Bacterial symbiosis forming laminated iron-rich deposits in Okuoku-hachikurou hot spring, Akita Prefecture, Japan

Banded iron formations are the most characteristic of Archean–Paleoproterozoic sediment records. Laminated textures resembling banded iron formations can be observed in modern hot‐spring environments. Using sedimentological and microbiological techniques, we investigated the processes of laminar formation and considered the origin of lamination textures. An iron‐rich deposit at the Okuoku‐hachikurou hot spring in Japan exhibits sub‐millimeter laminations consisting of bacteria‐induced ferrihydrite and aragonite. The ferrihydrite particles are spherical and exhibit fine lamination, up to 100 µm thick in ferrihydrite‐rich parts. In aragonite‐rich parts, ferrihydrite particles form filamentous textures with diameters of 10–30 µm, but not laminations. Textural analysis using scanning electron microscopy and phylotype analysis using 16S rRNA indicated the bacterial contribution to ferrihydrite precipitation. A sheath‐like fabric showing a meshwork of nanometer‐order organic filaments, and sheath‐forming bacteria were observed in the deposit specimen etched by citric acid. Phylotype analysis detected in the iron‐rich deposits some bacterial types related to cyanobacteria, purple bacteria, and iron‐oxidizing bacteria. Iron‐oxidizing bacteria probably were responsible for precipitation of the ferrihydrite. Chemolithoautotrophic iron‐oxidizing bacteria are microaerophilic and thrive on Fe(II) in a redox gradient, but dissolved oxygen was not detected in the Okuoku‐hachikurou hot spring. Thus, a certain supply of oxygen is needed for metabolism of the microaerophilic iron‐oxidizing bacteria. The distribution of photosynthetic pigments in the iron‐rich parts indicates that the most likely source of oxygen is photosynthesis by cyanobacteria. This symbiotic relationship between cyanobacteria and iron‐oxidizing bacteria can explain the laminated texture of iron‐rich deposits in the Okuoku‐hachikurou hot spring. These laminations may reflect changes in photosynthetic intensity. There is presently some debate about the bacterial groups that may have played roles in precipitation of banded iron formations. This study presents a new bacterial model for iron precipitation and may provide a mechanism for sub‐millimeter laminations in banded iron formations deposited in shallow water.