Coupled Dynamics of Carbon and Manganese in P. Putida-Birnessite Assemblages.

Biological oxidation and precipitation is the major pathway for Mn oxide formation; this process is widely spread among bacteria and fungi. Manganese oxides occur typically as nanoparticles that exhibit significant chemical and structural disorder. Furthermore, the mineral is embedded within a biomass matrix composed of microbial cells and extracellular polymeric substances, where the biomass not only provides reactive surfaces but cells remain metabolically active long into stationary phase. These properties of biomass-mineral assemblages raise two questions: To what extent does admixing with microbial biomass modify the reactivity of biogenic Mn oxide minerals? What is the role of abiotic and biotic redox transformations of the mineral phase? We have approached these questions through laboratory scale studies of model Pseudomonas putida-MnO2(s) assemblages, where the mineral phase was either accumulated in situ through enzymatic Mn2+ oxidation or physically admixed using an abiotic preparation of ?-MnO2(s). The extent and mechanisms of organo-mineral interactions as well as changes in the redox state of the mineral phase were monitored using a combination of chemical and spectroscopic methods. We found that admixing of mineral nanoparticles with biomass reduced the reactivity of the edge sites of Mn oxides through the attachment of organic moieties to the mineral particles and/or modification of the assemblage surface charge properties. In addition, the interaction of biomass components with MnIVO2 particles caused partial Mn(IV) reduction and biomass oxidation. In contrast, interaction of bacterial biomass with MnIII,IVO2 led to an increase in the Mn(IV) content of the mineral, presumably due to biotic oxidation. Our results demonstrate the ability of microbial biomass to directly and indirectly sustain the redox cycling of Mn between its three different oxidation states. The mechanisms identified in our work have wide implications in understanding organic matter-mineral interactions in soil systems, which are central to metal attenuation and elemental cycling.