How endosymbionts escape dead tubeworms and repopulate live specimens

(Phys.org)—Deep within oceanic hydrothermal vents, the thiotrophic gamma-proteobacterial endosymbiont Candidatus Endoriftia lives in happy symbiosis with its host organism, the sessile tubeworm Riftia pachyptila: The tubeworm provides all the substrates the endosymbiont needs for chemosynthesis and, in return, Endoriftia releases fixed organic carbon for the tubeworm host, which is gutless, and would otherwise have no source of nutrition.

In order to propagate and persist, Endoriftia needs to transmit to other tubeworm individuals, but until recently, researchers were unsure how this was achieved, as Riftia has no body openings. This mode of intergenerational persistence is called horizontal transmission, as opposed to vertical transmission, in which hosts transmit endosymbionts directly to their offspring.

A group of researchers recently explored Endoriftia transmission experimentally to determine how and when symbionts were released into the unique environment of hydrothermal vents, and what surfaces it occupies until populating a clump of new, aposymbiotic tubeworms. They have published their results in the Proceedings of the National Academy of Sciences.

The researchers hypothesize that Endoriftia escapes the tissues of the newly dead tubeworms into the surrounding environment. To discover the mode of transmission, the researchers simulated the high-pressure, high-temperature conditions of a hydrothermal vent in a laboratory setting.

To collect tubeworm samples, the researchers embarked on four cruises that included excursions in a submersible. They deployed colonization devices at several sites and collected them once they were populated with specimens. Live worms were dissected and samples placed in specially designed capsules, which were incubated in sterile, filtered seawater in high-pressure flowthrough vessels.

All samples were analyzed via fluorescence in situ hybridization (FISH) analysis. The results revealed that Endoriftia rapidly abandons dead host tissue in very large numbers and recruits to such surfaces found in hydrothermal vent conditions.

While populating the tubeworms, Endoriftia lives inside cells called bacteriocytes. Though the researchers found free Endoriftia in the seawater, they found no evidence of bacteriocytes, a strong indication that Endoriftia makes an active escape from the bacteriocyte following the death of the host. They note that while the endosymbiont does not express flagella while associated with the tubeworm, they do produce flagella while independent of a host.

Further, the researchers explored the degradation processes by evaluating the integrity of the host and symbiont membranes. They found that under deep-sea conditions, most of the symbiont and bacteriocyte membranes remained intact for about 10 days. They note, however, that "even after six days of vent incubation, a few morphologically intact symbionts were detected, but in most symbionts, membranes were distorted." This suggests a rough timeframe for the repopulation of free-living symbionts.

Thus, the study uniquely demonstrates Endoriftia escaping from dead tubeworm hosts and seeding the surrounding environment in the singularly dynamic and fluctuating deep-sea hydrothermal environment. The researchers conclude, "Now that this principal mode of host-associated and free-living symbiont connectivity is known, we can start to decipher the stability of this mutualism in situ in the framework of population genetics and metapopulation ecology."