Symbiont Relations

All R. pachyptila populations contain the same symbiont species, according to DNA profiling. Despite the vast distances between populations and the transient nature of their habitats R. pachyptila is apparently highly selective in its bacterial association, only incorporating a single species of sulfur oxidizing bacteria, among the many available in its vent habitat, into its troposome.  R. pachyptila and its symbiont also have a higher number of base pairs in common than would be expected of species so distantly related. This suggests that genes, especially those governing the synthesis and utilization of proteins, have been shared between the host and its symbiont. In an unprecedented twist, Edwards and Nelson discovered that R. pachyptila’s symbiont enters into symbiotic relationships with another species of tubeworm, Tevnia jerichonana, too (Edwards and Nelson1991).

The symbiont itself, according to RNA sequence comparisons, belongs in the “purple photosynthetic bacteria” grouping. It has close phylogenetic relationships with heterotrophic, chemosynthetic, and photosynthetic bacteria. Perhaps its diverse evolutionary background provided it with the tools necessary to use the vastly disparate energy sources it incorporates, both as a symbiont and as a free living bacterium (Stahl et al. 1984).