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Calorimetric measurements of heat flow and total heat prodn. can inform on the energetics of microbial activity. Nanocalorimetry permits such investigations in environments (natural or lab.) where energy supplies are extremely limited and/or very few cells are metabolically active. We will present nanocalorimetry data obtained during incubations of unamended formation fluids from the oceanic crust (∼300 m deep at the Juan de Fuca Ridge flank) and the continental subsurface (∼1500 m deep at the Sanford Underground Research Facility in South Dakota). Enthalpic responses by the resident microbial communities to addn. of org. carbon, inorg. electron donors, and nutrients will also be discussed. We further used nanocalorimetry on lab. expts. with model organisms to det. the energetics assocd. with carbon, electron donor, and nutrient limitations. For example, with a sulfate reducing bacterium, we quantified the significant cost during the metabolic transition from ammonium assimilation to N_2 fixation. In another set of expts., with an evolving population of E. coli mutants that express the GASP (growth advantage in stationary phase) phenotype, we recorded distinct heat events assocd. with the emergence of nascent mutant populations.