Download The Thermodynamics of Chemosynthetic Life

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