Ciliates are common hosts of a huge variety of bacteria, which are either carried as epibionts on the cell body surface, or endosymbionts in the cell cytoplasm as is the case of Euplotes species. Francisella γ-proteobacteria (which may colonize and harm a variety of hosts, mammals included) were originally isolated as endosymbionts from an Antarctic population of E. petzi, assigned to F. adeliensis sp. nov. (now re-named Parafrancisella adeliensis), and later identified in other Antarctic E. focardii and E. nobilii populations. In consideration of the bipolar distribution of E. nobilii, we inquired whether and to which extent non-Antarctic conspecific populations host P. adeliensis endosymbionts. Screened in PCR amplifications and in-situ hybridizations with a P. adeliensis-specific 16S rRNA gene probe, E. nobilii populations from coastal sites of southern Patagonia, Svalbard Islands, eastern and western Greenland and northern Alaska all proved to be positive to P. adeliensis, thus supporting the conclusion that E. nobilii may act as a trans-tropical P. adeliensis vector. Insights on the E. nobilii/P. adeliensis symbiotic relationships were next obtained by sequencing and analyzing the genomes of the two species. While the P. adeliensis genome lacks genes encoding enzymes for the synthesis of essential amino acids such as lysine, cysteine, methionine and tyrosine, the E. nobilii genome lacks genes encoding methionine sulfoxide reductase of type A which is an essential enzyme to repair diastereomeric S-form of methionine sulfoxide in oxidized proteins. It thus likely that P. adeliensis relies on E. nobilii cytoplasm to recover essential amino acids, and that E. nobilii relies on P. adeliensis antioxidant enzymes released from the Type-6 Secretion System to face damage from oxidative stress imposed by the high (saturated) oxygen concentrations of the polar sea waters
