ABSTRACT

Several icy moons of the outer solar system carry subsurface oceans containing many times the volume of liquid water on Earth and may provide the greatest volume of habitable space in our solar system. Functional sub-ice polar ecosystems on Earth provide compelling models for the habitability of extraterrestrial sub-ice oceans. A key feature of sub-ice environments is that most of them receive little to no solar energy. Consequently, organisms inhabiting these environments must rely on chemical energy to assimilate either carbon dioxide or organic molecules to support their metabolism. Methane can be utilized by certain bacteria as both a carbon and energy source. Isotopic data show that methane in Earth’s polar lakes is derived from both biogenic and thermogenic sources. Thermogenic sources of methane in the thermokarst lakes of the north slope of Alaska yield supersaturated water columns during winter ice cover that support active populations of methanotrophs during the polar night. Methane in the permanently ice-covered lakes of the McMurdo Dry Valleys, Antarctica varies widely in concentration and is produced either by contemporary methanogenesis or is a relic from subglacial flow. Rate measurements revealed that microbial methane oxidation occurs beneath the ice in both the arctic and Antarctic lakes. The first samples collected from an Antarctic subglacial environment beneath 800 m of ice (Subglacial Lake Whillans) revealed an active microbial ecosystem that has been isolated from the atmosphere for many thousands of years. The sediments of Lake Whillans contained high levels of methane with an isotopic signature that indicates it was produced via methanogenesis. The source of this methane appears to be from the decomposition of organic carbon deposited when this region of Antarctica was covered by the sea. Collectively, data from these sub-ice environments show that methane transformations play a key role in microbial community metabolism. The discovery of functional microbial ecosystems in Earth’s sub-ice aquatic environments together with what we know about the geochemistry of extraterrestrial ice-covered water worlds provide a compelling case that sub-ice oceans, such as those on Europa and Enceladus, may support microbial life.

About the Speaker:

John Priscu is a leading expert on cold climate biology. He has spent 30 field seasons in Antarctica conducting research on life under ice-shelves, the southern ocean, sea ice, permanently ice covered lakes, and life beneath the vast Antarctic ice sheet. Priscu has taken more than 200 undergraduate, graduate and postgraduate students to Antarctica and the arctic as part of his research efforts during this period. His work also includes the study of life on other icy worlds such as Mars, Europa, Enceladus and Titan and was a principal investigator on a NASA Astrobiology Institute “The Astrobiology of Icy Worlds” and a NASA project “Endurance” that tested the use of robots to examine life under the ice on these worlds. The field work for these NASA projects focused on both poles of our planet. He has published more than 200 scholarly articles, 2 books, and has served on numerous national and international committees, including the National Academy of Sciences Committee on the Origin and Evolution of Life. He led the first winter expeditions to study the lakes of the McMurdo Dry Valleys. Priscu is chief scientist on a recently funded 5 year project to explore the habitat beneath a portion of the West Antarctic Ice Sheet. Antarctica has more than 400 lakes buried thousands of meters below ice. Priscu, a Fellow of the American Association for the Advancement of Science and a Fellow of the American Geophysical Union, has received numerous awards for his research, including a valley and a stream in Antarctica named after him, the Goldwaithe Medal for his work on polar ice sheets, and the International Medal for Scientific Excellence from the Scientific Committee on Antarctic Research. He is currently a Professor of Ecology at Montana State University-Bozeman.