Microbially mediated manganese oxides enriched in yttrium and rare earth elements in the Ytterby mine, Sweden

Sammanfattning: Microorganisms are able to manipulate redox reactions and thus exert extensive control on chemical speciation and element partitioning in nature, affecting the formation and dissolution of certain minerals. One of these redox active elements is manganese (Mn), which in its oxidized states (III/IV), commonly forms Mn oxide-hydroxide minerals. A microbially mediated birnessite-type Mn oxide enriched in yttrium (Y) and rare earth elements (REE) has been studied in our research. The YREE-enriched birnessite was found in a tunnel to the main shaft of the former Ytterby mine in Sweden, well known as the place of discovery of scandium, yttrium, tantalum and five of the REEs. The thesis aims to define preconditions and processes leading to the formation of this Ytterby birnessite, with particular focus on microbial involvement and the potentially promoting role of biofilms. Dynamics and mineral products of the natural system are studied in combination with analyses of Mn phases produced in vitro by microbes isolated from this system. In addition, the nature of the YREE association with the birnessite-type Mn oxides is investigated.Natural birnessite has the composition Mx(Mn3+, Mn4+)2O4•(H2O)n  with M ususally being (Na,Ca) and x=0.5. An empirical formula based on element analyses for the Ytterby birnessite has been assessed as M = (Ca0,37-0,41YREE0.02Mg0.04Other metals0.02-0.03), with [Mn3+]/[Mn4+] = 0.86-1.00 to achieve charge balance. We find that there is a preference for the trivalent YREEs over divalent and monovalent metals. There is also a preferential uptake of light rare earth elements (LREE) relative to heavy rare earth elements (HREE), likely due to mineralogical preferences for charge and ionic radius. The YREEs are strongly bound to the mineral structure and not merely adsorbed on the surface. The Mn deposit subsystems (fracture water, Mn oxide precipitating biofilm and bubble biofilm) are phylogenetically significantly different and the microbial community composition of the feeding water has little impact on the derived biofilms. The signature microbial groups of the Mn oxide producing biofilm Rhizobiales (e.g., Pedomicrobium), PLTA13 Gammaproteobacteria, Pirellulaceae, Blastocatellia and Nitrospira are adapted to the specific characteristics of the biofilm: an emerging extreme environment (low temperature, no light, high metal concentration) which is in part generated by the biofilm components themselves. Known Mn oxidizers are identified among these niched microbial groups and four of the isolated bacterial species (Hydrogenophaga sp., Pedobacter sp., Rhizobium sp. and Nevskia sp.) as well as one fungal species (Cladosporium sp.) are involved in Mn oxide production. Hydrogenophaga sp. and Pedobacter sp. produce Mn oxides independently while results imply a synergistic relationship between Rhizobium sp. and selected species. Members of the Pedobacter and Nevskia genera are previously not known to oxidize Mn. Microstructural characterizaton show that the growth pattern of the birnessite-type Mn oxides is either dendritic/shrublike or spherulitic/botryoidal. Nucleation takes place in close association to the biofilm and initial Mn precipitates are observed at different locations depending on the mediating species. Encrustations of cells and other organic structures by Mn precipitates serve as stable nuclei for further growth. The close relationship appears to decrease in importance as the aggregates of poorly crystalline precipitates grow. In the more developed crystals, a repetitive pattern, Liesegang-type of rings, suggests that abiotic factors take over.