Insights into crystallinity-dependent microbial reduction of ferrihydrites

Microbial dissimilatory reduction of iron minerals is a central process regulating iron cycling, coupled biogeochemical processes, and pollutant fate in anoxic environments. Naturally-occurring iron minerals span a wide range of crystallinities in response to hydrogeological conditions. Although crystallinity is widely recognized as a key factor controlling the reactivity of iron minerals, its mechanistic role in microbial dissimilatory iron reduction remains poorly understood. Here, we systematically investigate the effect of crystallinity on microbial reduction efficiency of ferrihydrite. Over 48 h, microbial reduction of 2-line low-crystallinity ferrihydrite proceeded approximately 2.0-fold faster than that of 6-line high-crystallinity ferrihydrite, yielding 525.4 ± 36.4 μM versus 280.5 ± 21.5 μM Fe(II), respectively, and promoting the formation of secondary minerals such as goethite and magnetite. Mechanistically, dynamic force spectroscopy reveals significantly stronger adhesion between low-crystallinity ferrihydrite and microbial extracellular electron shuttles. This enhanced adhesion originates from the higher density of intrinsic under-coordinated Fe sites on low-crystallinity ferrihydrite, which elevates surface potential and facilitates interfacial electron exchange, as confirmed by electrochemical measurements and first-principles density functional theory calculations. The coupled surface structural and electrochemical characteristics synergistically drive the accelerated microbial reduction of low-crystallinity ferrihydrite. Our study provides a direct, mechanistic link between mineral crystallinity and biogeochemical reactivity, with potential implications on iron cycling and associated element transformations in natural waters and soils.