Trophosome

Initially, the trophosome in frenulates and vestimentiferans, which are now classified as members of the Siboglinidae, had been identified as a mesodermal tissue. The discovery of bacteria inside the trophosomal tissue only occurred in 1981 when the ultrastructure of trophosome of several frenulate species and of Sclerolinum brattstromi was studied. The bacteriocytes and symbionts composed of 70.5% and 24.1% of the trophosome's volume respectively. Symbionts were often embedded separately in the symbiosome membrane adjacent to the bacterial cell wall except when they are proliferating. In frenulates In frenulates, the trophosome is limited to the post-annular portion of the trunk. == Trophosome color ==

The host lacks entirely a digestive system but derives all the essential nutrients from its endosymbiont . The host in turn provides the endosymbiont with all necessary inorganic compounds for chemolithoautotrophy. Inorganic elements, such as hydrogen sulphide, are oxidized by bacteria to produce energy for carbon fixation. Therefore, the energetic state of the symbiosis can be specifically interpreted from the color of the trophosome. == Trophosome growth ==

Trophosome tissue development happens by stem cells in the center of each lobule, contributing to new lobules as well as the regeneration of bacteriocytes circulating from the center to the periphery of each lobule through which apoptosis happens. The trophosome tissue thus not only shows high levels of proliferation but also fairly small levels of apoptosis. Furthermore, symbionts in the periphery are constantly digested and replaced by separating symbionts in the middle. Lysophosphatidylethanolamines and free fatty acids are the products of phospholipid hydrolysis by phospholipases through the normal degradation of the membranes. The presence of fairly high levels of lysophosphatidylethanolamines and fatty acids in trophosome indicate the high turnover of host and symbiont cells in the trophosome contributing to tissue and membrane degradation. == Chemolithoautotrophy ==

In both these animals, the symbiotic bacteria that live in the trophosome oxidize sulfur or sulfide found in the worm's environment and produce organic molecules by carbon dioxide fixation that the hosts can use for nutrition and as an energy source. This process is known as chemosynthesis or chemolithoautotrophy. Carbon transfer Two different modes of carbon transfer from the symbionts to the host have been suggested. • The transfer of nutrients through digestion of bacteria. This model is supported by the ultrastructural studies of the trophosome showing symbionts in various stages of lysis. The only strong evidence for this hypothesis is the discovery by Felbeck and Jarchow (1998) that the distilled symbionts release substantial quantities of succinate and, to a lesser degree, glutamate in vitro, indicating that these could be the main compounds transmitted from the symbionts to the host in vivo. Furthermore, rhodanese, APSreductase, and ATP-sulfurylase are involved in adenosine triphosphate synthesis using the energy found in sulfur compounds such as hydrogen sulphide. These findings contribute to the conclusion that the symbiont of R. pachyptila is capable of producing ATP by means of sulfide oxidation, and that ATP energy could be used to fix carbon dioxide. == Glycogen storage in trophosome ==

In Riftia pachyptila, the glycogen content of 100 μmol glycosyl units g−1 fresh wt determined in the trophosome is divided equally between host and symbionts. Although the symbionts take up only 25% of the trophosome, glycogen content is distributed equally between both partners, and this ratio remains similar for up to 40 h of hypoxia. Thus, host and symbiont each contain about 50 μmol glycosyl units g−1 fresh wt of trophosome. This amount is comparable to that in other host tissues of R. pachyptila, e.g. in the body wall (35 μmol glycosyl units g−1 fresh wt) or the vestimentum (20 μmol glycosyl units g−1 fresh wt), to that of other chemoautotrophic symbiotic animals and to that of nonsymbiotic animals known to be adapted to long-term anoxic periods. == Host-microbe interaction ==

Protection against oxidative damage Higher concentration of oxygen in the trophosome, (partial) anaerobic metabolism of the host, and host ROS-detoxifying enzymes in this tissue will not only shield the symbionts from oxidative damage but also minimize competition between the host and its oxygen symbionts. Symbiont population control Symbiont population control can be largely the result of symbiont digestion, which essentially prevents symbionts from escaping from their compartments and/or overgrowing the host. The ankyrin repeat proteins could interact directly with the host proteins in order to modulate endosome maturation and interfere with host symbiont digestion. == See also ==