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• Characteristic genus: Prymnesium
  
• Abundance: primarily marine group, highest diversity in oligotrophic water at greater depth
• Appearance: Naked cells, mostly single and < 20 µm; motile, 2 flagella and 1 haptonema (therefore also referred to as Haptophytes)
• Coccolithophorids cells with scales of calcium carbonate (coccoliths) on the cell surface (Coccolithophorids)
  
• Coccoliths contribute ~25% of annual carbon transport to the deep-sea
• Fossil records because coccoliths are well preserved in sediments; coccolith deposits form white carbonate cliffs, e.g. White Cliffs of Dover
  
• Phototrophic and mixotrophic forms; phagotrophy abundant in cells that lack coccoliths
• Secondary plastids with 4 membrane envelopes
• Pigments: Chl.a, Chl.c, b-carotene, diatoxanthin, diadinoxanthin, fucoxanthin, 19-hexanoylfucoxanthin, 19-butanoylfucoxanthin; high diversity in accessory pigments, no pigment characteristic for all prymnesiophytes

• Coccosphere: outer wall of coccoliths
• Coccoliths are either produced with in the cell by the Golgi apparatus (heterococcoliths) or outside the cell membrane (holococcoliths)
• Haptonema is no flagellum, does not possess 9+2 microbubuli structure; it can be as long as 160 µm
  
• Phagotrophy: long haptonema are used for prey capture and attachement to substrates; short haptonema mostly without function
  
  
• Coccolithophorids lack haptonema 

• Organic scales occur in a few species together with calcareous coccoliths
  
• Holococcoliths: produced externally, simple shapes; 
  
• heterococcoliths are produced internally and possess a complex three-dimensional structure
  
• Great variety of coccolith forms, used for taxonomy and geological identifications
  
• Function: defense of small grazers and infection by pathogenes; light focusing into the cell has been discussed
• Coccolith production is highly dependent on photosynthesis
• Calcification is not a net sink for CO2!
• 2 HCO₃^- + Ca²⁺ = CaCO₃ + CO₂ + H₂O
• Atmospheric CO₂ partial pressure is higher than normal above oceanic coccolithophorid blooms

• Prominent blooms in temperate and polar areas of both hemispheres
• Water discoloration can be seen from satellites
  
• Three life-cycle stages: non-motile C cells with coccoliths, non-motile naked N-cells, motile S-cells with scales; DNA analyses indicate that S-cells are haploid and C-cells are diploid
  
  
• Sexual reproduction appears likely, given the presence of diploid and haploid stages, but gamete fusion has not been observed
• DMS: Emiliania huxleyi produces DMSP (dimethylsulfoniumpropionate); converted to DMS (dimethylsulfide) upon release to air
• Climate effects: DMS increases cloud formation; increased clouds and coccolithsincrease water albedo ? cooling effect

• Bloom forming in the North Sea, Gulf of Maine, Antarctic Ocean
• Foam accumulates on the beaches during dense blooms because wave action destroys the cells and wips the cells‘ proteins to foam 
  
• Biogeochemically of interest due to DMS production; ca. 10% of global DMS is produced by Phaeocystis
• Growth forms: as large (up to 1 cm), hollow, gelatinous colonies with round cells lacking flagella, haptonema, and scales; or biflagellate, single cells with short haptonema; blooms are formed by colonies
  
• Toxicity is not reported from such blooms, but fish evades blooms, probably by detection of the DMS in the water

Fisheries are affected by net clogging