Symbiotic adaptation to hadal environment -Metabolic functions of intracellular symbions from the hadal thyasirid clam Marithyas hadalis-

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The thyasrid clam Maorithyas hadalis is a conspicuous species in seep environments at depths of 7, 200-7,400 meters in the Japan Trench. Two distinct phylotypes of endosymbiotic bacteria were shown and spatial partitioning between the phylotypes was observed within the gill tissues.The purpose of this study was to clarify the metabolic functions of the symbionts of M. hadalis and to understand the adaptation mechanism of the thyasirid clam to its hadal reducing environment. In 2005, detection of genes regarding sulfur oxidation was conducted from M. hadalis gills. The specific primers for PCR amplification of SoxB gene of symbionts from the vesicomyid clam Calyptogena okutanii were designed because of the similarity of 16S ribosomal RNA gene sequences of between the M. hadalis and C. okutanii symbionts. In 2007, SoxB gene was successfully amplified from the total extracted DNA of M. hadalis gills using the specific primers. Therefore, the sulfur oxidizing capability of one of the symbionts was supported by not only phylogeny but also metabolism.For understanding the microdistribution of the SoxB gene in the host tissue, fluorescence in situ hybridization experiments were conducted in 2006. To determine the experimental conditions, the whale-fall mussel Adipicola pacifica was applied because flesh individuals of the mussel were easily available from our aquaria. The optimum conditions were set for fixation time and fixative. Flesh specimens were required for FISH experiments but there was no chance to collect new samples because the ROV Kaiko, which was the only ROV to reach the M. hadalis habitat, had been lost. Developments of new deep-diving ROV can be dissolve this problem.This study showed the metabolic function regarding sulfur oxidation in the M. hadalis symbionts. It is important to recognize the wide distribution range of chemosynthetic symbioses between invertebrates and bacteria for understanding of their biogeography and evolution.