Molecular systematics of the Pleurochrysidaceae, a family of coastal coccolithophores (Haptophyta)

Within the marine phytoplankton community, coccolithophores are important players in the carbon cycle, because of their great abundance and unusual ability to form an external cover of calcium carbonate. Most described species are oceanic, and so little attention has been devoted to coastal species, the majority of which belong to two families: Pleurochrysidaceae and Hymenomonadaceae. Here, we study the biodiversity and evolutionary relationships within the Pleurochrysidaceae, for which we have compiled DNA sequences from a total of 18 strains plus three members of the Hymenomonadaceae. Most of the analysed sequences have been obtained for this study and belong to three genetic fragments: 18S rDNA, elongation factor Tu (tufA) and internal transcribed spacers 1 and 2, which include the 5.8S rDNA. We find some discordance between the current taxonomy of the group and our molecular analyses. First, we observe that several different recognized species are in fact highly similar taxa, probably belonging to the same evolutionary lineages. Secondly, we show some unexpected phylogenetic relationships. In the light of these results, we suggest that, when possible, DNA sequences be considered in future taxonomic classifications of the Pleurochrysidaceae and other planktonic organisms.