the thermodynamics of chemosynthetic life -...
TRANSCRIPT
The Thermodynamics ofChemosynthetic Life
Tom McCollom
Vast majority of life on Earth derivesenergy (directly or indirectly) from sunlight(i.e., photosynthesis)
but … unlikely energy source for veryearliest organisms on Earth or onMars & Europa
earliest organisms probably relied onchemical sources of metabolicenergy (“chemosynthesis”)
“Chemosynthetic” organisms (more properlychemolithoautotrophs):
derive metabolic energy frominorganic chemical reactions &produce biomass from inorganiccompounds known for well over a century
are ubiquitous on Earth and live in a widevariety of habitats (e.g., marinesediments, cow stomachs, sewage systems)
most involved indirectly in decay ofphotosynthetic organic matter
(reductant)
+ Energy(oxidant)
=electron donor electron acceptor
Chemical energy for autotrophic life
Chemical energy sources involve electronexchange reactions
(a.k.a. oxidation-reduction, or “redox”)
(reductants) (oxidants)
Fe2+ (FeO)Mn2+ (MnO)SºH2SH2
NH4+
H2AsO3-
CH4
Organic compounds
O2
NO3-
Fe3+ (FeO1.5)Mn4+ (MnO2)SO4
2- Sº CO2
H2O
Electron donors Electron acceptors
Energy*
*Requires: - chemical disequilibrium - favorable thermodynamics
(reductant)
+(oxidant)
=electron donor electron acceptor
electron donors electron acceptors
(from Nealson, Journal of Geophysical Research, 1997)
Fe2+ (FeO)Mn2+ (MnO)SºH2SH2
NH4+
H2AsO3-
CH4
O i d
O2
NO3-
Fe3+ (FeO1.5)Mn4+ (MnO2)SO4
2- Sº CO2
H2O
Hot springs(Earth’s crust)
Atmosphere,groundwater,
ocean
(reductants) (oxidants)
Electron donors Electron acceptors
Fe2+ (FeO)Mn2+ (MnO)SºH2SH2
NH4+
H2AsO3-
CH4
Organic compounds
O2
NO3-
Fe3+ (FeO1.5)Mn4+ (MnO2)SO4
2- Sº CO2
H2O
(reductants) (oxidants)
Electron donors Electron acceptors
Energy!
Hot springs(Earth’s crust)
Atmosphere,groundwater,
ocean
(from M
acur et al., G
eobiology, 2
004)
Succession SpringNorris Geyser Basin
(from M
acur et al., G
eobiology, 2
004)
Available chemical energy sources inSuccession Spring
(from S
pear et al., PNAS, 2
005)
Chemical energy & microbiology inObsidian Pool
Ava
ilable e
nerg
y (k
ilojo
ules
)
