Table 1.

Examples of Potential Redox Couples for Life on Past and Present-Day Mars

Electron donor	Electron acceptor	Name	Comment
Photosynthesis
Fe2+		Photoferrotrophy	Depends on clement surface conditions. Unlikely since the Noachian.
S/S2-		Anoxygenic sulfur photosynthesis	Depends on clement surface conditions. Unlikely since the Noachian. Sulfide suggested in Curiosity data (Ming et al.,2014). Sulfur suggested at Gusev Crater (Morris et al.,2007).
H2O		Oxygenic photosynthesis	Cannot be discounted on early Mars, but no atmospheric evidence for this reaction on present-day Mars.
Chemolithotrophy
Fe2+		Anaerobic iron oxidation	Distribution of on Mars not known although fixed nitrogen is inferred (Ming et al.,2014).
Fe2+	perchlorates	Anaerobic iron oxidation	Perchlorate can be used to oxidize iron but is not shown to be used for growth in organisms. It is included to highlight the need for investigation of perchlorate-containing redox couples.
H2	CO2	Methanogenesis, acetogenesis	Hydrogen inferred from presence of olivine and serpentine—substrates and products for H2-evolving water-rock reactions.
H2	Fe3+	Iron reduction	As above for hydrogen.
H2		Sulfate reduction	As above for hydrogen.
H2	oxidized nitrogen species		Distribution of oxidized nitrogen species on Mars not known.
S		Sulfur oxidation	Sulfur suggested at Gusev Crater (Morris et al.,2007).
S	Fe3+	Anaerobic sulfur oxidation	Occurs in acidic conditions.
CO		Anaerobic carboxydotrophy	Carbon monoxide in atmosphere.
Chemoorganotrophy
organics	Fe3+	Iron reduction	The accessibility and state of organics on Mars is not known, but they are expected to arrive within carbonaceous chondrites.
organics		Sulfate reduction	As above for organics.
organics		Nitrate reduction	As above for organics.
organics	perchlorate	Perchlorate reduction	As above for organics.
Fermentation (disproportionation)
organics	organics	Fermentation	As above for organics.

Red denotes a half-reaction that is known to exist or has existed on Mars, green a half-reaction for which there is strong reason to suspect its presence (meteoritic organics and hydrogen). Reactions are selected from anoxic redox couples. The use of O₂ as an electron acceptor for microaerophilic reactions such as hydrogen oxidation on past or present-day Mars is not explicitly ruled out. The table does not include many redox reactions that use different oxidation states of nitrogen (e.g., anaerobic ammonium oxidation with nitrite), since the fixed state of nitrogen in the martian crust is not known. Note that redox couples involving the oxidation and reduction of iron can be performed with other variable valence cations (e.g., Mn, U) that could be present in varying oxidation states in the martian crust.