化学合成がうみだすもうひとつの深海生態系 化学合成共生システム‐ベントスと共生細菌との密接な関係

Abstract

Endosymbiotic associations between invertebrates and chemosynthetic bacteria are common in deep-sea hydrothermal vent and cold seep environments. Since characterization of the endosymbionts has been difficult due to their resistance to cultivation once removed from their hosts, the chemotrophic endosymbioses were initially confirmed by electron microscopic observation, stable isotope analysis, lipid analysis, and enzymatic characterization. They were classified into two types: one using sulfide, oxygen, and carbon dioxide as an electron donor, acceptor, and carbon source, respectively; another using methane as an energy and carbon source. Molecular phylogenetic analysis based on 16S ribosomal RNA gene sequences have revealed that most host species contain a single phylotype of endosymbionts belonging to a limited domain of the γ-subdivision of Proteobacteria despite the wide taxonomic range of hosts. Most symbionts are characterized as thioautotrophs or methanotrophs. Symbiont transmission mechanisms have been reported in several host taxa. Vertical transmission has been shown in vesicomyid clams and deep-sea mussels. Evidence is accumulating to suggest environmental transmission in vestimentiferans and lucinid clams. In the case of solemyid clams, several lines of evidence seem to conflict. A hypothesis is suggested that bacterial chemotrophic endosymbionts influence host distributions in deep-sea chemosynthetic ecosystems. The distribution of methanotrophic symbioses between deep-sea mussels and their symbionts correlates with high methane concentrations in deep-sea chemosynthetic ecosystems. A thyasirid clam living in the deepest yet known chemosynthetic environment has an unusual intracellular dual symbiosis, which is likely essential for the survival of host bivalves at great depth in reduced environments. These results indicate that bacterial chemotrophic endosymbionts influence host distributions in deep-sea chemosynthetic ecosystems.