Between a rock and a hard place: geomicrobial electron transfer

Abstract

The success of microbial life on Earth can largely be attributed to their collective ability to squeeze energy from a broad range of electron donor and acceptor couples including many where the electrochemical potential between the couples is marginally favorable (i.e, small ΔG). Many of the redox couples that microbiologists employ to cultivate their favorite organism(s) involve compounds that are relatively water soluble, such as glucose and O2, and can readily diffuse to and from cells. In contrast, many organic and inorganic substrates, either electron donors or acceptors, exist as solids that are poorly soluble or insoluble. Certain heterotrophic microorganisms have overcome this problem by secreting enzymes outside the cell that can break down insoluble polymers, such as chitin and cellulose, to soluble subunits that are readily accessed by cells. Over the past few decades an increasing number of microorganisms have been isolated and studied that are capable of utilizing transition metal ions such as Fe and Mn as electron acceptors or donors. In their most oxidized form Fe(III) and Mn(IV) exist predominantly as metal oxides of varying morphology and composition but have in common low solubility in neutral pH environments and in the absence of complexing ligands. What clevermore » mechanisms have microorganisms evolved to facilitate electron transfer to and from metal ions that are sequestered in minerals?« less