Different trophic requirement in the host-symbiont interactions of both Calyptogena clams and Bathymodiolus mussels based on nitrogen stable isotopic composition of amino acids

We investigated trophic hierarchy in the host-symbiont interaction for Calyptogena clams and Bathymodiolus mussels by estimating the trophic position (TP) using stable nitrogen isotopic composition of amino acids to understand transfer and metabolic flow of amino acids in the host-symbiont interaction. The Estimation of TP based on the isotopic composition of amino acids has recently been used as an important and useful tool to understand the prey-predator interaction among various marine organisms, which will be applicable for chemotrophic food webs with the host-symbiont interaction.
In this method, the TP is estimated based one different 15N-enrichment between two common amino acids, glutamic acid and phenylalanine, because the isotope ratio of glutamic acid is significantly increased with a shift of the TP but that of phenylalanine is slightly increased and show almost the same value to that of the primary producers at the bases of food web. The TP of organisms is therefore calculated according to the formula: TPGlu/Phe={(δ15NGlu-δ15NPhe-3.4)/7.6}+1 (Chikaraishi et al. 2009). Unlike the estimation of TP based on traditional bulk nitrogen and carbon isotope analyses, this new method allows more accurate estimation of the TP without characterizing the adequate or mean isotopic composition of the primary producers.
In this study, we elucidated interspecific relationships in the TP for two representative host-symbiont interactions in Calyptogena clams and Bathymodiolus mussels. In the case of Calyptogena clams, TP is increased by ~1.2 from symbiotic bacteria to muscle. It therefore seems that the host Calyptogena (muscle) feeds on symbiotic bacteria like a grazing prey-predator interaction, although digestions of bacteria was not observed so far. On the other hand, surprisingly, Bathymodiolus mussels have the almost same TP values (~1.3) for all tissues examined including the host Bathymodiolus (muscle), the symbiont-bearing gills, and even the symbiotic bacteria.
We also found different signatures in the bulk nitrogen and carbon isotopic compositions between these organisms that significant 15N- but little 13C-enrichments occurred in Calyptogena clams but little 15N- and significant 13C-enrichments occurred in Bathymodiolus mussels. Thus Bathymodiolus and Calyptogena clearly had difference in strategy of amino acid assimilation and mechanisms including different metabolic flow rate, associated isotopic fractionation between symbiotic bacteria and host organisms, and acquisition of nutrients and energy in the interaction with symbiotic bacteria.

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First Name: 
Masashi
Last Name: 
Tsuchiya
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+81-46-867-9793
Affiliation: 
Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
First Name: 
Takao
Last Name: 
Yoshida
Affiliation: 
Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
First Name: 
Yoshito
Last Name: 
Chikaraishi
Affiliation: 
Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
First Name: 
Nanako
Last Name: 
O. Ogawa
Affiliation: 
Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
First Name: 
Yuki
Last Name: 
Hongo
Affiliation: 
Tokyo University of Marine Science and Technology/Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
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Trophic relations (including symbiosis)
Abstract ID: 
CBE5-113