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The Thermodynamics of Chemosynthetic Life Tom McCollom Vast majority of life on Earth derives energy (directly or indirectly) from sunlight (i.e., photosynthesis) but … unlikely energy source for very earliest organisms on Earth or on Mars & Europa earliest organisms probably relied on chemical sources of metabolic energy (“chemosynthesis”) “Chemosynthetic” organisms (more properly chemolithoautotrophs): derive metabolic energy from inorganic chemical reactions & produce biomass from inorganic known for well over a century compounds are ubiquitous on Earth and live in a wide variety of habitats (e.g., marine sediments, cow stomachs, sewage systems) most involved indirectly in decay of photosynthetic organic matter Chemical energy for autotrophic life Chemical energy sources involve electron exchange reactions (a.k.a. oxidation-reduction, or “redox”) electron donor + electron acceptor = Energy (reductant) (oxidant) Electron donors (reductants) Fe2+ (FeO) Mn2+ (MnO) Sº H 2S H2 NH4+ H2AsO3CH4 Organic compounds Electron acceptors (oxidants) O2 NO3Fe3+ (FeO1.5) Mn4+ (MnO2) SO42Sº CO2 H 2O electron donor + electron acceptor = Energy* (reductant) (oxidant) *Requires: - chemical disequilibrium - favorable thermodynamics electron donors electron acceptors (from Nealson, Journal of Geophysical Research, 1997) Hot springs (Earth’s crust) Atmosphere, groundwater, ocean Electron donors Electron acceptors (reductants) Fe2+ (FeO) Mn2+ (MnO) Sº H 2S H2 NH4+ H2AsO3CH4 O i (oxidants) d O2 NO3Fe3+ (FeO1.5) Mn4+ (MnO2) SO42Sº CO2 H 2O Hot springs (Earth’s crust) Electron donors (reductants) Fe2+ (FeO) Mn2+ (MnO) Sº H 2S H2 NH4+ H2AsO3CH4 Organic compounds Energy! Atmosphere, groundwater, ocean Electron acceptors (oxidants) O2 NO3Fe3+ (FeO1.5) Mn4+ (MnO2) SO42Sº CO2 H 2O (from Macur et al., Geobiology, 2004) Succession Spring Norris Geyser Basin (from Macur et al., Geobiology, 2004) Available chemical energy sources in Succession Spring Available energy (kilojoules) (from Spear et al., PNAS, 2005) Chemical energy & microbiology in Obsidian Pool