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A symbiotic relationship between the marine ciliate Euplotes magnicirratus and an alpha-proteobacterium has recently been described. The bacterial symbiont, identified by molecular analysis as a new species of the genus Devosia, is referred to with the provisional name 'Candidatus Devosia euplotis'. This association is constant (it was found in different ciliate strains sampled in different geographical regions), and durable (it persisted in laboratory conditions for years). The physiological significance of this association was investigated through antibiotically produced aposymbiotic E. magnicirratus and attempts to grow the endosymbionts outside their host. Aposymbiotic protozoa completely lost the ability to digest the green alga Dunaliella tertiolecta, their usual food in laboratory conditions, and died. An investigation into the different steps of the digestive process showed that acidification of the digestive vacuoles occurred normally in both symbiotic and aposymbiotic ciliates, but that subsequent activation of acid phosphatase did not occur in aposymbiotic cells. Therefore, the ciliate depends on the symbiont for digestion of its green algal food and, as a consequence, for its survival. Attempts to grow bacterial endosymbionts on diluted culture media and on ciliate homogenate were unsuccessful. The results indicate that this represents a new type of symbiosis between prokaryotes and ciliates, which is mutualistic, and most likely obligatory for both partners.KEY WORDS: Alpha-proteobacteria · Ciliates · Devosia · Digestive process · Euplotes · Lysis · SymbiosisResale or republication not permitted without written consent of the publisher

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