The Hidden World of Rickettsiales Symbionts: Candidatus Spectririckettsia obscura, a Novel Bacterium Found in Brazilian and Indian Paramecium caudatum

Symbioses between bacteria and eukaryotes are widespread and may have significant impact on the evolutionary history of symbiotic partners. The order Rickettsiales is a lineage of intracellular Alphaproteobacteria characterized by an obligate association with a wide range of eukaryotic hosts, including several unicellular organisms, such as ciliates and amoebas. In this work, we characterized the Rickettsiales symbionts associated with two different genotypes of the freshwater ciliate Paramecium caudatum originated from freshwater environments in distant geographical areas. Phylogenetic analyses based on 16S rRNA gene showed that the two symbionts are closely related to each other (99.4% identity), belong to the family Rickettsiaceae, but are far-related with respect to previously characterized Rickettsiales. Consequently, they were assigned to a new species of a novel genus, namely Candidatus Spectririckettsia obscura. Screening on a database of short reads from 16S rRNA gene amplicon-based profiling studies confirmed that bacterial sequences related to the new symbiont are preferentially retrieved from freshwater environments, apparently with extremely scarce occurrence (<0.1% positive samples). The present work provides new information on the still under-explored biodiversity of Rickettsiales, in particular those associated to ciliate host cells.