Function, Structure, and Gene Expression in Electroactive Bacteria

Electroactive bacteria connect biology to electricity, acting as livingelectrochemical catalysts. In nature, these bacteria can respire insoluble compounds like iron oxides, and in the laboratory, they are able to respire an electrode and produce an electrical current. This document investigates two of these electroactive bacteria: Geobacter sulfurreducens and Thermincola ferriacetica. G. sulfurreducens is a Gramnegative iron-reducing soil bacterium, and T. ferriacetica is a thermophilic, Grampositive bacterium that can reduce iron minerals and several other electron acceptors. Respiring insoluble electron acceptors like metal oxides presents challenges to a bacterium. The organism must extend its electron transport chain from the inner membrane outside the cell and across a significant distance to the surface of the electron acceptor. G. sulfurreducens is one of the most-studied electroactive bacteria, and despite this there are many gaps in knowledge about its mechanisms for transporting electrons extracellularly. Research in this area is complicated by the presence of multiple pathways that may be concurrently expressed. I used cyclic voltammetry to determine which pathways are present in electroactive biofilms of G. sulfurreducens grown under different conditions and correlated this information with gene expression data from the same conditions. This correlation presented several genes that may be components of specific pathways not just at the inner membrane but along the entire respiratory pathway, and I propose an updated model of the pathways in this organism. I also characterized the composition of G. sulfurreducens and found that it has high iron and lipid content independent of growth condition, and the high iron content is explained by the large abundance of multiheme cytochrome expression that I observed. I used multiple microscopy techniques to examine extracellular respiration in G. sulfurreducens, and in the process discovered a novel organelle: the intracytoplasmic membrane. I show 3D reconstructions of the organelle in G. sulfurreducens and discuss its implications for the cell’s metabolism. Finally, I discuss gene expression in T. ferriacetica in RNA samples collected from an anode-respiring culture and highlight the most abundantly expressed genes related to anode-respiring metabolism.