- © The Mineralogical Society Of America
Coccolithophores are a group of unicellular plant plankton, which produce exoskeletons of minute calcite plates, called coccoliths. Despite their small size (1–10 μm across), coccoliths are remarkably elaborate biomineral structures characterized by precise control of both nucleation and growth of calcite by the organic system. They are also of considerable environmental and applied geological importance. Coccolithophores occur in enormous abundances, forming a significant proportion of total marine primary production and carbon fixation, especially in open ocean environments. Their biogeochemical impact is magnified by export of coccoliths to the ocean floor, where coccoliths are the largest single component of deep-sea sediments, forming vast accumulations of calcareous oozes and chalks, including the Late Cretaceous chalks of Northwestern Europe. Moreover, coccoliths are extensively employed by geologists as marker fossils to determine the ages of sediments, especially from drill cores. More recently, satellite imaging of immense coccolithophore blooms has led to extensive multi-disciplinary research into their biology and impact on the marine ecosystem and global carbon cycle (e.g., Westbroek et al. 1993; Paasche 2002; Thierstein and Young in press).
Coccolithophores belong to the algal division (or phylum) Haptophyta. Other haptophytes are non-calcifying and include the well-known genera Phaeocystis, Prymnesium, Pavlova and Chrysochromulina. They are all characterized by possessing golden-brown chloroplasts, two smooth flagella and a third flagellum-like organelle, the haptonema. The haptonema shows distinctive coiling behavior and has a quite different microtubular sub-structure from flagella. Molecular genetic data have shown that the haptophytes are a group discrete from other algal protists that probably originated during the Precambrian (>600 Ma) protist radiation and acquired chloroplasts subsequently (possibly in the Late Palaeozoic, ca. 300–400 Ma) as a result of secondary endosymbiosis (Medlin et al. 1997).
Molecular genetic phylogenies of the haptophytes are still being developed, but all results to date (Edvardsen …