Microbial community in a sediment-hosted CO₂ lake of the southern Okinawa Trough hydrothermal system

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 CO₂ 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 CO₂ 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 CO₂ lake at the Yonaguni Knoll IV hydrothermal field, southern Okinawa Trough. We found high abundances (>10⁹ cm^-3) of microbial cells in sediment pavements above the CO₂ lake, decreasing to strikingly low cell numbers (10⁷ cm^-3) at the liquid CO₂/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 ¹³C-depleted archaeal and bacterial lipid biomarkers suggest that microorganisms assimilating CO₂ and/or CH₄ dominate the liquid CO₂ and CO₂-hydrate-bearing sediments. Clearly, the Yonaguni Knoll is an exceptional natural laboratory for the study of consequences of CO₂ disposal as well as of natural CO₂ reservoirs as potential microbial habitats on early Earth and other celestial bodies.