ABSTRACT
The
Rickettsiales
(
Alphaproteobacteria
) are obligate intracellular bacteria that colonize a wide range of eukaryotic hosts, including diverse metazoa and protists. Here, we characterize rickettsial endosymbionts discovered in the cytoplasm of the algivorous amoeboflagellates
Viridiraptor invadens
and
Orciraptor agilis
(Viridiraptoridae, Cercozoa, Rhizaria), supplying evidence of free-living, phagotrophic members of the Cercozoa serving as hosts for
Rickettsiales
. According to 16S rRNA gene phylogenies, the bacteria represent two closely related but distinct genotypes within a deep-branching rickettsial clade, which contains the genera “
Candidatus
Odyssella,” “
Candidatus
Paracaedibacter,” and “
Candidatus
Captivus.” Using the full-cycle rRNA approach, we detected the novel bacteria in four of nine viridiraptorid strains tested. Furthermore, two specific oligonucleotide probes with a single-nucleotide-difference discriminated both bacterial genotypes by fluorescence
in situ
hybridization (FISH). We establish the candidate species “
Candidatus
Finniella inopinata” (found in
Viridiraptor invadens
) and “
Candidatus
Finniella lucida” (found in
Orciraptor agilis
) for the novel bacteria and propose a new, provisional family of
Rickettsiales
, “
Candidatus
Paracaedibacteraceae.”