Environmental Uptake of Bacterial Symbionts by Giant Tubeworms

Deep sea hydrothermal vents are dominated by marine invertebrates called tubeworms (Riftia pachyptila). The environments they live in are in complete darkness and very little organic material reaches to these depths. How they manage to maintain their fast growth rate when organic compounds are so scarce was a mystery for many years. After several investigations it was discovered that these animals contain a large internal organ, which was later named the trophosome, instead of a digestive system. The tubeworm uses gills to filter O2, CO2 and H2S from the surrounding water and transports them to the trophosome where Gammaproteobacteria uses them to produce organic molecules for themselves and for their host. The tube worm survival is completely reliant on these chemoautotrophic bacteria but how and where do these bacterial symbionts come from?

A study by Nussbaumer et al (2006) tried to identify how this symbiosis develops. In many obligate symbioses this is achieved via vertical transfer from parent to offspring. However, another unusual discovery was that the larval stage of R. pachyptila did have a digestive system and no trophosome. Transmission electron microscopy and FISH of specimens removed from a hydrothermal vent in the Pacific confirmed there were no symbiotic bacteria present in young larvae. At a certain size of development (250-400μm) the bacteria suddenly appeared. Further evidence suggested that the bacteria entered through the host’s skin rather than through the mouth, almost like an infection. Once inside the bacteria initiated the growth of the trophosome organ and apoptosis of the cells that make up the digestive system.

Harmer et al (2008) tried to further prove the idea that tubeworms acquire their symbiont via environmental uptake rather than vertical transfer. Evidence from other studies have found no evidence of symbiont presence in larvae but whether the bacteria is present in the surrounding environment for up take by tubeworm juveniles was unknown. In this study, samples of seawater and biofilms were collected from the Tica hydrothermal vent in the Pacific Ocean at a depth of ~2,600m. The biofilm samples were collected on bacteria settlement devices that were placed in clusters of R. pachyptila, adjacent to the clusters, 10m and 100m away from the clusters. Sadly, the only water samples taken were 1m away from the clusters. PCR of 16 rRNA and FISH were used to test for the presence of the symbiont. All samples confirmed their presence but with one exception, biofilm samples 100m away from a R. pachyptila which had been collected after only one month. Other samples at this distance that had been left for a year did show symbiont presence. FISH indicated that bacterial abundance was greatest the closer to the clusters, but symbionts were still clearly visible on 100m samples. The authors did comment however, that FISH is only semi-quantitative and so only gave an indication of their numbers.

A number of related papers have been published since these two articles but I picked them because they do high light the very unusual way in which R. pachyptila develops a symbiosis with bacteria from its environment instead of vertical transmission. Without this relationship, the tubeworms would simply not have enough energy to survive.

A Review of:
Harmer, T. Rotjan, R. Nussbaumer, A. Bright, M. Ng, A. DeChaine, E and Cavanaugh, C (2008) Free-Living Tube Worm Endosymbionts Found at Deep-Sea Vents. Applied and Environmental Microbiology, 74, 3895-3898.

Additional Reference:
Nussbaumer, A. Fisher, C and Bright, M (2006) Horizontal endosymbiont transmission in hydrothermal vent tubeworms. Nature, 441, 345-348.