Abstract
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.
Highlights
Ciliates are a highly diverse group of heterotrophic protists [1], and crucial members of the marine microbial loop that predominates in the oligotrophic ocean [2]
The most widespread consortia were of the diatoms Chaetoceros dadayi and C. tetrastichon attached to the lorica of Eutintinnus apertus and E. pinguis, respectively
The morphology of the diatom frustules is well preserved in fixed plankton samples, but the ciliate soft bodies are distorted or lost, and the identification of the diatom hosts is easier than the naked ciliates
Summary
Ciliates are a highly diverse group of heterotrophic protists [1], and crucial members of the marine microbial loop that predominates in the oligotrophic ocean [2]. Tintinnid ciliates have vase-shaped hyaline proteinaceous shells called lorica that survive preservation and facilitate species identification. There is more information at the species level of tintinnids (loricate ciliates) as compared with the oligotrichs (naked ciliates) [3,4,5], the identification exclusively based on the lorica features is sometimes problematic due to the polymorphic character of some species [6,7,8]. Diatoms have a characteristic silica cell wall composed of two valves known as the frustule [10]. They reach high abundances in eutrophic environments such as well-mixed coastal or upwelling regions and, in oligotrophic waters, diatoms can utilize ecological adaptations such as the symbiotic interactions with diazotrophic cyanobacteria [11,12]. Diatoms are potential prey for ciliates [13], but there are examples of symbiotic relationships between diatoms and ciliates
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