KPL-2:

Dark Energy Ecosystem: Looking through the modern deep ocean to the ancient and the extraterrestrial ones

Takai, Ken^1,2
1Department of Subsurface Geobiological Analysis and Research (D-SUGAR), 2Laboratory of Ocean-Earth Life Evolution Research (OELE), Japan Agency for Marine-Earth Science & Technology (JAMSTEC), Japan

Over the past 35 years, researchers have explored seafloor deep-sea hydrothermal vent environments around the globe and studied
a number of microbial ecosystems there, which is now called as Dark Energy Ecosystems. Bioinformatics and interdisciplinary geochemistry-microbiology approaches have provided new ideas on the diversity and community composition of microbial life living in deep-sea vents. In particular, recent investigations have revealed that the community structure and productivity of chemolithotrophic microbial communities in the deep-sea hydrothermal environments are controlled primarily by variations in the geochemical composition of hydrothermal fluids. This was originally predicted by a thermodynamic calculation of energy yield potential of various chemolithotrophic metabolisms in a simulated hydrothermal mixing zone. The prediction has been finally justified by the relatively quantitative geomicrobiological characterizations in various deep-sea hydrothermal vent environments all over the world. Thus, there should be a possible principle that the thermodynamic estimation of chemolithotrophic energy yield potentials could predict the realistic chemolithotrophic living community in any of the deep-sea hydrothermal vent environments in this planet. Once such a principle is realized, the principle can be applied n
ot only to exploration of extant dark energy ecosystem but also to understanding of the most ancient dark energy ecosystem in the Earth and even the likely extraterrestrial dark energy ecosystems in our solar system.

Thermdynamic calculations of mixing between Hadean seawater and hydrothermal fluid were carried out to predict distribution of mineral precipitates and redox reactions that could occur in Hadean submarine alkaline hydrothermal systems associated with the serpentinization of ultramafic rocks. The modeling indicates that potential mineral precipitates in the mixing zone (chimney structures and hydrothermal deposits) could consist mainly of iron sulfides but also of ferrous serpentine and brucite, siderite, and ferric iron-bearing minerals such as goethite, hematite and/or magnetite as minor phases. The precipitation of ferric iron-bearing minerals suggests that chemical iron oxidation could be caused by pH shift even under anoxic condition. Hydrogenotrophic methanogenesis and acetogenesis – long considered the most ancient forms of biological energy metabolisms – are able to achieve higher maximum energy yield (>0.5 kJ per kg hydrothermal fluid) than those in the modern serpentinization-associated seafloor hydrothermal systems (e.g., Kairei field)
In 2005, a spacecraft Cassini discovered a water vapour jet plume from the sole pole area of the Saturnian moon Enceladus. The chemical composition analyses of Cassini’s mass spectrometer strongly suggested that the Enceladus could host certain extent of extraterrestrial ocean beneath the surface ice sheet and possible ocean-rock hydrothermal systems. An experimental study simulating the reaction between chondritic material and alkaline seawater reveals that the formation of silica nanoparticles requires hydrothermal reaction at high temperatures. Based on these findings, we attempt to build a model of possible hydrothermal fluid/rock reactions and bioavailable energy composition in the mixing zones between the hydrothermal fluid and the seawater in the Enceladus subsurface ocean. The results indicate that the pH of fluid should be highly alkaline and H₂ concentration in the fluid is elevated up to several tens mM through the water/rock reaction. The physical and chemical condition of the extraterrestrial ocean environments points that the abundant bioavailable energy is obtained maximally from redox reactions based on CO₂ and H₂ but not from with other electron accepters such as sulfate and nitrate. Our model strongly suggests that the abundant living ecosystem sustained by hydrogenotrophic methanogensis and acetogenesis using planetary inorganic energy sources should be present in the Enceladus hydrothermal vent systems and the ocean.