Microbial community in a sediment-hosted CO sub(2) lake of the southern Okinawa Trough hydrothermal system

Increasing levels of CO sub(2) 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 sub(2) disposal options are discussed, including injection into the deep ocean. Because the dissolution of CO sub(2) in seawater will decrease ambient pH considerably, negative consequences for deep-water ecosystems have been predicted. Hence, ecosystems associated with natural CO sub(2) reservoirs in the deep sea, and the dynamics of gaseous, liquid, and solid CO sub(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 sub(2) lake at the Yonaguni Knoll IV hydrothermal field, southern Okinawa Trough. We found high abundances (>10 super(9) cm super(-3)) of microbial cells in sediment pavements above the CO sub(2) lake, decreasing to strikingly low cell numbers (10 super(7) cm super(-3)) at the liquid CO sub(2)/CO sub(2)-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 super(13)C-depleted archaeal and bacterial lipid biomarkers suggest that microorganisms assimilating CO sub(2) and/or CH sub(4) dominate the liquid CO sub(2) and CO sub(2)-hydrate-bearing sediments. Clearly, the Yonaguni Knoll is an exceptional natural laboratory for the study of consequences of CO sub(2) disposal as well as of natural CO sub(2) reservoirs as potential microbial habitats on early Earth and other celestial bodies.