A trophosome is a highly vascularised organ found in some animals that houses symbiotic bacteria that provide food for their host. Trophosomes are located in the coelomic cavity in the vestimentiferantube worms and in symbiotic flatworms of the genus Paracatenula.
Organization
Initially, the trophosome in frenulates and vestimentiferans had been identified as a mesodermal tissue. The discovery of bacteria inside the trophosomal tissue only occured in 1981 when the ultrastructure of trophosome of several frenulate species and of Sclerolinum brattstromi was studied. The bacteriocytes and symbiontscomposed of 70.5% and 24.1% of the trophosome's volume respectively. Generally, trophosome extends over the entire trunk region between the two longitudinal blood vessels from immediately posterior to the ventral ciliary band of the forepart to the posterior end of the trunk deliniated by the septum between trunk and first opisthosomal segment. The trophosome can be differentiated between anterior and a posterior area due to incremental changes in host tissue organization, the amount of bacteriocytes, the size and shape of symbionts. The trophosome consisted anteriorly of a small number of bacteriocytes and extensive mesenchyma, while the posterior of trophosome subsequently consisted of a large population of bacteriocytes and a peripheral peritoneum.
In frenulates, the trophosome is limited to the post-annular portion of the trunk. While a structural variant of the frenulate trophosome seems to occur, this organ typically consists of two epithelias and blood spaces sandwiched between the basal matrix of the epithelia in which the inner one is composed of bacteriocytes and the outer one is the coelomic lining. The trophosome of Sclerolinum brattstromi consists of a centre of bacteriocytes surrounded by blood space and epithelium.
In vestimentiferans
The trophosome of vestimentiferans is a complex, multi-lobed body with a vascular blood system that covers the entire trunk region. Each lobule consists of a tissue of bacteriocytes enclosed by an aposymbiotic coelothel. It is traversed by an axial efferent blood vessel, and is supplied with ramifying peripheral afferent blood vessels.
In osedax, only female has the trophosome. The trophosome is Osedax is made up of non symbiotic bacteria that reside between the muscle layer of the body's wall and the peritoneum in the ovisac and root regions; therefore, it is derived from the somatic mesoderm.
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 diff?erent 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 transfer of nutrients through small nutritive molecules released by bacteria. The only strong evidence for this hypothesis is the discovery by Felbeck and Jarchow that the distilled symbionts release substantial quantities of succinate and, to a lesser degree, glutamatein vitro, indicating that these could be the main compounds transmitted from the symbionts to the host in vivo.
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 or the vestimentum, to that of other chemoautotrophic symbiotic animals and to that of nonsymbiotic animals known to be adapted to long-term anoxic periods.
Lower oxygen concentration in the trophosome, anaerobic host metabolism, and host ROS-detoxifying enzymes in this tissue would not only protect the symbionts from oxidative damage but also decrease the competition between the host and its symbionts for oxygen.
Symbionts population is controlled by host
To a large degree, symbiont population control may result from symbiont digestion, which effectively prevents symbionts from escaping from their compartments and/or overgrowing the host. The immune system may however be involved in phage protection and symbiont recognition during establishment of the symbiosis.
Communication between Host and Microbe
Eukaryote-like protein structures in the symbiont might be involved in host communication. More than 100 of these symbiont proteins in the trophosome samples, which points to a symbiosis-relevant function. Ankyrin repeats were found to mediate protein-protein interactions. The ankyrin repeat-containing proteins could directly interact with host proteins to modulate endosome maturation and thus to interfere with symbiont digestion by the host.
