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A growing body of work on microbial life in deep subsurface environments has altered our perspective on the limits of living organisms and challenged our understanding of their need for nutrients and energy.  Microbial cells in these very stable and oligotrophic settings apparently catabolize 10-4 to 10-6 fold more slowly than organisms in nutrient-rich cultures and thereby subsist with energy fluxes orders of magnitude below what are considered to be “maintenance” levels.  Such organisms may in fact represent a truly basal state of metabolism, and a corresponding basal power requirement, that is not easily reproduced in culture.   Do these organisms have extraordinary properties beyond our current understanding of microbial energy metabolism, and not represented in cultured organisms, or is the capability to subsist on extremely low energy fluxes an inherent property of many microorganisms?  What are the energetic requirements and limits to life, how are they affected by environment, and how can we characterize them more fully in both natural and culture-based systems?  These questions bear on a physiological state that is pervasive and meaningful in the environment – prokaryotic cells in the terrestrial or marine sub-surface comprise a significant fraction of all living microorganisms on Earth and, at the interface between the inhabited and uninhabited realms of our planet, they represent the ultimate biological arbiters of chemical exchange between the biosphere and the geosphere. Much remains to be done in understanding this physiological state and its consequences and implications for microbial ecology and biogeochemistry.

To address these and many other challenging questions, we organize the 3rd International Workshop on Microbial Life under Extreme Energy Limitation to explore the biological demand for energy, with a specific focus on microorganisms. This workshop follows up on the outcome of an earlier 2nd International Workshop on the same theme which was organized 2012 in Aarhus, Denmark, by Tori Hoehler and Bo Barker Jørgensen. The new understanding of slow microbial life that emerged from that workshop is currently under publication in a comprehensive review authored by the rapporteurs of the working groups and the organizers of the workshop.

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