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International audienceCoccolithophores have influenced the global climate for over 200 million years1. These marine phytoplankton can account for 20 per cent of total carbon fixation in some systems2. They form blooms that can occupy hundreds of thousands of square kilometres and are distinguished by their elegantly sculpted calcium carbonate exoskeletons (coccoliths), rendering them visible from space3. Although coccolithophores export carbon in the form of organic matter and calcite to the sea floor, they also release CO2 in the calcification process. Hence, they have a complex influence on the carbon cycle, driving either CO2 production or uptake, sequestration and export to the deep ocean4. Here we report the first haptophyte reference genome, from the coccolithophore Emiliania huxleyi strain CCMP1516, and sequences from 13 additional isolates. Our analyses reveal a pan genome (core genes plus genes distributed variably between strains) probably supported by an atypical complement of repetitive sequence in the genome. Comparisons across strains demonstrate that E. huxleyi, which has long been considered a single species, harbours extensive genome variability reflected in different metabolic repertoires. Genome variability within this species complex seems to underpin its capacity both to thrive in habitats ranging from the equator to the subarctic and to form large-scale episodic blooms under a wide variety of environmental conditions

The trafficking and function of cell surface proteins in eukaryotic cells may require association with detergent-resistant sphingolipid-and sterol-rich membrane domains. The aim of this work was to obtain evidence for lipid domain phenomena in plant membranes. A protocol to prepare Triton X-100 detergent-resistant membranes (DRMs) was developed using Arabidopsis (Arabidopsis thaliana) callus membranes. A comparative proteomics approach using two-dimensional difference gel electrophoresis and liquid chromatography-tandem mass spectrometry revealed that the DRMs were highly enriched in specific proteins. They included eight glycosylphosphatidylinositol-anchored proteins, several plasma membrane (PM) ATPases, multidrug resistance proteins, and proteins of the stomatin/prohibitin/hypersensitive response family, suggesting that the DRMs originated from PM domains. We also identified a plant homolog of flotillin, a major mammalian DRM protein, suggesting a conserved role for this protein in lipid domain phenomena in eukaryotic cells. Lipid analysis by gas chromatography-mass spectrometry showed that the DRMs had a 4-fold higher sterol-to-protein content than the average for Arabidopsis membranes. The DRMs were also 5-fold increased in sphingolipid-to-protein ratio. Our results indicate that the preparation of DRMs can yield a very specific set of membrane proteins and suggest that the PM contains phytosterol and sphingolipid-rich lipid domains with a specialized protein composition. Our results also suggest a conserved role of lipid modification in targeting proteins to both the intracellular and extracellular leaflet of these domains. The proteins associated with these domains provide important new experimental avenues into understanding plant cell polarity and cell surface processes.Biological membranes consist of a perplexing number of lipids (Edidin, 2003a). The classical model of membranes assumes that these lipids form a homogeneous fluid-like or liquid-disordered (l d ) phase, which allows free diffusion of individual molecules and resident proteins (Edidin, 2003b). However, numerous recent studies on model membranes have demonstrated that certain lipids, in particular sphingolipids and cholesterol, may form relatively stable clusters by tight self-association, thus segregating them from surrounding phospholipids (Schroeder et al., 1994; Ahmed et al., 1997;Dietrich et al., 2001;Silvius, 2003). The association of rigid sterol molecules with the long and saturated acyl chains of sphingolipids results in the formation of a more organized, liquidordered (l o ) phase; l o and l d phases can coexist in the same membrane (Brown and London, 1998;Edidin, 2003b). The lipid raft hypothesis postulates that a sterol-and sphingolipid-rich l o phase is also present in cell membranes and that it forms discrete microdomains or lipid rafts that diffuse in the bulk of the l d phospholipid phase (Simons and Ikonen, 1997;Mayor and Rao, 2004).There is substantial evidence supporting the existence of plasma membrane (PM) domains in ...