Diversity, environmental distribution, and molecular variability of the bacterial endosymbiont "Candidatus Megaira" widespread in ciliates and other protists

In the last years, the recently described bacterial genus “Candidatus Megaira” (Rickettsiales) caught the attention of many researchers from different fields. This genus comprises obligatory intracellular bacteria, which are closely phylogenetically related to the pathogen Rickettsia. Many diverse organisms have been colonized by bacteria of the genus “Candidatus Megaira”, both unicellular (e.g. ciliates, amoebae, and algae), but also multicellular (cnidarians, porifers, plants, and worms). Herein, we report the multidisciplinary description of a novel “Candidatus Megaira” species inhabiting the cytoplasm of Paramecium bursaria and Paramecium nephridiatum, together with the characterization of several new isolates of the type species “Candidatus Megaira polyxenophila” in different ciliate species. Our ultrastructural investigations showed that these bacteria display variations in dimensions like other members of Rickettsiaceae; and the novel species presents polymorphic features such as the presence of flagella, and the predilection of being closely associated to endosymbiontic algae of Paramecium bursaria. Moreover, we performed several trans-infection experiments in order to understand how this bacterium is spread in the aquatic environment, how it influences the host fitness within the same population of ciliates, and how it interacts with other symbiotic bacteria already present or invading the same host cell. In addition, we also screened 16S rDNA amplicon databases to see host and environmental distribution of members of the “Candidatus Megaira” genus. We propose an updated phylogeny of the genus based both on phylogenetic analysis and 16S rRNA gene diversity, thus subdividing the genus in four distinct and separated species. All together our findings give support to the hypothesis that this poorly investigated bacterium may be transmitted by protists in aquatic ecosystems.