Gutless flatworm host unique chemosynthetic bacteria

The marine flatworm Paracatenula has an obligate symbiosis with Candidatus Riegeria, a chemoautotrophic bacterium, due to its lack of mouth and gut. Paracatenula is completely dependent on its intracellular endosymbiont for nutrition, this symbiosis has been vertically transmitted for around 500 million years resulting in a highly specialised relationship.

Using techniques including CARD-FISH, gene expression and image analysis, this study investigated the genome and mechanisms of Ca. Riegeria to elucidate the physiology and evolution of the symbiosis.

They found that Ca. R. has a genome about a third of the size of its free living relatives, this reduced genome included specialised energy efficient pathways for essential functions such as sulphide and carbon fixation which were linked to its Rhodospirillaceae ancestry. Ca. R. provides energy storage for the host flatworm, accounting for up to a third to half of Paracatenula’s biomass, unusually for a chemosynthetic symbiosis, nutrition is secreted to the host via outer membrane vesicle (OMV). 

This paper intricately and eloquently examined the relationship between Paracatenula and Ca. R., their findings were complex and interesting. Combining the results and discussion helped breakdown key findings and comment on individual facets of their work.

Jäckle, O., Seah, B.K.B., Tietjen, M., Leisch, N., Liebeke, M., Kleiner, M., Berg, J.S., Gruber-Vodicka, H.R. (2019) Chemosynthetic symbiont with a drastically reduced genome serves as primary energy storage in the marine flatworm Paracatenula. Proceedings of the National Academy of Sciences, 116(7), pp 8505-8514.