Particle size effect and the mechanism of hematite reduction by the outer membrane cytochrome OmcA of Shewanella oneidensis MR-1
The cycling of iron at the Earth’s near surface is profoundly influenced by dissimilatory metal reducing microorganisms, and many studies have focused on unraveling electron transfer 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 cytochromes responsible 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 Shewanella oneidensis and nanoparticulate hematite, examining the reduction rate as a function of particle size and reaction products 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 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 pore 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. X-ray absorption spectroscopies reveal incipient magnetite on the reacted surfaces of the hematite nanoparticles after reaction. The collective findings establish the importance of accessibility of physical contact between the terminal reductases and iron oxide surfaces, and through apparent consistency of observations help reconcile behavior reported at the larger more complex scale of whole cell studies.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1373010
- Report Number(s):
- PNNL-SA-121314; 48590; 49381; KP1702030
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 193, Issue C; ISSN 0016-7037
- Publisher:
- The Geochemical Society; The Meteoritical Society
- Country of Publication:
- United States
- Language:
- English
Similar Records
Specific Bonds between an Iron Oxide Surface and Outer Membrane Cytochromes MtrC and OmcA from Shewanella oneidensis MR-1
The X-ray crystal structure of Shewanella oneidensis OmcA reveals new insight at the microbe-mineral interface