Mechanisms for Fe(III) Oxide Reduction in Sedimentary Environments

The reduction of Fe(III) is one of the most significant reactions that takes place as anaerobic conditions develop in soils and sediments. The mechanisms by which Fe(III) is reduced in sediment and by pure cultures was examined in the research. Other than by direct contact between Fe(III) reducing microorganisms and Fe(III) oxide minerals, there are two ways Fe(III) oxide can be reduced. Fe(III) oxide reduction can occur without contact via electron shuttling by an environmental or microbially produced molecule or by chelation of Fe(III) by an environmental or microbially produced molecule. Both the environment and cultures were examined for these phenomena. Humics and humic analogs were found to serve as electron shuttles between Fe(III) oxides and Fe(III) reducing microorganisms in aquifer sediments while, other compounds found to serve as electron shuttles in culture did not function as electron shuttles in soils and sediments. Environments where humus materials are capable of serving as electron shuttles were located. Environments also existed where there were appreciable amounts of soluble Fe(III), indicating that chelators are present in these environments. The Fe(III) reducing microorganism Geobacter metallireducens was found to reduce Fe(III) oxides only via direct contact. The method used previously to assay for reduction of Fe(III) minerals without contact, dialysis tubing, was shown to be invalid. Another method was developed, by which Fe(III) oxides were entrapped in alginate beads. This method together with, cell suspensions, thin layer chromatography and soft agar plates were used to assay for the electron shuttling capacity. Both Geothrix ferrnentans and Shewanella alga strain BrY where found to produce both electron shuttles and Fe(III) chelators, thus eliminating the need for contact between Fe(III) mineral and these Fe(III) reducing microorganisms thus exhibiting methods of Fe(III) reduction different from Geobacter metallireducens