@article {16055,
	title = {Particle size effect and the mechanism of hematite reduction by the outer membrane cytochrome OmcA of Shewanella oneidensis MR-1},
	journal = {Geochimica et Cosmochimica Acta},
	volume = {193},
	year = {2016},
	pages = {160-175},
	publisher = {Pergamon},
	abstract = {The cycling of iron at the Earth{\textquoteright}s near surface is profoundly influenced by dissimilatory metal reducing microorganisms, and many studies have focused on unraveling\&nbsp;electron transfer\&nbsp;mechanisms between these bacteria and Fe(III)-(oxyhydr)oxides. However, these efforts have been complicated by the fact that these minerals often occur in the micro- to nanosize regime, and in relevant natural environments as well as in the laboratory are subject to aggregation. The nature of the physical interface between the cellular envelope, the outer-membrane\&nbsp;cytochromesresponsible for facilitating the interfacial electron transfer step, and these complex mineral particulates is thus difficult to probe. Previous studies using whole cells have reported reduction rates that do not correlate with particle size. In the present study we isolate the interaction between the decaheme outer-membrane cytochrome OmcA of\&nbsp;Shewanella oneidensisand nanoparticulate\&nbsp;hematite, examining the reduction rate as a function of particle size and\&nbsp;reaction products\&nbsp;through detailed characterization of the electron balance and the structure and valence of iron at particle surfaces. By comparison with abiotic reduction via the smaller molecule\&nbsp;ascorbic acid, we show that the reduction rate is systematically controlled by the sterically accessible interfacial contact area between OmcA and hematite in particle aggregates; rates increase once\&nbsp;pore\&nbsp;throat sizes in aggregates become as large as OmcA. Simultaneous measure of OmcA oxidation against Fe(II) release shows a ratio of 1:10, consistent with a cascade OmcA oxidation mechanism heme by heme.\&nbsp;X-ray\&nbsp;absorption spectroscopies\&nbsp;reveal incipient\&nbsp;magnetite\&nbsp;on the reacted surfaces of the hematite\&nbsp;nanoparticles\&nbsp;after reaction. The collective findings establish the importance of accessibility of physical contact between the terminal reductases and\&nbsp;iron oxide\&nbsp;surfaces, and through apparent consistency of observations help reconcile behavior reported at the larger more complex scale of whole cell studies.},
	isbn = {0016-7037},
	author = {Juan* Liu and Pearce, Carolyn I and Shi, Liang and Wang, Zheming and Shi, Zhi and Arenholz, Elke and Rosso, Kevin M*}
}
