Microbiologically influenced corrosion of steel in anaerobic environments has been attributed to hydrogenotrophic microorganisms. A sludge sample collected from the bottom plate of a crude-oil storage tank was used to inoculate a medium containing iron (Fe 0 ) granules, which was then incubated anaerobically at 37°C under an N 2 -CO 2 atmosphere to enrich for microorganisms capable of using iron as the sole source of electrons. A methanogen, designated strain KA1, was isolated from the enrichment culture. An analysis of its 16S rRNA gene sequence revealed that strain KA1 is a Methanococcus maripaludis strain. Strain KA1 produced methane and oxidized iron much faster than did the type strain of M. maripaludis, strain JJ T , which produced methane at a rate expected from the abiotic H 2 production rate from iron. Scanning electron micrographs of iron coupons that had been immersed in either a KA1 culture, a JJ T culture, or an aseptic medium showed that only coupons from the KA1 culture had corroded substantially, and these were covered with crystalline deposits that consisted mainly of FeCO 3 .Iron (Fe 0 ) is an inexpensive metal and is widely used in many industrial processes and industrial/commercial products. When iron contacts an aqueous electrolyte, it readily corrodes. This happens because, as a result of metallurgical and environmental heterogeneities, the electrolytes are not evenly distributed across the surface of the metal and consequently the electric potential is also unevenly distributed. Therefore, electrons flow within the metal from an area of higher electrical potential (the anode) to an area of lower electrical potential (the cathode). At the anode, iron atoms lose electrons and dissolve into ferrous ions (Fe 2ϩ ), whereas cations or elements dissolved in solution (e.g., H ϩ under anaerobic conditions or O 2 under aerobic conditions) are reduced by electrons at the cathode.The corrosion of structures that contain iron is economically devastating. It has been estimated that in the United States alone, the cost of corrosion is 276 billion dollars annually (17). Iron is corroded not only by physiochemical processes but also by the metabolic activity of microorganisms; this metabolic process is termed microbiologically influenced corrosion (MIC). Some 10% of all corrosion damage may be the result of microbial activity (15), and sulfate-reducing bacteria (SRB) are widely regarded as the causative agents of MIC in anaerobic environments (11,12,18,21). The mechanism by which SRB stimulate iron corrosion may occur via the uptake of electrons at the cathodic surface of iron (cathodic depolarization) in conjunction with sulfate reduction (8e Ϫ ϩ SO 4 2Ϫ ϩ 10H ϩ 3 H 2 S ϩ 4H 2 O) (27), while at the anionic surface, iron atoms are oxidized to ferrous ions (Fe 3 Fe 2ϩ ϩ 2e Ϫ ). In fact, certain SRB use not only hydrogen but also iron as a source of electrons for sulfate reduction (1, 9, 22). Because not all SRB grow as fast in the presence of iron as they do in the presence of hydrogen (9), fast-growing SRB...
dMicrobiologically influenced corrosion (MIC) of metallic materials imposes a heavy economic burden. The mechanism of MIC of metallic iron (Fe 0 ) under anaerobic conditions is usually explained as the consumption of cathodic hydrogen by hydrogenotrophic microorganisms that accelerates anodic Fe 0 oxidation. In this study, we describe Fe 0 corrosion induced by a nonhydrogenotrophic nitrate-reducing bacterium called MIC1-1, which was isolated from a crude-oil sample collected at an oil well in Akita, Japan. This strain requires specific electron donor-acceptor combinations and an organic carbon source to grow. For example, the strain grew anaerobically on nitrate as a sole electron acceptor with pyruvate as a carbon source and Fe 0 as the sole electron donor. In addition, ferrous ion and L-cysteine served as electron donors, whereas molecular hydrogen did not. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain MIC1-1 was a member of the genus Prolixibacter in the order Bacteroidales. Thus, Prolixibacter sp. strain MIC1-1 is the first Fe Metallic iron (Fe 0 ) and stainless steel corrosion causes a severe economic burden. The annual cost of corrosion worldwide has been estimated to exceed 3% of the world's GDP (http://www .nace.org/uploadedFiles/Publications/ccsupp.pdf). Under aerobic conditions, molecular-oxygen-mediated corrosion may be predominant, whereas under anaerobic conditions, microbiologically influenced corrosion (MIC) is believed to be a major cause of corrosion-related failures (1). Specifically, sulfate-reducing bacteria (SRB) are considered to be major causative microorganisms of MIC in anaerobic environments because FeS has frequently been observed as a major corrosion product (2).The mechanism underlying Fe 0 corrosion by SRB has been explained on the basis of the cathodic-depolarization theory: Fe 0 oxidation occurs at the anode (Fe 0 ¡ Fe 2ϩ ϩ 2e Ϫ ), while protons are reduced to molecular hydrogen at the cathode (2e Ϫ ϩ 2H ϩ ¡ H 2 ). In the presence of SRB, a hydrogenase of the organism consumes either molecular hydrogen or electrons at the cathode, thereby accelerating the anodic Fe 0 oxidation (3, 4). Recently, a methanogenic archaeon and an iron-oxidizing bacterium were shown to cause Fe 0 corrosion (5-7). These reports suggest that diverse microorganisms contribute to metal corrosion.MIC of metallic materials imposes a heavy economic burden, and technologies for MIC control are being actively investigated. However, the process of MIC is still poorly understood, and further research is required. The characterization of Fe 0 -corroding microorganisms and a clear understanding of the mechanism of MIC are required for effective MIC prevention and control. In this study, we isolated a novel Fe 0 -corroding bacterium from a crude-oil sample collected at an oil well and investigated its Fe 0 corrosion activity under a variety of culture conditions. MATERIALS AND METHODSSampling of crude oil. Crude oil was sampled from an oil well in Akita Prefecture, Japan, on 16 December...
Elemental iodine is produced in Japan from underground brine (fossil salt water). Carbon steel pipes in an iodine production facility at Chiba, Japan, for brine conveyance were found to corrode more rapidly than those in other facilities. The corroding activity of iodide-containing brine from the facility was examined by immersing carbon steel coupons in "native" and "filter-sterilized" brine samples. The dissolution of iron from the coupons immersed in native brine was threefold to fourfold higher than that in the filter-sterilized brine. Denaturing gradient gel electrophoresis analyses revealed that iodide-oxidizing bacteria (IOBs) were predominant in the coupon-containing native brine samples. IOBs were also detected in a corrosion deposit on the inner surface of a corroded pipe. These results strongly suggested the involvement of IOBs in the corrosion of the carbon steel pipes. Of the six bacterial strains isolated from a brine sample, four were capable of oxidizing iodide ion (I(-)) into molecular iodine (I(2)), and these strains were further phylogenetically classified into two groups. The iron-corroding activity of each of the isolates from the two groups was examined. Both strains corroded iron in the presence of potassium iodide in a concentration-dependent manner. This is the first report providing direct evidence that IOBs are involved in iron corrosion. Further, possible mechanisms by which IOBs corrode iron are discussed.
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