Host-Microbe Interactions in the ChemosyntheticRiftiapachyptilaSymbiosis

The deep-sea tubewormRiftia pachyptilalacks a digestive system butcompletely relies on bacterial endosymbionts for nutrition. Although the symbionthas been studied in detail on the molecular level, such analyses were unavailable forthe animal host, because sequence information was lacking. To identify host-symbiont interaction mechanisms, we therefore sequenced theRiftiatranscriptome,which served as a basis for comparative metaproteomic analyses of symbiont-containing versus symbiont-free tissues, both under energy-rich and energy-limitedconditions. Our results suggest that metabolic interactions include nutrient alloca-tion from symbiont to host by symbiont digestion and substrate transfer to the sym-biont by abundant host proteins. We furthermore propose thatRiftiamaintains itssymbiont by protecting the bacteria from oxidative damage while also exerting sym-biont population control. Eukaryote-like symbiont proteins might facilitate intracellu-lar symbiont persistence. Energy limitation apparently leads to reduced symbiontbiomass and increased symbiont digestion. Our study provides unprecedented in-sights into host-microbe interactions that shape this highly efficient symbiosis