AME 70:233-243 (2013)  -  DOI: https://doi.org/10.3354/ame01657

ABSTRACT: A monophyletic group of freshwater and brackish ciliate species belonging to the genus Euplotes is known to harbor bacterial endosymbionts of the class Betaproteobacteria permanently in the natural environment. One or other of 2 different species of bacteria have been shown to be present: the obligate symbiont Polynucleobacter necessarius or a recently described betaproteobacterium. While the association between P. necessarius and its Euplotes hosts has been well studied in the past, knowledge about the relationship between the newly discovered symbiont and its Euplotes hosts is still very poor. Here we present the SSUrRNA gene characterization of both host and symbiont for an additional brackish-water population of Euplotes hosting the newly discovered symbiont. We also provide the symbiont genome size determination and ultrastructural description. Attempts to cultivate the new symbiont outside its host, using many different methods, failed. This result indicates that, as in the case of the symbiotic P. necessarius, we are probably dealing with an obligate symbiont, unable to grow outside the cytoplasm of its natural host. On the basis of the results obtained we propose the new symbiont as a candidate new species with the name of ‘Candidatus Protistobacter heckmanni’, according to the current rules of prokaryotic nomenclature.

KEY WORDS: Polynucleobacter · Euplotes · 16S rRNA · 18S rRNA · Symbiosis · Coevolution