The nature of the plankton symbioses between ciliates and diatoms has been investigated from the tropical South Atlantic Ocean, and Mediterranean and Caribbean Seas. The obligate symbioses of the diatoms
Chaetoceros dadayi or
C. tetrastichon with the tintinnid
Eutintinnus spp., and
Chaetoceros coarctatus with the peritrich ciliate
Vorticella oceanica are the most widespread, and the consortium of
Chaetoceros densus and
Vorticella sp. have been rediscovered. Facultative symbioses between
Eutintinnus lususundae and
Chaetoceros peruvianus,
Hemiaulus spp., and
Thalassionema sp. are less frequent, often containing three or four partners because
Hemiaulus can also harbor the diazotrophic cyanobacteria
Richelia intracellularis. Another three-partner consortium is the peritrich ciliate
Zoothamnium pelagicum, ectobiont bacteria, and the diatom
Licmophora sp. The predominantly oligotrophic conditions of tropical seas do not favor the survival of large diatoms, but large species of
Coscinodiscus and
Palmerina in facultative symbiosis with
Pseudovorticella coscinodisci have a competitive advantage over other diatoms (i.e., reduction of sinking speed and diffusive boundary layer). Symbioses allow sessile peritric ciliates to extend their distribution in the pelagic environment, permit boreal-polar related diatoms such as
C. coarctatus or
Fragilariopsis doliolus to inhabit tropical seas, and help large diatoms to extend their survival under unfavorable conditions.
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