ULTRASTRUCTURE AND SUB-SEAFLOOR PRESENCE OF BACTERIOGENIC IRON OXIDES FROM AXIAL VOLCANO, JUAN DE FUCA RIDGE, NORTH-EAST PACIFIC OCEAN

Iron oxides from the caldera of Axial Volcano, a site of hydrothermal vent activity along the Juan de Fuca Ridge, were found to consist predominantly of microbial structures in hydrated whole mounts examined using an environmental scanning electron microscope (ESEM). Novel observations were made of the iron oxides revealing the spatial relationships of the bacteria within to be more consistent with microbial mats than mineral precipitates. The bacterial structures are attributed to the sheaths of Leptothrix sp. and the stalks of Gallionella ferruginea, based on the distinctive morphological characteristics of these two iron oxidizing bacteria. Energy dispersive X-ray spectroscopy (EDS) revealed the presence and distribution of Fe, Si, O and Cl on the bacterial sheaths and stalks. The iron oxides were identified by X-ray diffraction to be 2-line ferrihydrite, a poorly ordered iron oxyhydroxide. Adsorption of Si in particular to the bacterial iron oxide structures likely contributes to the stability of 2-line ferrihydrite on the sea floor, and may also be a preservation mechanism creating bacterial microfossils. Presumptive evidence of the sub-seafloor presence of Leptothrix and G. ferruginea at Axial Volcano was obtained from the presence of these bacteria on a trap that had been placed on an active vent, and also in a vent fluid sample. The presence of these iron oxidizing bacteria in the sub-seafloor may be an indication that the surface expression of iron oxide deposits at Axial Volcano is minimal in comparison to what exists beneath the seafloor.