At several hundred meters below our feet or below the sea floor, the energy flux and the theoretical growth rate of bacteria are orders of magnitude below anything we can understand from research on cultivated microorganisms. Studies of the carbon and energy turnover deep beneath the seafloor and in the terrestrial subsurface indicate that the prokaryotic cells living here subsist at an energy flux that barely allows cell growth over tens to thousands of years. It remains unexplained whether the organisms have properties beyond our current understanding of microbial life and whether these organisms in fact represent the predominant mode of microbial life on our planet – or whether energy sources may be available that have not yet been identified. The limits of microbial life and the exploration of the biological demand for energy is the focus of the International Workshop on Microbial Life under Extreme Energy Limitation (co-sponsored by C-DEBI), held 7-11 September at Sandbjerg Manor near Sønderborg, Denmark. We invite researchers and students from different relevant disciplines to participate in the workshop in order to discuss microbial energy requirements and stimulate new thinking and new approaches. The deadline for abstract submissions is April 1, 2020.