CAREER: Oxygen & Iron Isotopic Analyses of Bacterially-Produced Iron Oxides in the Laboratory & in Acid Mine Drainage: A Watershed Approach to Classroom Instruction & Research

Colorado School Of Mines, Golden CO

Investigators

Abstract

9985234 Mandernack Oxygen and Iron Isotopic Analyses of Bacterially-Produced Fe Oxides in the Laboratory and in Acid Mine Drainage: A Watershed Approach to Classroom Instruction and Research Bacteria serve as important catalysts in many geochemical cycles, including the oxidation of Fe(II) and Mn(II) in aquatic and terrestrial environments. As catalysts, bacteria influence the mobilization and precipitation of these metals. The formation of insoluble Fe(III) and Mn(III, IV) oxides that result from bacterial oxidation in turn can control the fate of other trace metals, as these oxides can actively adsorb metal cations. In soils and in freshwater and marine nodules, iron and manganese oxides often coexist as ferromanganese deposits. In the case of acid mine drainage (AMD) where pH is low (e.g., <3.0), Mn(II) oxidation is inhibited and bacterial Fe(II) oxidation is favored. Therefore, pure (Mn-free) Fe(III) oxides of bacterial origin are likely to be formed in AMD environments, and in turn can control the fate and transport of other trace metals that are common to AMD waters. The importance of bacteria in the formation of AMD and in the precipitation of iron oxides is well documented. The bacterial influence on Fe(II) oxidation may be preserved in the oxygen isotopic signatures of the Fe oxide products. Previous results of mine from oxygen isotope analyses of Mn(IV) manganates indicated that the d18O values of chemically and biologically produced manganates may be discerned due to differences in isotopic fractionation and oxidation pathways. Furthermore, evidence suggests that the binding of Mn2+ or Fe2+ to the bacterial cell wall may be the rate limiting step for subsequent oxidation and mineral formation, and thus exert an important kinetic control on the final d18O value of bacterially-produced Fe and Mn oxides. Coexisting Fe and Mn oxides in a freshwater nodule had very different d18O values as a result of different oxidation pathways . Consequently, the d18O values of iron and manganese oxides may independently reflect different types of information (Fe oxides can reflect temperature and the d18O of water, whereas Mn oxides have been shown to reflect bacterial origins by incorporation of molecular O2, and consequently may also serve as paleo-oxygen indicators). The d18O signatures of mixed phases of iron and manganese oxides, therefore, may reflect abundant information for paleo-environmental studies. This will be examined from additional d18O studies of deep-sea ferromanganese nodules that are part of the extensive collection at the Smithsonian Natural History Museum. Although, iron oxides are the most abundant metal oxide in soils and sediments, there has been no systematic laboratory study of the bacterial influences on the d18O signatures of Fe(III) oxides. Therefore, this study will examine the d18O and d 56Fe values of Fe(III) oxides produced from laboratory cultures of Fe2+-oxidizing bacteria, as well as those produced in AMD environments. AMD environments provide a convenient location to study the activities of certain iron oxidizing bacteria that are important in AMD formation and to collect bacterially-produced Fe(III) oxides for stable isotopic anlysis. The close proximity of AMD sites that drain into the Clear Creek watershed near the Colorado School of Mines Campus provides an ideal opportunity for students to participate in this research and be engaged in an active learning experience as part of my ongoing courses in environmental microbiology. The students will receive instruction in microbiological principles that directly apply to a geochemical problem common in Colorado watersheds. The focus of my classes will shift from a classic lecture-style approach, to hands-on instruction whereby students learn microbiological principles as they apply to a local watershed. This will provide the students with an experiential and highly visual means for learning microbiology while engaged in original research.