In surface waters, many acantharian species harbour symbiotic microalgae in their cytoplasm (golden cells on the pictures), representing about 80% of the acantharian biomass (Stoecker et al., 1996). These particular acantharians play a dual role in marine ecosystems: they are predators on a wide diversity of preys, from bacteria to zooplankton, but they also participate indirectly to primary production through their microalgal symbionts, sometimes significantly -up to 20% – in oligotrophic waters (Michaels, 1988). Taylor (1982) observed in the Sargasso Sea that most chlorophyll above the Deep Chlorophyll Maximum (DCM) could be represented by Acantharia, after the small microalgae (picoplankton) and filamentous cyanobacteria. Depending on the host species, Acantharia can have between 10 to 100 of microalgal cells. The number of these cells increases throughout the ontogeny of the Acantharia  (Michaels, 1991),  but we ignore whether they keep dividing in their host and/or they are continuously acquired from the environment.

In the 70’s, based on some  diagnostic criteria of the plast,  electronic microscopy observations of the microalgae in hospite indicated that symbionts of Acantharia would be related to two main lineages, the haptophytes and dinoflagellates (Hollande et Carré, 1974; Febvre et Febvre-Chevalier, 1979). Haptophyte symbionts have been observed on only three acantharian species  (Lithoptera muelleri, Acanthometra pellucida et Amphilonche elongata) belonging to the order Arthracanthida et Symphiacanthida that contain more than 100 species. As for the dinoflagellates, the host species was not identified in the study.  But overall, the identity of the symbiotic microalgae remains unaccurate, and no genetic information is available  in order to have an identification at the genus or at the species level. Moreover, specificity and biogeography of the symbiotic relationship is poorly understood with only three host species sampled in a single location (Mediterranean Sea). We also ignore whether symbiosis occurred several times during the evolution of Acantharia.

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Related papers to symbiosis in Acantharia:

Decelle J, Probert I, Bittner L, Desdevises Y, Colin S, de Vargas, Galí M, Simó R, Not F (2012) An original mode of symbiosis in open ocean planktonPNAS 109: 18000-18005.  doi: 10.1073/pnas.1212303109  PDF

Decelle J, Siano R, Probert I, Poirier C, Not F (2012) Multiple microalgal partners in symbiosis with the Acantharia Acanthochiasma sp. (Radiolaria) – Symbiosis  doi:10.1007/s13199-012-0195-x  PDF