Endosymbionts escape death!

The horizontal acquisition of symbionts by fauna and flora has always thought to be coupled with a release mechanism, to continue the intergenerational persistence of the mutualistic. For horizontally transmitted symbionts, escape from hosts is essential for the persistence over host generations. Riftia pachyptila (Vestimentiferan worms) is entirely nourished by their internal symbionts, which are acquired horizontally; however, the release of these symbionts back into the wild has never been demonstrated. This paper (Klose et al., 2015) aims to experimentally show this release of symbionts upon host death, and explain the mechanism behind the facilitation and intergenerational persistence of this cooperation.

The R.pachyptila symbiont is a thiotrophic, gamma-proteobacteria (candiditus) named Endoriftia persephone. Interestingly, it exhibits an identical 16srRNA sequence to the co-occurring hydrothermal vent vestimentiferan symbionts of the worm species – Tevnia jerichonana, and nearly identical sequence to the symbionts of Oasisia alviniae and Ridgeia piscesae.  R. pachptila (the host) provides all substrates necessary for chemosynthesis to the symbiont, enabling Endorifitia (the endosymbiont) to thrive and grow up to >1 billion symbiont cells g^-1; in return, the symbiont fixes organic carbon and thus nourishes the gutless host.  The symbionts are found within bacteriocytes deep within the host’s body with no openings towards the exterior. In many symbiotic relationships, the release of symbionts occurs whilst the host is alive, e.g the release of Vibrio fishceria from Bob tail squid; however in R.pachyptila there is no support for such release, and it has therefore been hypothesised that symbionts escape from decaying host tissues.

Potential symbiont escape from decaying host was measured using a custom-designed set of recruitment plates (SRPs). Live worms were dissected, and 0.4g wet weight trophosomal tissue was placed in capsules and incubated in sterile filtered seawater, with hydrothermal vent conditions being maintained. A trophosome was fixed in 100% ethanol prior to incubations to test for dead symbionts and dead bacteriocytes (containing dead symbionts) being rinsed out of the capsules passively; this was the control. Water samples and coverslips from SRPs were analysed with FISH, using a mixture of symbiont-, host-, and bacteria-specific oligonucleotide probes and epifluorescence microscopy to localise symbionts and other microbes on the surface and in the water. FISH analyses revealed that Endorifita left the dead host tissues quickly, recruiting to surfaces (under deep-sea vent conditions). Endoriftia was not detected by FISH in sampled incubation water before the experiment, and was only detected afterwards; indicating that under experimental conditions, only living Endoriftia can leave the dead host and colonise surfaces. Host cells (or partial cells containing host DNA) were not detected by either FISH, or by DAPI staining in the water, nor on coverslips outside of the trophosome in any treatment; suggesting that bacteriocytes housing symbionts, were not passively flushed into the water during experiments; indicating an active escape process.

Although the adult tube worms are sessile, the larval dispersal stage is pelagic, which facilitates the recruitment to new and already-existing vents. Likewise, the symbionts must colonise new vents; hence both host and symbiont arrive at new vent sites separately. The potential pulse release of symbionts on host death (as demonstrated in this paper) may ensure reliable colonisation of new/nearby vent sites. The experiment indicates successfully that a tremendous amount of Endoriftia symbionts may escape dead tubeworms and increase the density of the free-living population. 

The paper concludes with a few summative lines, and suggests that their research can be used to help decipher the stability of this mutualism in situ; in relation to population genetics, metapopulational ecology, and dispersal mechanisms in the deep sea.

This paper, I feel, has succeeded in answering its hypothesis as well as being equally engaging. The methodology is described concisely; in a manner which is easy to follow. The results are also exciting, as symbiotic relationships in the deep sea are poorly understood – and the high homogeneity between hydrothermal worm symbionts potentially indicates that this method of pulse-escaping upon death may be seen in other species. 

Reviewed paper:Klose, J., Polz, M.F., Wagner, M., Schimak, M.P., Gollner, S. and Bright, M., 2015. Endosymbionts escape dead hydrothermal vent tubeworms to enrich the free-living population. Proceedings of the National Academy of Sciences, 112(36), pp.11300-11305.