Electron transport and protein secretion pathways involved in Mn ( III ) reduction by S hewanella oneidensis

Summary Soluble Mn ( III ) represents an important yet overlooked oxidant in marine and freshwater systems. The molecular mechanism of microbial Mn ( III ) reduction, however, has yet to be elucidated. Extracellular reduction of insoluble Mn ( IV ) and Fe ( III ) oxides by the metal‐reducing γ‐proteobacterium S hewanella oneidensis involves inner ( CymA ) and outer ( OmcA ) membrane‐associated c ‐type cytochromes, the extracellular electron conduit MtrCAB , and GspD , the secretin of type II protein secretion. CymA , MtrCAB and GspD mutants were unable to reduce Mn ( III ) and Mn ( IV ) with lactate, H 2 , or formate as electron donor. The OmcA mutant reduced Mn ( III ) and Mn ( IV ) at near wild‐type rates with lactate and formate as electron donor. With H 2 as electron donor, however, the OmcA mutant was unable to reduce Mn ( III ) but reduced Mn ( IV ) at wild‐type rates. Analogous Fe ( III ) reduction rate analyses indicated that other electron carriers compensated for the absence of OmcA , CymA , MtrCAB and GspD during Fe ( III ) reduction in an electron donor‐dependent fashion. Results of the present study demonstrate that the S . oneidensis electron transport and protein secretion components involved in extracellular electron transfer to external Mn ( IV ) and Fe ( III ) oxides are also required for electron transfer to Mn ( III ) and that OmcA may function as a dedicated component of an H 2 oxidation‐linked Mn ( III ) reduction system.