Teruhiko Kashiwabara

Prokaryotic Abundance and Community Composition in a Freshwater Iron-Rich Microbial Mat at Circumneutral pH

The abundance, diversity and composition of bacterial and archaeal communities in a freshwater iron-rich microbial mat were investigated using culture-dependent and culture-independent methods. The sampling site is a mixing zone where ferrous-iron-rich fluids encounter oxygen-rich environments. Quantitative PCR analysis shows that Bacteria dominated the mat community (>99% of the total cell numbers). Phylotypes related to iron-oxidizers in Gallionellaceae, methano/methylotrophs in Methylophilaceae and Methylococcaceae, sulfide-oxidizers in Sulfuricurvum and an uncultured clone group, called Terrestrial group I or the 1068 group, in the Epsilonproteobacteria were detected in the clone library from the original sample and/or the enrichment cultures. This result suggests that these members may play a role in Fe, S and C cycling in the mixing zone. Although Archaea were minor constituents numerically, phylogenetic analysis indicates that unique and diverse yet-uncultivated Archaea are present in the iron-rich mat. The phylotypes of these yet-uncultivated Archaea belong to environmental clone groups that have been recovered from other mixing zones in terrestrial and marine environments, and some of our phylotypes have significantly low similarity (80% or lower) with the archaeal clones reported previously. Our results provide further insights into the bacterial and archaeal communities in a microaerobic iron-rich freshwater environment in mixing zones.

Systematics of Distributions of Various Elements Between Ferromanganese Oxides and Seawater from Natural Observation, Thermodynamics, and Structures

Metal oxides including iron oxides, manganese oxides, and ferromanganese oxides have been frequently found at seafloor as a result of the release of dissolved iron and manganese from various sources including hydrothermal activities. These precipitates can adsorb or incorporate various elements, which can affect the behavior of the elements in marine environment. In addition, these precipitates can be resources of rare metals due to their high abundances in ferromanganese oxides. In this review, our aims are (i) to summarize distribution of various trace elements between ferromanganese oxides and seawater, (ii) to understand the distributions based on thermodynamic parameters, and (iii) to show the relationship between the distribution and structural information of the species adsorbed onto the ferromanganese oxides. For this purpose, our original data of chromate adsorption on ferrihydrite was also included. These attempts enable us to obtain systematic explanation of the solid-water distributions of various elements in marine environment, which in turn gives us clearer view on (i) the mechanism of isotopic fractionation during adsorption which is linked to the understanding of paleoenvironment based on the isotope geochemistry and (ii) prediction of abundances of various elements in the ferromanganese oxides that are important from the viewpoint of exploration of marine resources.

Biotic manganese oxidation coupled with methane oxidation using a continuous-flow bioreactor system under marine conditions

Biogenic manganese oxides (BioMnOx) can be applied for the effective removal and recovery of trace metals from wastewater because of their high adsorption capacity. Although a freshwater continuous-flow system for a nitrifier-based Mn-oxidizing microbial community for producing BioMnOx has been developed so far, a seawater continuous-flow bioreactor system for BioMnOx production has not been established. Here, we report BioMnOx production by a methanotroph-based microbial community by using a continuous-flow bioreactor system. The bioreactor system was operated using a deep-sea sediment sample as the inoculum with methane as the energy source for over 2 years. The BioMnOx production became evident after 370 days of reactor operation. The maximum Mn oxidation rate was 11.4 mg L-1 day-1. An X-ray diffraction analysis showed that the accumulated BioMnOx was birnessite. 16S rRNA gene-based clone analyses indicated that methanotrophic bacterial members were relatively abundant in the system; however, none of the known Mn-oxidizing bacteria were detected. A continuous-flow bioreactor system coupled with nitrification was also run in parallel for 636 days, but no BioMnOx production was observed in this bioreactor system. The comparative experiments indicated that the methanotroph-based microbial community, rather than the nitrifier-based community, was effective for BioMnOx production under the marine environmental conditions.

Post-drilling hydrothermal vent and associated biological activities seen through artificial hydrothermal vents in the Iheya North field, Okinawa Trough

In 2010, IODP Expedition 331 was conducted in the Iheya North Field, the Okinawa Trough and drilled several sites in hydrothermally active subseafloor: e.g., the active hydrothermal vent site and sulfide-sulfate mound at North Big Chimney (NBC) and three sites east of NBC at different distances from the active vents [1]. In addition, during the IODP Expedition 331, four new hydrothermal vents were created. These post-drilling artificial hydrothermal vents provide excellent opportunities to investigate the physical, chemical and microbiological characteristics of the previously unexplored subseafloor hydrothermal fluid reservoirs, and to monitor and estimate how the anthropogenic drilling behaviors affect the deep-sea hydrothermal vent ecosystem. The IODP porewater chemistry of the cores pointed to the density-driven stratification of the phase-separated hydrothermal fluids and the natural vent fluids were likely derived only from the shallower vapor-enriched phases. However, the artificial hydrothermal vents had deeper fluid sources in the subseafloor hydrothermal fluid reservoirs composed of brine phases. The fluids from the artificial hydrothermal vents were sampled by ROV at 5, 12, 18 and 25 months after the IODP expedition. The artificial hydrothermal vent fluids were slightly enriched with Cl as compared to the natural hydrothermal vent fluids. Thus, the artificial hydrothermal vents successfully entrained the previously unexplored subseafloor hydrothermal fluids. The newly created hydrothermal vents also hosted the very quickly grown, enormous chimney structures, of which mineral compositions were highly variable among the vents. In addition, the IODP drilling operation not only created new hydrothermal vents but also induced the newly generated diffusing flows by many short drillings in the seafloor where no apparent hydrothermal fluid discharge was observed. The new widespread diffusing flows altered the habitat condition, and provided postdrilling propagation and colonization of indigenous hydrothermal chemosynthetic animals.