A microbial driver of clay mineral weathering and bioavailable Fe source under low-temperature conditions

Biotic and abiotic Fe(III) reduction of clay minerals (illite IMt-1) under low-temperature (0 and 4°C, pH 6) was studied to evaluate the effects of bioalteration on soil properties including clay structure and elemental composition. The extent of Fe reduction in bioreduced samples (∼3.8 % at 4°C and ∼3.1 % at 0°C) was lower than abiotic reduction (∼7.6 %) using dithionite as a strong reductant. However, variations in the illite crystallinity value of bioreduced samples (°Δ2θ = 0.580-0.625) were greater than those of abiotic reduced samples (°Δ2θ = 0.580-0.601), indicating that modification of crystal structure is unlikely to have occurred in abiotic reduction. Moreover, precipitation of secondary-phase minerals such as vivianite [Fe²⁺ ₃(PO₄)₂ ^⋅8H₂O] and nano-sized biogenic silica were shown as evidence of reductive dissolution of Fe-bearing minerals that is observed only in a bioreduced setting. Our observation of a previously undescribed microbe-mineral interaction at low-temperature suggests a significant implication for the microbially mediated mineral alteration in Arctic permafrost, deep sea sediments, and glaciated systems resulting in the release of bioavailable Fe with an impact on low-temperature biogeochemical cycles.