CHEMICAL SPECIATION AND MICROBIAL CHARACTERISTICS OF FERRO-MANGANESE CRUSTS IN THE WESTERN PACIFIC SEAMOUNTS

Ferromanganese (Fe-Mn) crusts are a kind of marine chemical sediment composed of Fe and Mn oxy-hydroxides occurring on the slopes of seamounts and oceanic plateaus at depths from 400 to 5500 mbsl. Fe-Mn crusts contain extremely high concentrations of useful minor metals such as Co, Ni, Te, REEs, and thus are expected as submarine mineral resources. Also, Fe-Mn crusts record the change of chemical compositions of seawater during last several tens million years, which allow us to obtain clues to clarify the paleoenvironmental conditions at the time of its deposition.

Through the elemental speciation in Fe and Mn oxides by synchrotron X-ray analyses, it is revealed that element concentration in Fe-Mn crusts are well constrained by mode of adsorption on Fe (ferrihydrite) and Mn oxides (Kashiwabara et al., 2008, 2011, 2013, 2014). As for homologous W and Mo, Fe-Mn crusts are distinctly more enriched with W than Mo compared with their relative abundance in seawater. Kashiwabara et al. (2013) proposed its mechanism based on the chemical speciation data as follows: W forms an inner-sphere complex both on Fe and Mn oxides, while Mo forms an inner-sphere complex on Fe oxide and forms an outer-sphere complex on Mn oxide. Such difference in mode of adsorption leads to difference in concentrations of W and Mo in Fe-Mn crusts. Hence, the difference in mode of adsorption may be one of the primary cause of the lower seawater W concentration than that of Mo.

The abundance and phylogenetic diversity of the microbial community in the Fe-Mn crust, sediment and overlying seawater were investigated using a culture-independent molecular analysis based on the 16S rRNA gene (Nitahara et al., 2011). Based on quantitative PCR analysis, Archaea occupy a significant portion of the prokaryotic communities in the Fe-Mn crust and the sediments, while Bacteria dominated in the seawater samples. Comparative analysis indicates that over 80% of the total phylotype richness estimates for the crust community were unique as compared with the sediment and seawater ones. Phylotypes related to Nitrosospira belonging to the Betaproteobacteria and those related to Nitrosopumilus belonging to MGI Crenarchaeota were detected in the crust, suggesting that the ammonia-oxidizing chemolithoautotrophs play a role as primary producers in the microbial ecosystem of crusts.