Successful Maintenance of Chemosynthetic Symbiotic Bacterial Abundance in Hydrothermal Mussels During Long-Term Rearing Experiments Exceeding 1000 Days

Symbiosis with chemosynthetic bacteria is a biological phenomenon that has enabled animals to adapt to deep-sea environments. The deep-sea vent mussel <i>Bathymodiolus septemdierum</i> harbors sulfur-oxidizing symbionts in its gills, which serve as its main source of nutrients. However, the establishment and maintenance of this symbiosis process remain poorly understood, partly because culturing deep-sea mussels and their symbionts is difficult and experimental studies are rare. In the present study, we aimed to establish a rearing method for <i>B. septemdierum</i> and investigate the effects of sodium sulfide (Na2S) addition and gas concentration control in rearing tanks. Three tanks were prepared: a normal tank (tank N), one with Na2S (tank S), and one with Na2S and control dissolved oxygen (tank SO). Mussels were maintained for more than 1000 days, and host survival, gill histology, and the abundance of symbionts were investigated. In tanks N and S, symbionts were depleted within a short period, whereas in tank SO, gill morphology and symbiont abundance were maintained at levels comparable to those of freshly collected individuals, even after 1000 days. However, the survival rate of the host did not correspond to symbiont maintenance. Phylogenetic analysis revealed that the symbiont RuBisCO exists in Form II, which is generally adapted to low-O2 and high-CO2 environments. These results suggest that the long-term maintenance of <i>B. septemdierum</i> symbionts is possible by adding Na2S and controlling the dissolved gas concentration. Further improvements in rearing methods could contribute to a deeper understanding of host-symbiont interactions in this unique deep-sea organism.