Increasing levels of CO2 in the atmosphere are expected to cause climatic change with negative effects on the earth's ecosystems and human society. Consequently, a variety of CO 2 disposal options are discussed, including injection into the deep ocean. Because the dissolution of CO2 in seawater will decrease ambient pH considerably, negative consequences for deep-water ecosystems have been predicted. Hence, ecosystems associated with natural CO2 reservoirs in the deep sea, and the dynamics of gaseous, liquid, and solid CO 2 in such environments, are of great interest to science and society. We report here a biogeochemical and microbiological characterization of a microbial community inhabiting deep-sea sediments overlying a natural CO 2 lake at the Yonaguni Knoll IV hydrothermal field, southern Okinawa Trough. We found high abundances (>10 9 cm ؊3 ) of microbial cells in sediment pavements above the CO2 lake, decreasing to strikingly low cell numbers (10 7 cm ؊3 ) at the liquid CO2͞CO2-hydrate interface. The key groups in these sediments were as follows: (i) the anaerobic methanotrophic archaea ANME-2c and the Eel-2 group of Deltaproteobacteria and (ii) sulfur-metabolizing chemolithotrophs within the Gamma-and Epsilonproteobacteria. The detection of functional genes related to one-carbon assimilation and the presence of highly 13 C-depleted archaeal and bacterial lipid biomarkers suggest that microorganisms assimilating CO2 and͞or CH4 dominate the liquid CO2 and CO 2-hydrate-bearing sediments. Clearly, the Yonaguni Knoll is an exceptional natural laboratory for the study of consequences of CO 2 disposal as well as of natural CO2 reservoirs as potential microbial habitats on early Earth and other celestial bodies.anaerobic oxidation of methane ͉ chemolithotroph ͉ CO2 disposal ͉ CO 2 hydrate ͉ liquid CO2

[1] We determined the chemical and isotopic compositions of the liquid CO 2 found on Yonaguni IV knoll hydrothermal site, as well as those in hydrothermal fluid venting from the surrounding chimneys. The d 13 C of both CO 2 and CH 4 in the liquid CO 2 almost coincide with those in the hydrothermal fluid, suggesting that the liquid CO 2 must be derived from the hydrothermal fluid. While showing homogeneous d 13 C, the hydrothermal fluids exhibit wide variation in gas contents. Active phase separation must be taking place within the conduits. Besides, H 2 -depletion in the liquid CO 2 suggests formation of solid CO 2 -hydrate must also precede the venting of liquid CO 2 . In conclusion, liquid CO 2 must be produced through following subseafloor processes: phase separation of hydrothermal fluid due to boiling, formation of solid CO 2 -hydrate due to cooling of vapor phase, and melting of the solid CO 2 -hydrate to liquid CO 2 due to a temperature increase within the sedimentary layer.

The Yonaguni Knoll IV hydrothermal vent fi eld (24°51 ′ N, 122°42 ′ E) is located at water depths of 1370 -1385 m near the western edge of the southern Okinawa Trough. During the YK03 -05 and YK04 -05 expeditions using the submersible Shinkai 6500 , both hydrothermal precipitates (sulfi de/sulfate/carbonate) and high temperature fl uids (Tmax = 328°C) presently venting from chimney-mound structures were extensively sampled. The collected venting fl uids had a wide range of chemistry (Cl concentration 376 -635 mmol kg -1 ), which is considered as evidence for sub-seafl oor phase separation. While the Cl-enriched smoky black fl uids were venting from two adjacent chimney-mound structures in the hydrothermal center, the clear transparent fl uids sometimes containing CO 2 droplet were found in the peripheral area of the fi eld. This distribution pattern could be explained by migration of the vapor-rich hydrothermal fl uid within a porous sediment layer after the sub-seafl oor phase separation. The collected hydrothermal precipitates demonstrated a diverse range of mineralization, which can be classifi ed into fi ve groups: (i) anhydrite-rich chimneys, immature precipitates including sulfi de disseminations in anhydrite; (ii) massive Zn-Pb-Cu sulfi des, consisting of sphalerite, wurtzite, galena, chalcopyrite, pyrite, and marcasite; (iii) Ba-As chimneys, composed of barite with sulfi de disseminations, sometimes associated with realgar and orpiment overgrowth; (iv) Mn-rich chimneys, consisting of carbonates (calcite and magnesite) and sulfi des (sphalerite, galena, chalcopyrite, alabandite, and minor amount of tennantite and enargite); and (v) pavement, silicifi ed sediment including abundant native sulfur or barite. Sulfi de/sulfate mineralization (groups i -iii) was found in the chimney -mound structure associated with vapor-loss (Cl-enriched) fl uid venting. In contrast, the sulfi de/carbonate mineralization (group iv) was specifi cally found in the chimneys where vapor-rich (Cl-depleted) fl uid venting is expected, and the pavement (group v) was associated with diffusive venting from the seafl oor sediment. This correspondence strongly suggests that the subseafl oor phase separation plays an important role in the diverse range of mineralization in the Yonaguni IV fi eld. The observed sulfide mineral assemblage was consistent with the sulfur fugacity calculated from the