Mid-ocean ridge bacteriogenic iron oxides from Axial Volcano, Juan de Fuca Ridge, north-east Pacific Ocean

An investigation was performed on iron oxides from the caldera of Axial Volcano, a site of hydrothermal vent activity along the Juan de Fuca Ridge in the northeast Pacific Ocean, to determine their microbial, geochemical and transformation characteristics. The iron oxides, found to be 2-line ferrihydrite (termed simply herein as ferrihydrite), are characterized by abundant bacterial structures that closely resemble the sheaths of Leptothrix ochracea, the stalks of Gallionella ferruginea and the filaments of a novel iron oxidizing PV-1 strain. The occurrence of bacteria in the sub-seafloor, the microbial mat-like fabric of the mineral precipitates and isotopic fractionation of carbon and has led to the hypothesis that bacteria play two causal roles in the formation of ferrihydrite at Axial Volcano. First, they are likely metabolically increasing the rate of Fe(II) oxidation, and second, they are lowering the degree of supersaturation required for ferrihydrite precipitation by their reactive surfaces behaving as heterogeneous nucleation sites. The high partitioning values of the diverse suite of adsorbed trace metals to ferrihydrite from Axial Volcano is attributed to the presence of 3–5 heterogeneous surface functional groups. Binding of ferrihydrite to bacterial surfaces inhibits the phase transition of ferrihydrite to more structurally ordered iron oxides, which may enhance the importance of ferrihydrite to marine trace metal cycling and prolong the long term survival of the bacterial structures as fossils. Moreover, the fossilized bacterial structures and fractionated C-isotopes in the ferrihydrite deposits from Axial Volcano imparts bacteriogenic iron oxides with a great deal of potential for use in paleontology and astrobiology.