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The formation process of hydrous manganese oxide deposit can hardly be demonstrated in natural environments, namely, hot and cold springs and deep-sea hydrothermal vents. We discovered a unique phenomenum that hydrous manganese oxide is now actively depositing from the hot spring water at the Yuno-taki Falls, Hokkaido, Japan. The chemical and the biological factors that potentially control the actual manganese deposition in the natural environment were examined.Pure manganese oxide deposit (1.1 ton/year calculated as Mn02) is currently being formed from weakly acidic hot spring water by the mediation of microbial consortium consisting of manganese-oxidizing bacteria and filamentous algae. A comparison of the mineral deposition at this location with other terrestrial and oceanic deposits suggests that they were formed in a similar manner.

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