Life After Death for Tubeworm Endosymbionts

Riftia pachyptila, better known as tubeworms, are deep sea invertebrates that live in symbiosis with what seems to be a single species of symbiotic bacteria: Candidatus Endoriftia persephone, better known as Endoriftia, that live in a specially created organ in the tubeworm called a trophosome. While the worm is alive, it provides these Endoriftia with substrates for chemosynthesis (the fixation of 1-carbon compounds into organic carbon using chemical energy) and in return the symbiont gives the host organic carbon. But what happens when the worm dies? The general consensus was that the symbionts die too, but how can this be when this symbiosis has lasted for so long? This study by Klose et al. investigates the fate of the symbionts once the tubeworm has died.

The researchers found that 97% of the endosymbionts in their experimental setup did in fact escape the dead tubeworm tissue in a process similar to how they initially enter the host (through the skin); estimating 7 million symbionts released per 10 dead worms, though this is an interesting figure as later in the paper it is stated that 3g of trophosome in a 20g tubeworm can house 1.11x10^10 symbionts, what is causing this loss? Potentially autolytic enzymes or loss of symbionts that are not able to pass back through the skin of the worm. Furthermore, the symbiont metagenome revealed the capacity for flagellar motility. I wonder if these Endoriftia are able to leave their host pre-death should conditions become unfavourable to them, similar to a coral bleaching event.

Though they live in symbiosis, Endoriftia are also able to live in a planktonic state, but the distance to travel to a new hydrothermal vent must be unachievable for these micro-organisms. A thought of mine is that once these symbionts escape their dead host and establish a free living population, they resume chemosynthesis. This production of organic carbon might attract larger organisms or even their next host to ingest them or otherwise pick them up and carry them to another vent site. Ocean current dispersal is a much more feasible idea however.
In order to acquire the data, special recruitment plates were built with 0.4g tubeworm trophosome at the bottom and 5 glass cover slides stationed at distances away from the flesh to investigate the post-death dispersion of the Endoriftia. The plates were kept under simulated environmental conditions from where the worms were collected from and incubated for up to six days. after the experimental period and then analysed with FiSH and TEM.

We knew before that these symbionts were transmitted horizontally but this paper further shows how and on what magnitude  they are released into the environment once their host dies or can no longer provide for them, giving us insight into the life history of these symbionts and their potential impact on Carbon chemistry of the local water after their host dies.

Reference:

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. PNAS, 112 (36), 11300-11305

Available at: 

http://www.pnas.org.plymouth.idm.oclc.org/content/112/36/11300.full