Table 1.

Potential Energy Sources for Chemotrophic Life on Mars (Adapted from Cockell, 2014, and Rummel et al., 2014)

Electron donor	Electron acceptor	Metabolism	Comment
Chemolithotrophy
H2	CO2	Methanogenesis; acetogenesis; C fixation via Wood–Ljungdahl pathway	Hydrogen from hydrothermal alteration of mafic/ultramafic minerals (e.g., olivine) and microbial mediation of H2 from mineral alteration (Parkes et al., 2011)
H2	Fe3+	Iron reduction	Hydrogen from sources mentioned above
H2	SO42−, S0	Sulfate reduction	Hydrogen from sources mentioned above
H2	O2	Hydrogen oxidation
H2	ClO4−	Perchlorate reduction
CH4	(Mn4+, Mn3+)	Birnessite reduction
CH4	Fe3+	Ferrihydrite reduction
CH4	NO3−	Anaerobic methane oxidation
CH4	SO42−	Sulfate reduction
CO	H2O	Carbon monoxide oxidation
CO	O2	Aerobic carbon monoxide oxidation	“CO oxidizers” are bacteria capable of growing with CO as a sole carbon and energy source
CO	NO3−	Aerobic methane oxidation
CO	SO42−	Sulfidogenesis
CO	CO2	Methanogenesis; acetogenesis
Fe2+	CO2	Carbon dioxide reduction
Fe2+(basalt glass)	O2, NO3−	Iron oxidation	Not confirmed for terrestrial microorganisms
Fe2+(aqueous)	O2, NO3−	Microaerobic iron oxidation
Fe2+(biotite)	O2, NO3−	Aerobic iron oxidation
Fe2+, Fe3+(magnetite)	NO3−	Aerobic iron oxidation
FeS2	MnO2, NO3−	Anaerobic pyrite oxidation
S2−	(Mn4+, Mn3+)	Anaerobic sulfides oxidation
HS−(aqueous)	O2, NO3−
S0	NO3−	Sulfur oxidation
S0	Fe3+	Anaerobic sulfur oxidation	Occurs in acid conditions
S0	MnO2	Sulfur oxidation
H2S, HS−, S0, S2O32−	O2	Oxidation of reduced sulfur compound
NH3	O2	Oxidation of ammonia	Part of the nitrification process
NO2−	H2O	Oxidation of the nitrite	Part of the nitrification process
NH4+	NO2−	Anammox
Chemoorganotrophy
Organics	Fe3+	Iron reduction	Carbon from abiotic/prebiotic sources as well as biogenic; hydrogen from sources mentioned above
Organics	SO42−	Sulfate reduction	Carbon and hydrogen sources as above
Organics	Perchlorates
Fermentation (disproportionation)
Organics	Organics	Fermentation	Carbon sources as above