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In some thiotrophic symbioses the digestive system is completely reduced, for example, in siboglinid tubeworms and gutless oligochaetes (see Dubilier et al., 2008). Here, the entire food should come from the symbiont. In other systems the digestive system still functions, additionally allowing for “normal” feeding. The microzooids in ''Z. niveum'' also have a functioning digestive system (Bauer-Nebelsick et al., 1996a,b). The degree to which host nourishment depends on symbionts or ingested prey has not been studied in any system yet. However, cultivation experiments in ''Z. niveum'' show that host fitness (measured as host growth and life span) was considerably decreased when symbionts were forced to defect. ''Cand''. Thiobios zoothamnicoli could not fix carbon under normoxic culture conditions without sulfide (Rinke et al., 2007). The only means of nourishment left for the host were symbiont digestion and food uptake from the surrounding seawater. This indicates that a considerable portion of food comes from the symbionts.
 
In some thiotrophic symbioses the digestive system is completely reduced, for example, in siboglinid tubeworms and gutless oligochaetes (see Dubilier et al., 2008). Here, the entire food should come from the symbiont. In other systems the digestive system still functions, additionally allowing for “normal” feeding. The microzooids in ''Z. niveum'' also have a functioning digestive system (Bauer-Nebelsick et al., 1996a,b). The degree to which host nourishment depends on symbionts or ingested prey has not been studied in any system yet. However, cultivation experiments in ''Z. niveum'' show that host fitness (measured as host growth and life span) was considerably decreased when symbionts were forced to defect. ''Cand''. Thiobios zoothamnicoli could not fix carbon under normoxic culture conditions without sulfide (Rinke et al., 2007). The only means of nourishment left for the host were symbiont digestion and food uptake from the surrounding seawater. This indicates that a considerable portion of food comes from the symbionts.
  
Sulfide is highly toxic to aerobic eukaryotes (National Research Council, 1979). It inhibits cytochrome c oxidase, the eukaryote terminal enzyme of the mitochondrial electron transport chain (Dorman et al., 2002). Accordingly, the hosts of thiotrophic symbionts are challenged in providing their symbionts with sulfide while at the same time avoiding poisoning. Detoxification of sulfide through uptake and oxidation by symbionts has been proposed several times (Somero et al., 1989). Short incubations with Na<sup><small>235</small></sup>S and autoradiographic analysis in the stilbonematid ''Eubostrichus dianae'' showed that most uptake was in the thiotrophic ectosymbionts (Powell et al., 1979). Future studies are urgently needed using aposymbiotic hosts exposed to sulfide in order to determine whether symbiont presence (with their sulfide oxidation capabilities) affects host fitness.
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Sulfide is highly toxic to aerobic eukaryotes (National Research Council, 1979). It inhibits cytochrome c oxidase, the eukaryote terminal enzyme of the mitochondrial electron transport chain (Dorman et al., 2002). Accordingly, the hosts of thiotrophic symbionts are challenged in providing their symbionts with sulfide while at the same time avoiding poisoning. Detoxification of sulfide through uptake and oxidation by symbionts has been proposed several times (Somero et al., 1989). Short incubations with Na<sup><small>235</small></sup>S and autoradiographic analysis in the stilbonematid Eubostrichus dianae showed that most uptake was in the thiotrophic ectosymbionts (Powell et al., 1979). Future studies are urgently needed using aposymbiotic hosts exposed to sulfide in order to determine whether symbiont presence (with their sulfide oxidation capabilities) affects host fitness.
  
 
Access to oxygen and sulfide for thiotrophic ectosymbionts is generally facilitated by the host’s behavior (Ott et al., 2004). Migrations through the chemocline in sediments have been reported in the ciliate Kentrophoros ssp. (Fenchel and Finlay, 1989), the stilbonematin nematodes (Ott et al., 1991) and the gutless oligochaetes (Giere, 1992). Polz et al. (1999, 2000) observed the shrimp Rimicaris exoculata swimming in and out of hydrothermal vent fluid as well as ventilation of the chamber in which its symbionts reside. In ''Z. niveum'', the host contracts and expands continuously, facilitating switches between sulfidic and oxygenated seawater (Ott et al., 1998). The symbionts on the host’s surface were suggested to overcome the diffusion limitations of their substrate supply by two processes: feeding currents generated by the host, and the pulsed advection of sulfidic seawater from the peat caused by interactions of the boundary layer flow with groups of ciliates (Vopel et al., 2005). Interestingly, all the symbionts exposed to the feeding currents are larger and coccoid in shape, while the symbionts on the other host part are less favored and thus remain smaller and rod-shaped (Rinke et al., 2007). This emphasizes the importance of host-generated ciliary currents.
 
Access to oxygen and sulfide for thiotrophic ectosymbionts is generally facilitated by the host’s behavior (Ott et al., 2004). Migrations through the chemocline in sediments have been reported in the ciliate Kentrophoros ssp. (Fenchel and Finlay, 1989), the stilbonematin nematodes (Ott et al., 1991) and the gutless oligochaetes (Giere, 1992). Polz et al. (1999, 2000) observed the shrimp Rimicaris exoculata swimming in and out of hydrothermal vent fluid as well as ventilation of the chamber in which its symbionts reside. In ''Z. niveum'', the host contracts and expands continuously, facilitating switches between sulfidic and oxygenated seawater (Ott et al., 1998). The symbionts on the host’s surface were suggested to overcome the diffusion limitations of their substrate supply by two processes: feeding currents generated by the host, and the pulsed advection of sulfidic seawater from the peat caused by interactions of the boundary layer flow with groups of ciliates (Vopel et al., 2005). Interestingly, all the symbionts exposed to the feeding currents are larger and coccoid in shape, while the symbionts on the other host part are less favored and thus remain smaller and rod-shaped (Rinke et al., 2007). This emphasizes the importance of host-generated ciliary currents.

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