Laminated iron texture by iron-oxidizing bacteria in a calcite travertine

An iron-rich travertine at Shionoha hot-spring in western Japan, displays a sub-millimetre order lamination consisting of microbially-incduced ferrihydrite and calcite-matrix. The ferrihydrite occurs as 5- to 10-μm thick filaments that extend and apparently branch upwards. A sheathed morphotype having a meshwork of a rod-like organic substance and phylogenetically identified species of genus Siderooxidans were responsible for precipitation of the ferrihydrite. The iron oxidizers are microaerophilic and thrive on Fe(II) and a redox gradient, that are available at the study site near the vent. Bacterial activity enhanced ferrihydrite deposition at rate of ∼10 μm/day, and formed the laminated texture. The bacteria increased their density upward in each lamina and suddenly decreased the density at the top of the lamina. This change may have resulted from a deficiency of metabolic substances at the sediment-water interface when the iron-oxidizers became very dense or when other chemoautotrophs, such as methanotrophs, consumed oxygen on the surface. The metabolism of the microaerophilic iron-oxidizing bacteria growing in neutral pH environments contribute significantly to the precipitation of iron mineral deposits. Because the laminated textures observed in this study have a great preservation potential, they help to identify the contributions of iron-oxidizers to ancient BIFs and provide an idea for pO₂ and pH of the ancient ocean.