Re-evaluation of nutrient sources for deep-sea wood-boring bivalves using the isotopic composition of bulk C, N, S, and amino acid nitrogen

Wood-boring bivalves (families Xylophagaidae and Teredinidae) are unique bivalves that are peculiarly adapted to feed on terrigenous woody materials, even though they inhabit the deep-sea floor far from land. Previous studies of their metabolic processes suggested the importance of symbiotic microbes that secrete cellulase to access woody carbon (carbohydrates), and supply organic nitrogen via nitrogen fixation. Since nitrogen is generally depleted in woody materials, dissolved dinitrogen in seawater has been proposed as a plausible nitrogen source for wood-boring bivalves. We evaluated the food ecology of wood-boring bivalves (genus: Xyloredo) obtained from the deep seafloor off the Ryukyu Islands by considering their bulk carbon, nitrogen, and sulfur isotope composition and potential dietary sources (i.e. logs, particulate organic matter, and surface sediments). We also investigated the trophic interactions between wood-boring bivalves and logs based on the amino acid nitrogen isotopic composition. The bulk isotope data revealed that in wood-boring bivalves these elements are derived mainly from the logs in which they live. These results were consistent with the trophic hierarchy calculated from the nitrogen isotopic composition of amino acids. Ecologically, wood-boring bivalves are one step higher than logs in terms of their trophic position. Based on these data, we propose that terrigenous woody materials are the major dietary sources for wood-boring bivalves and the same standard trophic interaction exists between diet and consumer species as in the grazing food web. The symbionts may aid the digestion of woody materials, but they do not supply nitrogen via nitrogen fixation in this case.