Subtype variation among bacterial endosymbionts of tubeworms (Annelida: Siboglinidae) from the Gulf of California.

Symbiosis involving chemoautotrophic bacteria allows vestimentiferan tubeworms to thrive in sulfidic marine environments. This study examined genetic variation among endosymbionts associated with three vestimentiferan species from the Gulf of California. Small subunit (16S) rRNA sequences identified two evolutionary lineages of -Proteobacteria in these worms. Phylotype-II bacteria associated with the hydrothermal vent species Riftia pachyptila exhibited no subtype variation upon examination of form II (cbbM) RuBisCO, whereas the phylotype-I bacteria associated with two cold-seep species, Escarpia spicata and Lamellibrachia barhami, were polymorphic. Bacterial subtypes distinguished by three RuBisCO alleles occurred at similar frequencies in both host species when sampled together from tubeworm clusters, offering, therefore, no evidence for host-specificity. Instead, the frequencies of these subtypes varied significantly among patchily distributed tubeworm clusters. Subtype variation on small spatial scales is consistent with prior evidence that vestimentiferans acquire their symbionts locally from the environment in which they settle as larvae. Adult vestimentiferans are nourished by endosymbiotic bacteria that oxidize inorganic sulfides and fix carbon via the Calvin-Benson cycle (1). These essential bacteria infect vestimentiferans de novo in each generation by penetrating the epidermis of trochophore larvae that settle on benthic substrates (2). Previous studies (summarized in reference 3) revealed two related phylotypes (i.e., a clade defined by 16S rRNA sequences) of -Proteobacteria associated with vestimentiferans worldwide. The two phylotypes segregate geographically and according to the kind of chemosynthetic habitat in which the hosts settle. Phylotype-I is found in cold-seep vestimentiferans worldwide and in hydrothermal vent vestimentiferans from the western Pacific. Phylotype-II has been found only in vestimentiferans from eastern Pacific hydrothermal vents. When distinct vestimentiferan species co-occur in a habitat, they host the same symbiont phylotype. Conversely, when a single host species can occur in multiple habitats—as does Escarpia spicata, which is found at seeps, vents, and whale-falls—it acquires the symbiont phylotype associated with that habitat. Such “opportunistic environmental acquisition” of symbionts is hypothesized to be an ecologically flexible strategy that allows an invertebrate host to incorporate locally adapted microbial strains (4). Nonetheless, the ribosomal sequences defining these bacterial phylotypes are highly conserved and may not reveal subtype variation that is physiologically or ecologically relevant. Indeed, co-occurring vestimentiferan host species that share the same bacterial phylotype can exhibit distinct ITS sequences and rep-PCR fingerprints (3). Thus, some degree of host-recognition or specificity might exist among genetic strains of these bacterial phylotypes; alternatively, apparent subtype associations with host species might be a consequence of sampling hosts that obtained their symbionts in different places or times from a spatially patchy and temporally varying environment (3). The goal of this study was to examine how host-specificity and small-scale environmental patchiness affect the subtype composition of vestimentiferan symbionts. Vestimentiferans were sampled with a robotic submersible from hydrothermal vent and cold-seep environments in the central Gulf of California (Fig. 1A; Table 1). We Received 18 January 2007; accepted 6 March 2007. * To whom correspondence should be addressed. E-mail: [email protected] Abbreviations: 16S, small-subunit ribosomal RNA; rbc, form II (cbbM) ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO); ST, Southern Trough; TF, Transform Fault. Reference: Biol. Bull. 212: 180–184. (June 2007) © 2007 Marine Biological Laboratory