Sources of organic electron donors for microbial iron reduction in the subsurface of Earth and Mars

The microbial mediation of iron reduction reactions in the subsurface is a significant contributor to the global cycling of Carbon and Iron on Earth. It also represents a compelling candidate metabolism operating in the subsurface of Mars. However, our knowledge of the limits and metabolic capabilities of microbial iron reduction is confined, largely caused by research bias towards practical applications, such as bioremediation and microbial fuel cells. Iron reducing bacteria (IRB) typically utilise organic compounds as electron donors for the reduction of ferric iron. In order to constrain the habitability of Mars for such life, it is critical to better understand the ability of iron reducing microorganisms to utilize complex ancient sources of organic compounds, such as shales, kerogens and carbonaceous meteorites. In this talk I will present initial results from a series of growth experiments in which the ability of IRB to reduce iron with the concurrent oxidation of organic compounds in shales and kerogens is assessed. Results indicating that non-proteinogenic D-amino acids common to carbonaceous meteorites may inhibit microbial iron reduction will also be presented, and the implications for the search for iron-reducing microorganisms on Mars discussed.