Experiments were conducted with synthetic iron oxide nanoparticles to gain insight into the rate-limiting step during the initial stages of dissimilatory microbial iron oxide reduction (DIR) in the presence of excess electron donor for microbial respiration. Ten iron oxide phases with a range of surface areas and thermodynamic properties were reduced by Geobacter sulfurreducens under nongrowth conditions with excess H2 as an energy source. Analogous experiments were conducted in the presence of 100 mM of the soluble electron shuttling compound AQDS. Other experiments examined the short-term kinetics of abiotic iron oxide reduction by excess (10 mM) AH2DS. Longer term abiotic reduction experiments with 10 mM AH2DS were used to estimate the reduction potential (Eh0) of the different oxide phases. There was a significant correlation between oxide Eh0 and short-term surface-area normalized rates of abiotic reduction by AH2DS. In contrast, there was no correlation between oxide Eh0 and initial surface area-normalized DIR rates, either the presence or absence of AQDS. Separate experiments with amorphous hydrous ferric oxide showed that 1 mM AQDS was sufficient to saturate rates of DIR. Thus, the concentration of AQDS did not limit DIR in the reaction systems with 1 mM AQDS. These results, together with other data sets, suggest that the rate of electron flow from cytoplasmic metabolism to redox active components (i.e. multiheme cytochromes) in the outer membrane limits the rate of DIR in both the absence and presence of soluble electron shuttles. This conclusion is supported by the documented rapid electron transfer kinetics between isolated outer membrane multiheme cytochrome systems (e.g. mtrCAB in Shewanella oneidensis) and iron oxide nanoparticles. The analysis highlights the previously demonstrated (or inferred) importance of particle surface properties – as opposed to bulk thermodynamic properties – on the kinetics of biological electron transport to iron oxide surfaces.

Rate-limiting step of dissimilatory microbial iron oxide nanoparticle reduction

ACS Fall 2021

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