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

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Abstract
Increasing levels of CO 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 2 disposal options are discussed, including injection into the deep ocean. Because the dissolution of CO 2 in seawater will decrease ambient pH considerably, negative consequences for deep-water ecosystems have been predicted. Hence, ecosystems associated with natural CO 2 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 CO 2 lake, decreasing to strikingly low cell numbers (10 7 cm −3 ) at the liquid CO 2 /CO 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 13 C-depleted archaeal and bacterial lipid biomarkers suggest that microorganisms assimilating CO 2 and/or CH 4 dominate the liquid CO 2 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 CO 2 reservoirs as potential microbial habitats on early Earth and other celestial bodies.