Hydrothermal vents: Tubeworms and the uptake of their symbionts.

   Microbes play a particularly large role within the ecosystems of deep sea hydrothermal vents and cold seeps, mostly as symbionts which can utilise chemosynthesis as a way of deriving energy from the minerals and compounds that are projected from black smokers, or from the water around cold seeps. There have been many studies conducted which focus primarily upon the bacteria within the bacteriocytes of the organism, the diversity of these bacteria and the role they play within the organism.

This paper focuses more upon how the endosymbionts of Vestimentiferan worms (Hydrothermal vent tube worms) are taken into the host organism as it has been found that the larval stages of these worms do not contain the symbiotic microbes that the adult stages do. Instead the larval stages feature a buccal cavity, transient gut and terminal anus. When the larvae do become infected with the bacteria found within the trophosomes of the adult stages, the bacteria causes a metamorphosis within the animal, namely apoptosis of the skin cells and any other cells not essential to the new form of the worm.

One hypothesis that has been put forward to explain how the larvae become infected by the bacteria is that the larva graze upon the bacteria and the gut becomes the trophosome. The main problem with this theory is that the bacteria must first avoid being digested by the organism which, unless the digestive tract has highly specific enzymes that do not target the bacteria, is particularly difficult.

The alternative hypothesis that the author proposes is that infection occurs through a symbiont specific selection process across the epidermal cells of the larvae, the bacteria then migrate further down into the mesodermal tissue which the develops into the trophosome. This hypothesis makes more sense as it is more feasible that the larvae have some way of regulating the uptake of the bacteria, for example via extracellular proteins which would allow the active transport of the bacteria through the cell membrane. The hypothesis put forward by this paper is supported by the observations made by the authors, namely that the bacteriocytes that had developed were located in the dorsal mesentery between the dorsal blood vessel and the foregut. They also found that none of the symbiotic bacteria were found within the digestive system.

This paper has refined an otherwise feasible explanation for how the Vestimentiferan worms that were studied take up their symbiotic bacteria. Although further research is still needed in order to correctly define the nature of the selective symbiont uptake and kits mechanisms this paper has made steps to help lay the groundwork for such research.  

Nussbaumer AD., Fisher CR., Bright M., 2006, Horizontal endosymbiotic transmission in hydrothermal vent tubeworms, Nature, 441, 345-348