INVESTIGATING THE POTENTIAL FOR MICROBIAL REDUCTION OF STRUCTURALLY COORDINATED IRON FROM COMMON CLAY MINERALS

Recent experimental evidence has demonstrated the capacity of some Fe-reducing bacteria to reduce structurally-bound Fe(III) in the clay mineral nontronite, an iron-rich smectite. Structural Fe-reduction may be accompanied by the dissolution of a portion of the smectite lattice, the neoformation of authigenic illite, and the liberation of silica to solution. Since iron is a limiting nutrient in many settings in the world ocean, it is possible that the operation of this microbial pathway might affect rates of carbon cycling in clay-rich sediments. Previous experimental studies have focused on the clay mineral nontronite, a member of the smectite group that is rich in iron, but is rare in the oceans. At present, it is unknown if microbial reduction of structurally coordinated Fe(III) can occur in less Fe-rich smectite clays, which are abundant and widespread in the oceans. To investigate the potential for microbial reduction of structurally bound iron in common smectites, four different smectites were exposed to the Fe-reducer Shewanella oneidensis for time increments ranging from 7 to 90 days. Additionally, the same suite of clay minerals was exposed to an iron-reducing bacterium cultured from anaerobic sediments of the San Pedro Basin, CA. Following exposure, the degree of structural Fe-reduction in each sample was assessed by measuring the concentration of Fe (II) in solution. Microbially-induced changes in mineralogy were then characterized using X-Ray Diffraction, Scanning Electron Microscopy, and Energy Dispersive X-Ray Spectroscopy.