular calcification. Regulation of CO_2 fixation and growth of coccolithophorids is therefore very important for the balance of CO_2 between the ocean and the atmosphere . In this article the role of coccolithophorids in the global carbon cycle is also discussed.
