Coccolithophores are a group of calcifying unicellular algae that constitute a major fraction of oceanic primary productivity, play an important role in the global carbon cycle, and are key biostratigraphic marker fossils. Their taxonomy is primarily based on the morphology of the minute calcite plates, or coccoliths, covering the cell. These are diverse and include widespread fine scale variation, of which the biological͞taxonomic significance is unknown. Do they represent phenotypic plasticity, genetic polymorphisms, or species-specific characters? Our research on five commonly occurring coccolithophores supports the hypothesis that such variation represents pseudocryptic speciation events, occurring between 0.3 and 12.9 million years ago from a molecular clock estimation. This finding suggests strong stabilizing selection acting on coccolithophorid phenotypes. Our results also provide strong support for the use of fine scale morphological characters of coccoliths in the fossil record to improve biostratigraphic resolution and paleoceanographic data retrieval.

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Marine unicellular coccolithophore algae produce species-specific calcite scales otherwise known as coccoliths. While the coccoliths and their elaborate architecture have attracted the attention of investigators from various scientific disciplines, our knowledge of the underpinnings of the process of biomineralization in this alga is still in its infancy. The processes of calcification and coccolithogenesis are highly regulated and likely to be complex, requiring coordinated expression of many genes and pathways. In this study, we have employed cDNA microarrays to investigate changes in gene expression associated with biomineralization in the most abundant coccolithophorid, Emiliania huxleyi. Expression profiling of cultures grown under calcifying and noncalcifying conditions has been carried out using cDNA microarrays corresponding to approximately 2,300 expressed sequence tags. A total of 127 significantly up-or down-regulated transcripts were identified using a P value of 0.01 and a change of >2.0-fold. Real-time reverse transcriptase PCR was used to test the overall validity of the microarray data, as well as the relevance of many of the proteins predicted to be associated with biomineralization, including a novel gamma-class carbonic anhydrase ( Coccolithophores are a diverse group of calcifying unicellular marine algae, which includes the extremely abundant species Emiliania huxleyi. This group of phytoplankton has captured the attention of scientists from diverse disciplines, particularly because of the important roles it plays in the cycling of carbon and sulfur in the marine environment. The fixation of CO 2 via photosynthesis and the production of CO 2 in the calcification process affect global carbon cycling and thus link coccolithophores with climate and the chemical balance between atmosphere, hydrosphere, and geosphere (20). Furthermore, the striking morphological diversity of the more than 200 species of extant coccolithophores (13) and the seemingly endless range of fossil coccolith types (40) provide a unique resource with which to study biodiversity and speciation in the marine environment (23, 24). The biomineralized skeletons or "coccoliths" (Fig. 1A) produced by these microscopic organisms have also captured the attention of materials scientists interested in the microscale and nanoscale fabrication of three-dimensional biomineralized structures for novel applications related to telecommunications, optoelectronics, and medicine (31, 37).Although hypotheses continue to be debated in the literature, the function of the calcite skeletons and the role of biomineralization in coccolithophores remain unresolved. Knowledge of the mechanisms of calcification in coccolithophores is equally poor. Coccolithogenesis takes place within the coccolith vesicle, a specialized apparatus derived from the Golgi body, in a highly regulated and reproducible manner. The molecules involved in the acquisition and transport of calcium ions and inorganic carbon within the cell and those involved in the precipitation o...