Microbial community composition in iron deposits and manganese crusts formed in riverine environments around the Aso area in Japan

The Kurokawa river flowing through the Aso Caldera in Southwest Japan is influenced by groundwater containing high concentration of dissolved metals (Fe2+ and Mn2+) and is associated with orange deposits in the upstream and thick black crusts on the wall of artificial waterway tunnel with several km lengths in the midstream regions. X-ray fluorescence analysis revealed that the orange deposits contained up to 34.4% Fe and black crusts comprised up to 25.4% and 10.6% Mn and Fe, respectively. Although naturally occurring Mn-oxides and crusts are considered biogenic in pH-neutral freshwater environments, the whole microbial community in Mn crust and their contribution to Mn crust formation in these environments remain unclear. High-throughput molecular sequencing for bacteria and eukaryotes including fungi revealed black Mn crusts had a high abundance of Sphingomonas, Hyphomicrobium, Bacillus, Pseudomonas, and Mortierella, previously reported genera including several species with Mn-oxidizing activity in Mn crusts or nodules of other marine and freshwater environments. In addition, one bacterial isolate and one fungal isolate with Mn-oxidizing activity were obtained from black Mn crust. These results suggest that multiple Mn-oxidizing bacteria including Pseudomonas and fungi are involved in Mn crust formation on the wall of dark waterway tunnel in the oligotrophic environment of the Kurokawa river.