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Coccolithophore biominerals, the coccoliths, represent an important part of the Meso-Cenozoic sedimentary archive. Geochemical analyses of coccoliths can be used to unravel climatic fluctuations in the oceanic realm, but such reconstructions are complicated due to the problem of the “vital effect”. This concept refers to the modulation in the record of the physico-chemistry of seawater in calcite due to algal physiology. For decades, it was thought that the magnitude of the vital effect was species-specific and constant for a given species. Recent studies aiming at a mechanistic understanding of these processes point towards a plastic and environmental-dependent interplay between the physiology of coccolithophores and isotopic composition in coccolith calcite. This “mobilis in mobili” relationship opens the door to the possibility to explore the vital effects as palaeoenvironmental proxies undertaking an interspecies approach. New physiological parameters, such as the quantification of calcification rates, pH, and calcium and carbon pools in the coccolith vesicle would further help geologists to constrain the vital effect. Emerging “non-traditional” isotope systems will also contribute to refine the transfer functions between coccolith geochemistry, vital effect, and palaeoenvironments.
Genetic approaches to exploring in situ marine phytoplankton assemblages have revealed previously unsuspected diversity at different taxonomic levels. However, the phylogenetic species concept has rarely been compared to classical morphologically based taxonomy, which forms the basis of most current ecological, physiological, and paleontological knowledge of phytoplankton. Here we use the coccolithophores as a case study to test the relationship between these two taxonomic approaches. Analysis of 217 coccolithophore LSU rDNA sequences and 729 specimens observed by light and electron microscopy obtained from three water samples (Atlantic Ocean, Pacific Ocean and Mediterranean Sea) demonstrated that parallel analysis of morphological and genetic data highlights limitations inherent to each approach. Combined morpho-genetic analyses increase the scope of description of the composition, richness and structure of natural coccolithophore communities. Overall, genetic determined diversity exceeded morphological determined diversity, which may partly reflect methodological biases, but also probably reflects cryptic speciation and/or the presence of lightly- or non-calcifying species (or life cycle stages) within the coccolithophore clade. Focusing on six coccolithophore family or order level subgroups, we show that the genetic diversity within established morphospecies varied significantly in different environments. Critically, we find that the divergence threshold at which phylospecies corresponded to morphospecies varied between different natural communities, a factor that may have important implications with respect to evaluation of diversity by metagenomics approaches.
Gephyrocapsa oceanica is a cosmopolitan bloom-forming coccolithophore species belonging to the haptophyte order Isochrysidales and family Noëlaerhabdaceae. Exclusively pelagic, G. oceanica is commonly found in modern oceans and in fossil assemblages. Its sister species Emiliania huxleyi is known to possess a haplo-diplontic life cycle, the non-motile diploid coccolith-bearing cells alternating with haploid cells that are motile and covered by non-mineralized organic scales. Since the cytology and ultrastructure of other members of the Noëlaerhabdaceae has never been reported, it is not clear whether these features are common to the family. Here, we report on the ultrastructure of both the non-motile calcifying stage and the non-calcifying motile stage of G. oceanica. We found no significant ultrastructural differences between E. huxleyi and G. oceanica either in the calcifying diploid stage or the haploid phase. The similarities between these two morphospecies demonstrated a high degree of conservation of cytological features. We discuss the significance of these results in the light of the evolution of the Noelaerhabdaceae.
Jomonlithus littoralis is a coccolithophore exhibiting unusual coccolith morphology that has previously only been reported from Japanese coastal waters. Jomonlithus is a mono-specific genus that has remained incertae sedis within the Prymnesiophyceae since its original description. We isolated a culture of the calcifying (diploid) stage of this species from Mediterranean coastal waters that subsequently produced a non-calcifying life cycle stage in culture. The non-calcifying (haploid) stage of J. littoralis is described for the first time through light and electron microscope observations of cytology, scale ornamentation and ultrastructure. A 28s rDNA molecular phylogeny is presented and the systematic affinities of Jomonlithus are discussed. This genus is placed in the Hymenomonadaceae and the diagnosis of this family is emended.