Endosymbionts escape dead hydrothermal vent tubeworms to enrich the free-living population

Abstract

Theory predicts that horizontal acquisition of symbionts by plants and animals must be coupled to release and limited dispersal of symbionts for intergenerational persistence of mutualisms. For deep-sea hydrothermal vent tubeworms (Vestimentifera, Siboglinidae), it has been demonstrated that a few symbiotic bacteria infect aposymbiotic host larvae and grow in a newly formed organ, the trophosome. However, whether viable symbionts can be released to augment environmental populations has been doubtful, because (i) the adult worms lack obvious openings and (ii) the vast majority of symbionts has been regarded as terminally differentiated. Here we show experimentally that symbionts rapidly escape their hosts upon death and recruit to surfaces where they proliferate. Estimating symbiont release from our experiments taken together with wellknown tubeworm density ranges, we suggest a few million to 1.5 billion symbionts seeding the environment upon death of a tubeworm clump. In situ observations show that such clumps have rapid turnover, suggesting that release of large numbers of symbionts may ensure effective dispersal to new sites followed by active larval colonization. Moreover, release of symbionts might enable adaptations that evolve within host individuals to spread within host populations and possibly to new environments.symbiosis | mutualism stability | symbiont seeding | tubeworms | Vestimentifera T he evolution of cooperation between species (mutualism) represents a challenge for evolutionary theory (1-5). The host provides benefits to the symbiont by supporting its partner's offspring at the expenses of its own offspring (6, 7). To assure beneficial coexistence after establishment of an association, partner sanctions and/or partner fidelity feedback may act as postinfection mechanisms (1,(8)(9)(10)(11)(12). During vertical transmission, hosts transmit symbionts directly to offspring during reproduction (1,5,7,13). During horizontal transmission, partners must reassociate anew each host generation (13). Limited dispersal following release of the cooperating symbiont back into the environment might keep the offspring of both partners in close proximity (14-18) and thereby enhance the probability that the offspring of both partners can reassociate (7,14,16,(18)(19)(20). Therefore, the processes of symbiont release back into the environment are key factors in understanding interspecies cooperation in ecological and evolutionary timescales.In facultative horizontally transmitted pathogens, escape from the infected host and long survival in the environment are crucial to reinfect susceptible hosts, especially in cases when host density is low (21,22). Although some pathogens are capable of growing outside the host, others follow a sit-and-wait strategy in the absence of proliferation in the environment (21). Several studies have shown that host-associated and free-living beneficial symbiont populations apparently can rejoin. The majority of horizontally transmitted bioluminescent Vibrio fischeri housed in the lig...

Exploring the Ecology of Deep-Sea Hydrothermal Vents in a Metacommunity Framework

“…Notably, during the short observation periods after the eruptions in both Pacific regions, death of foundation species in small aggregations resulted from cessation of vent flow. These observations point to the high temporal variability of venting and community structure on faster-spreading centers (Tsurumi and Tunnicliffe, 2001;Gollner et al, 2013;Klose et al, 2015; described further in section Global Patterns of Vent Distributions, Disturbance Frequency and Longevity; Figure 3). …”

Insights into Symbiont Population Structure among Three Vestimentiferan Tubeworm Host Species at Eastern Pacific Spreading Centers

“…Endoriftia persephone" (4,6). Viruses are known to be abundant at deep-sea hydrothermal vents and are likely an important cause of mortality for free-living bacteria (47).…”

“…This local increase in homogeneity might be amplified or maintained in the presence of the tubeworm hosts through pseudovertical transfer of symbionts (6).…”

“…Free-living "Ca. Endoriftia persephone" symbionts have been detected in biofilms and seawater surrounding R. pachyptila aggregations (4), and it has been demonstrated recently that the Riftia symbionts can return to their free-living stage upon the death of the worm (6), thereby maintaining/sustaining environmental populations.…”

Horizontally transmitted symbiont populations in deep-sea mussels are genetically isolated

“…Furthermore, the geographic isolation of vertically transmitted bacteria leads to genetic isolation and to symbiont genome reduction over time as a consequence of genetic drift 15 . In contrast, dispersal is expected to be higher for horizontally transmitted bacteria, where host-associated populations are connected to one another through the environmental pool 16 . Nonetheless, the genetic diversity of horizontally transmitted microbial populations may also be reduced due to bottlenecks during symbiont transmission and host colonization.…”