ABSTRACT
One reason that certain groups of prokaryotic microorganisms develop the extracellular respiration capabilities is that some terminal electron acceptors exist as solids that are too large to enter the cells. is is especially true for Fe(III) and Mn(III/IV) oxides, which are insoluble in water at circumneutral pH and in the absence of strong complexing ligand. e solid-phase Fe(III) and Mn(III/IV) oxides cannot cross the microbial cell envelope and thus remain external to the microbial cells. Similarly, electrodes and high molecular weight humic substances are external to the microbial cells, and their respirations occur extracellularly. Extracellular respiration is also considered as a detoxication mechanism. Aer microbial reduction of Cr(VI), Se(VI/IV) and U(VI), their respective reduced products
8.3 Redox Proteins and Protein Complexes for Extracellular Electron Transfers in Geobacter spp. 201 8.3.1 PpcA: A Periplasmic c-Cyt 202 8.3.2 Pcc Protein Complex: Dierent Type of Trans-Outer Membrane
Porin-Cytochrome Electron Conduit 203 8.3.3 Geobacter Nanowires: Proposed Pilin-Based Electron Conduits 204
8.4 Putative Electron Transfer Pathways for Extracellular Respiration in Other Microorganisms 206
8.5 Conclusions 208 8.6 Acknowledgements 208 References 209
Cr2O3, Se(0) and UO2 are poorly soluble in water and oen form nano-sized particles that may interfere with the normal functions of microbial cells. us, extracellular respirations of Cr(VI), Se(VI/IV) and U(VI) could avoid the accumulation of these water-insoluble products inside the microbial cells to mitigate their negative impacts on the microbial functions (Belchik et al., 2011; Cologgi et al., 2011).
