Characterization of Bacteriogenic Iron Oxide Deposits from Axial Volcano, Juan de Fuca Ridge, Northeast Pacific Ocean

Iron oxides from the caldera of Axial Volcano, a site of hydrothermal vent activity along the Juan de Fuca Ridge, 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. These bacteria are commonly associated with iron-oxide precipitates and are proposed to play two causal roles in the formation of iron oxides at Axial Volcano. First, by increasing the rate of Fe 2+ oxidation, and second, by lowering the concentration of Fe 3+ required for precipitation by providing a reactive surface for heterogeneous nucleation. Rapid rates of oxidation and precipitation caused by the bacteria likely contribute to the poorly ordered nature of the iron oxides, determined to be primarily 2-line ferrihydrite, and in one case, poorly ordered goethite. The iron-oxide precipitates consist dominantly of iron, silicon (mostly diatoms), and organic carbon with a suite of sorbed trace metals. The high metal uptake affinity of these poorly ordered iron oxides may be important in the global cycling of trace elements throughout the world's oceans. Additionally, iron oxides precipitating on the surface of bacteria can preserve individual cells as microfossils making bacteriogenic iron oxides ideal proxies for paleoenvironmental and astrobiological studies.