Other Invertebrate Hosts

While this chapter focuses on the association between bathymodioline mussels and their endosymbionts, because those are the best understood of methane-based symbioses, the two other known examples, a pogonophoran tubeworm and a carnivorous sponge, illustrate the diversity of invertebrate taxa that can harbor methanotrophs.

The pogonophoran Siboglinum poseidoni from methane-rich reducing sediments is the only tubeworm known to host methanotrophic endosymbionts (Schmaljohann 1991). Siboglinum poseidoni is abundant at methane- and sulfide-rich sites of the Skagerrak basin (~200-400 m depth) off the coast of Denmark (Dando et al. 1994). Endosymbiosis of methanotrophs in the pogonophoran tissue was detected through TEM showing symbionts containing stacked intracellular membranes typical of type I methanotrophs (Schmaljohann and Fl├╝gel 1987), enzyme assays indicating methanol dehydrogenase and hexulosephosphate synthetase activity, 14CH4 uptake experiments and stable carbon isotope analysis showing that pogonophoran cell carbon was derived from biogenic methane (Schmaljohann et al. 1990). No genetic analysis was performed, precluding any phylogenetic comparison with other methanotrophs.

Fig. 2. Representative taxa involved in the mussel-chemosynthetic bacteria symbioses. A Bathymodiolus childressi mussels. B and C Transmission electron micrographs of B. heckerae gill epithelial tissue (from Cavanaugh et al. 1987). B Transverse section of host cells containing symbionts. Scale bar 5 |m. C Higher magnification of methanotropic (M) and chemoautotrophic (C) symbionts. Scale bar 0.3 |m

Fig. 2. Representative taxa involved in the mussel-chemosynthetic bacteria symbioses. A Bathymodiolus childressi mussels. B and C Transmission electron micrographs of B. heckerae gill epithelial tissue (from Cavanaugh et al. 1987). B Transverse section of host cells containing symbionts. Scale bar 5 |m. C Higher magnification of methanotropic (M) and chemoautotrophic (C) symbionts. Scale bar 0.3 |m

Methanotrophic bacteria are also found as endosymbionts associated with carnivorous sponges in the genus Cladorhiza (family Cladorhizidae), which inhabit deep-sea mud volcanoes near the Barbados accretionary prism (Vacelet et al. 1996). The presence of extracellular methanotrophs within the tissue of the sponge was revealed by TEM, with methanol dehydrogenase activity and 813C values consistent with the incorporation of carbon from biogenic methane into the sponge. As in the case of the pogonophoran, no genetic analysis was performed to determine the relationship among cladorhizid symbionts and other methanotrophs.

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