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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

)

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