PHYLOGENETIC ANALYSIS OF A HIGHLY SPECIFIC ASSOCIATION BETWEEN ECTOSYMBIOTIC, SULFUR-OXIDIZING BACTERIA AND A MARINE NEMATODE

The phylogenetic relationship of chemoautotrophic, sulfur-oxidizing, e ctosymbiotic bacteria growing on a marine nematode, a Laxus sp. (forme rly a Catanema sp.), to known endosymbionts and free-living bacteria w as determined. Comparative 16S rRNA sequencing was used to investigate the unculturable nematode epibionts, and rRNA-targeted oligonucleotid e hybridization probes were used to identify the ectosymbionts in situ . Both analyses revealed a remarkably specific and stable symbiosis. U nique hybridization of a specific probe to the ectosymbionts indicated that only one species of bacteria was present and growing on the cuti cle of the nematode. Distance and parsimony methods used to infer phyl ogenetic trees both placed the nematode ectosymbionts at the base of a branch containing chemoautotrophic, sulfur-oxidizing endosymbionts of three bivalve families and of the tube worm Riftia pachyptila. The mo st closely related free-living bacteria were chemoautotrophic sulfur o xidizers belonging to the genus Thiomicrospira. Furthermore, our resul ts suggested that a second, only distantly related group of thioautotr ophic endosymbionts has as its deepest branch surface-colonizing bacte ria belonging to the genus Thiothrix, some of which are capable of sul fur-oxidizing chemoautotrophic growth.