Nanosized Iron Oxide Colloids Strongly Enhance Microbial Iron Reduction

Microbial iron reduction is considered to be a significant subsurface process. The rate-limiting bioavailability of the insoluble iron oxyhydroxides, however, is a topic for debate. Surface area and mineral structure are recognized as crucial parameters for microbial reduction rates of bulk, macroaggregate iron minerals. However, a significant fraction of iron oxide minerals in the subsurface is supposed to be present as nanosized colloids. We therefore studied the role of colloidal iron oxides in microbial iron reduction. In batch growth experiments with Geobacter sulfurreducens, colloids of ferrihydrite (hydrodynamic diameter, 336 nm), hematite (123 nm), goethite (157 nm), and akaganeite (64 nm) were added as electron acceptors. The colloidal iron oxides were reduced up to 2 orders of magnitude more rapidly (up to 1,255 pmol h⁻¹ cell⁻¹) than bulk macroaggregates of the same iron phases (6 to 70 pmol h⁻¹ cell⁻¹). The increased reactivity was not only due to the large surface areas of the colloidal aggregates but also was due to a higher reactivity per unit surface. We hypothesize that this can be attributed to the high bioavailability of the nanosized aggregates and their colloidal suspension. Furthermore, a strong enhancement of reduction rates of bulk ferrihydrite was observed when nanosized ferrihydrite aggregates were added.Dissimilatory iron reduction is an important anaerobic respiration process in anoxic subsurface environments. However, the reactivity of ferric iron is mostly limited by the reduction kinetics of the poorly soluble, extracellular iron minerals. Electron transfer from microorganisms to iron oxides can occur via direct contact or by electron shuttling compounds (46). Transport of the electron shuttle between the redox partners is then assumed to occur via diffusion. For example, humic substances can serve as natural electron shuttles that can be reduced by microorganisms and subsequently chemically oxidized by the ferric oxide (18). Shewanella oneidensis excretes a flavin to stimulate hematite reduction, functioning in a similar manner (27). As another option, formation of conductive pili serving as nanowires was described as a possible way of transferring electrons to the oxide surface (15, 34). Nevertheless, direct attachment has been recognized as a major mode of accessing iron oxides as electron acceptors (12). Direct transfer between microbial outer membrane reductases and the ferric minerals, however, requires close contact of less than 14 Å between the terminal iron reductase on the cell surface and the iron oxide molecule at the mineral surface (19, 25), limiting the rates of electron transfer between cell and mineral.Several parameters have been discussed in this context as being decisive for the bioavailability and reactivity of iron oxides, such as, e.g., the mineral surface area (8, 41). Larger surface areas have been shown to be accompanied by higher initial reduction rates. Another parameter that might determine reactivity is the low solubility of ferric iron in water at neutral pH (20). Low solubility entails high crystallinity, which reduces reaction rates (4). Therefore, crystalline bulk iron phases such as goethite or hematite (9) are poorly reducible by microorganisms, in contrast to amorphous ferrihydrite (41). Naturally, well crystalline minerals have lower surface areas, and the effects of surface area and solubility cannot be distinguished sharply. Cell density, initial oxide and substrate concentrations, and ferrous iron adsorbed to the bulk mineral surface were also reported to control microbial reduction rates by exhibiting mutual saturation behavior in Michaelis-Menten-type kinetics (3, 22, 40).The latter studies also considered particle sizes, a parameter that has often been overlooked so far. All concepts mentioned above generally assumed a bulk state of the electron-accepting iron oxide. Indeed, iron oxides used in microbiological experiments appear mainly as coarse, flocculating macroaggregates, visible to the naked eye as sludge-like precipitates. In nature, however, nanosized iron oxides are abundant (32, 45) and play a vital role in many biogeochemical processes (2, 16, 28). Such nanoparticles may appear in stable colloidal suspension, even if aggregated as a stable cluster of multiple particles (13). Ferric oxide particles can appear in colloidal suspensions of different aggregate sizes and densities.Different particle aggregate sizes might influence the bioavailability of iron oxides in microbial reduction. Nanosized aggregates appearing in colloidal suspensions might be spatially more accessible for microorganisms than large aggregates flocculating as bulk phases. Therefore, the present study aims at assessing the reactivity and putative role of aggregate sizes of iron oxides in dissimilatory iron reduction. A set of ferrihydrite, hematite, goethite, and akaganeite colloids was compared to their respective noncolloidal bulk phases to evaluate this effect.