ABSTRACT

A mussel, clams, vestimentiferan tube worms and squat lobsters from vent-communities at three volcanically active sites, the Kaikata Caldera of Ogasawara Arc and the Minami-Ensei Knoll and the eastern Iheya Ridge of Okinawa Trough, were analyzed for the concentration and isotopic ratio of sulfur in their soft tissues. The results indicate that the mussel, clams and tube worms are nutritionally supported by endosymbiosis with chemolithotrophic sulfur-oxidizing bacteria which utilize hydrogen sulfide in venting fluid as energy source for organic carbon synthesis, while the squat lobsters live on the chemosynthetic food web. However, the sulfur isotopic ratios of these animals range from -10 to -26‰, much lower than the volcanic sulfur values of these areas (0∼+10‰). This implies that in all the three sites, hydrogen sulfide produced by sulfate-reducing bacteria is an important sulfur source for vent communities. The sulfur flux required for the biological communities is an order of magnitude or two greater than that encountered in normal organic-rich marine sediments. It is suggested that the high heat flow, porous volcanogenic sediments and organic materials accumulated by biological communities play important roles in maintaining high production rate of microbial hydrogen sulfide and its efficient transport to surface in volcanically active seafloor.