The redox cycling of metals and mineral formation by metal-reducing bacteria can play crucial roles in controlling the mobility fate of both inorganic and organic species in a range of environments and may offer a basis for a wide range of innovative biotechnological processes. The objectives of this study were to examine microbially induced mineralization and precipitation of various metals such as Fe(III), Ag(I), Cr(VI), and Se(VI) by the facultative metal-reducing bacteria (Haejae-2 and Suncheon-1) enriched from intertidal flat sediments in South Korea.
In Chapter I, Haejae-2, consisting mainly of Clostridium sp., reduced poorly crystalline or amorphous iron-oxides such as akaganeite and ferrihydrite, and transformed them to more stable phases via glucose fermentation. Akaganeite and ferrihydrite were transformed to goethite within 5 days, and then reduced to magnetite and siderite after 21 days. The formation of siderite was influenced by the iron reduction rate and bicarbonate concentration in the media. Therefore, the iron reduction rate and secondarily reduced minerals were affected by the iron-oxides crystallinity and the chemical composition in the culture media. The Clostridium sp. of Haejae-2 contributed to iron reduction via glucose fermentation while Shewanella sp. of PAH 93 reduced iron using glucose and lactate via fermentation and respiration under anaerobic conditions. The results indicated that the bacteria, Clostridium sp. and Shewanella sp., contributed to dissimilatory iron reduction through different pathways.
In Chapter II, the properties of microbially formed magnetite nanoparticles were investigated. The magnetite nanoparticles formed by Clostridium sp. had around 10 nm in size and were spherical in shape. Unlike chemically synthesized magnetite, the biogenic magnetite nanoparticles were coated with organic matter containing an abundance of reactive carboxyl groups (-COOH) without any chemical process for functionalizing them. The functional organic matter may have been formed by the cell surface materials or microbial secretions such as EPS. The results of FT-IR and XPS analyses showed that the binding states of the organic matter were chemically stable. The magnetite-organic complex nanoparticles immobilized albumin on top of the carboxylic groups located on the particles’ surfaces and showed the that biogenic magnetite has high potential for serving as a useful and applicable material in relevant medical technologies. Therefore, such microbial processes may facilitate simple preparation of functional magnetite-organic complex nanoparticles which have benefits for biomedical applications such as being used as contrast agents in magnetic resonance imaging (MRI), drug delivery systems (DDS), and protein immobilization due to the reactive functional groups on the organic membrane.
In Chapter III, microbially induced precipitation and mineralization were investigated with various metals to understand which mechanism is more favorable for metal reductions of Cr(VI), Se(VI), and Ag(I). As a result, the anaerobically enriched bacteria, Suncheon-1, consisting of Shewanella sp., Clostridium sp., and Vibrio sp. reduced potassium chromate (K2CrO4) and sodium selenate (Na2SeO4) by extracellularly forming chromium hydroxide (Cr(OH)3) precipitates and amorphous elemental Se(0) nanoparticles, respectively, via their metabolism. The results showed dissimilatory metal reduction by electrons generated from oxidation of carbon compounds during microbial respiration or fermentation processes caused by biotransformation from toxic states of heavy metals to less toxic and more immobile states in contaminated environments. While the Cr(VI) and Se(VI) reductions occurred and were promoted by bacterial growth, Ag(I) reduction prevented bacterial growth. However, silver nitrate (AgNO3) was rapidly reduced to elemental silver with living bacteria in a stationary phase which indicated that the reduction was probably due to the nitrate reductase participation as an electron shuttle. Suncheon-1 formed 5 - 15 nm sized Ag(0) nanoparticles by reduction of 0.5 ？ 1 mM silver nitrate under the conditions around 15 ？ 25℃ and pH 7.5 ？ 8.5 within 3 days of incubation. This study demonstrated that the formation rate of Ag(0) nanoparticles and the size of the nanoparticles could be manipulated by controlling microbial growth conditions such as temperature, concentration of silver nitrate, and pH in the medium. Therefore, these results not only show that metal-reducing bacteria enzymatically reduced Ag(I), but also offer new methods for microbial synthesis of homogeneous A(0) nanoparticles and Ag(0) recovery from natural environments.
These results suggest that effective methods of bioremediation and biomineralization in metal-contaminated sites should be selected depending on the metals present because each type of metal may have a main pathway which can be utilized to induce mineralization or precipitation. Therefore, the metal-reducing bacteria living in the intertidal flat sediment may play many important roles for bioremediation of heavy metals, and in the cycles of organic matter and metals in subsurface environments, as well as in the synthesis of nano-sized minerals through metal-reductive biomineralization which can be employed in various industrial applications.