Mic Of Steels By Iron Reducing Bacteria

Sulfate reducing bacteria (SRB) are generally recognized as the most destructive agents in microbiologically influenced corrosion (MIC) of steel under anaerobic conditions. However, other bacteria with different physiological properties are also considered to play roles in corrosion processes. These include the iron reducing bacteria (IRB), or "iron reducers", which reduce ferric ion to ferrous ion in anaerobic environments. The role and importance of IRB in MIC has been a matter of controversy. Some work indicate that they are directly important in causing or enhancing corrosion, while other work show that iron reducers may actually confer protection against biotic or abiotic aggressive agents. In this work we are giving two examples on the role of IRB in corrosion of carbon steel in chloride containing media: one is a laboratory case involving a marine vibrio (Vibrio alginolyticus) and SRB and the other is a field case in a water injection line of an oil platform located in the Gulf of Mexico where the IRB (Shewanella and Vibrio) are growing in biofilms with SRB and other marine bacteria.

Iron reducers derive benefit from reduction of Fe³⁺ by using this ion as a terminal electron acceptor in their metabolism. This can be done in two ways: either through the use of an electron transport chain in anaerobic respiration, or by the use of Fe³⁺ as an auxiliary "electron sink" during fermentation. Organisms using an electron transport chain under anaerobiosis produce more energy than can be produced through fermentation if reducible ions such as nitrate or ferric ion can be used as terminal electron acceptors in place of molecular oxygen. These organisms are usually facultative anaerobes which use oxygen aerobically, and switch to a reducible ion as a terminal electron acceptor under anaerobiosis. Thus, nitrate reducing and denitrifying bacteria, for example, and some ferric ion reducers can utilize this type of pathway to grow quite rapidly under anaerobic conditions. In laboratory media with high concentrations of soluble ferric ammonium citrate, this type of iron reducer is observed to rapidly reduce nearly all of the soluble iron in the medium. Reduction of Fe⁺³ is growth linked and for some of these bacteria significant anaerobic growth does not occur in the absence of Fe⁺³. Many of the bacteria using anaerobic respiration for reduction of ferric ion can also use nitrate as a terminal electron acceptor¹. The iron reducers which use ferric ion as a terminal electron acceptor in anaerobic respiration have been of great interest in steel corrosion studies. A good example is Shewanella putrefasciens². For most organisms grown in the absence of oxygen, a fermentative mechanism is used and the bacteria usually use an organic compound as their final electron acceptor. To augment the limited amount of ATP produced under fermentation, Fe³⁺ can be used under anaerobic conditions by many fermenting bacteria as an auxiliary electron acceptor r "electron sink" during fermentative processes³. Growth of these fermenters is enhanced under anaerobic conditions by addition of ferric ion to the medium.