2011 年 45 巻 4 号 p. 281-301
Seafloor hydrothermal systems are known to support a variety of biological communities that are sustained by primary production of chemolithoautotrophic microorganisms. The symbiotic and free-living chemolithoautotrophic microorganisms obtain energy from inorganic substances such as H2S, H2, and CH4 derived from hydrothermal vent fluids. Thus, the diversity and abundances of the hydrothermal vent-endemic biological communities are considered to be controlled by chemical compositions of hydrothermal fluids. In order to elucidate biogeochemical relationships between chemolithoautotrophic microbial activities and hydrothermal fluid chemistry in seafloor hydrothermal systems, the amount of metabolic energy available for primary production by chemolithoautotrophic microorganisms is evaluated using geochemical models, and the model results are compared to observed variability in microbial community in seafloor hydrothermal vents. The results of our investigations clearly show that H2 concentartions in hydrothermal fluids have a significant impact on wide range of not only anaerobic but also aerobic reactions. In addition, the concentrations of CH4 and Fe2+ also affect their oxydation reactions. On the other hand, because almost all hydrothermal vent fluids contain sufficient amounts of H2S, a variation in H2S concentration of hydrothermal fluids has essentially no effect on sulfuroxydation reactions, except only under low-temerature conditions less than 25℃. We also present a comparison of potential chemolithoautotrophic microbial activities between modern and possible early Earth's seafloor hydrothermal vents. Assuming that early Earth's seawater had only very low O2-levels, all aerobic reactions can not be available, and availability of some anaerobic reactions using SO4 are confined to low-temerature condisions. In striking contrast, methanogenesis utilizing H2 and CO2 is essentially unaffected by a variation in seawater O2-level, suggesting the importance of hydrogen and hydrogenotrophic methanogenesis for life on early Earth as well as other planets and moons.
