Symbiotic interactions between ciliates (Ciliophora) and diatoms (Bacillariophyceae)

Abstract

Introduction: Diatoms and ciliates are important components of the marine plankton community, and some species are able to develop symbiotic associations in the tropical seas. Objective: To describe the nature of the symbioses, the morphological adaptations of the members, and the possible ecological advantages of the symbiotic life versus the free-living forms. Methods: Plankton samples were collected from Mediterranean and Caribbean Seas, and the South Atlantic Ocean. Consortia were examined during laboratory incubations, including studies of the motility and the feeding currents by high-speed video recordings, and culture tests of the species as symbiotic or free-living forms. Results: The consortia of the diatoms Chaetoceros dadayi and C. tetrastichon with the tintinnid Eutintinnus spp., and C. coarctatus with the peritrich ciliate Vorticella oceanica are examples of an obligate mutualism. The cultures of the host diatoms as free-living organism were unsuccessful. The consortia between Eutintinnus lususundae and the diatoms Chaetoceros peruvianus, Hemiaulus hauckii, H. membranaceus, and Thalassionema sp. are facultative symbioses. These are examples of three or four partner consortia because Hemiaulus spp. is the host of the diazotrophic cyanobacteria Richelia intracellularis. Other example of facultative three partner consortium is the peritrich ciliate Zoothamnium pelagicum with an ectobiont bacteria, and the diatom Licmophora sp. The barrel-shaped chains of the diatom Fragilariopsis dolious encircled the lorica of Salpingella spp., while these chains were almost flat in the free-living stage. The peritrich ciliate Pseudovorticella coscinodisci lives on large pelagic diatoms such as Coscinodiscus and Palmerina. These symbioses are facultative for the diatoms, but they extended their survival under unfavorable conditions. High-speed video recordings of the consortium of Vorticella oceanica and Chaetoceros coarctatus revealed that during the stalk contraction the zooid of reached 5 400 body length s-1, being the fastest organism with respect to its size. The consortia of Chaetoceros densus and an undescribed species of Vorticella is re-discovered. Conclusions: These symbioses have allowed that the sessile peritric ciliates colonize the pelagic environment and the proliferation of diatoms with a polar origin in the tropical sea.