Limitations to a microbial iron cycle on Mars

Anaerobic microbial metabolisms found on the Earth are the most plausible candidates for understanding potentially analogous energy gathering metabolisms on Mars. The iron-rich nature of Mars raises questions on whether the planet could support energy acquisition by iron-cycling microorganisms. This review paper addresses what is known about the redox couples that support microbial iron cycling on Earth, and evaluates evidence to date of the presence or absence of relevant redox constituents on Mars. We give particular focus to iron reduction. These constituents include the presence and prevalence of ferric iron-bearing minerals that may serve as terminal electron acceptors, and the likelihood of organic compounds (exogenous and endogenous) or hydrogen residing in the near- or sub-surface as a source of electron donors. Whilst it is feasible that redox couples for iron cycling may exist, or have existed in the past, current knowledge suggests that for chemolithotrophs (iron oxidation) Mars may be an electron acceptor limited world and that for chemoorganotrophs (iron reduction) Mars may be limited in widespread, readily available electron donors, particularly in its subsurface. There are several major limitations in this assessment due to lack of experimental data on Earth, and lack of measurements on Mars. We outline a series of high priority in-situ measurements that are necessary to fully evaluate the potential for a Martian biological iron cycle. Our conclusions also apply to the search for a Martian biological sulphur cycle.