Anaerobic microbial iron corrosion due to conductive pili

Iron, a well-known material susceptible to rusting, can also undergo a process called electrobiocorrosion where it decomposes due to certain bacteria. Geobacter sulfurreducens, a bacterium that resides in sediment, utilizes electrically conductive protein threads to carry out this process. Through the production of magnetite from iron, Geobacter promotes further corrosion in a positive feedback loop. This discovery was reported by a team of researchers in the scientific journal Angewandte Chemie.

Bacterial biofilms, particularly those formed by Geobacter, are the leading cause of microbial metal corrosion, which results in more costly damages compared to other biofilm-related issues. Geobacter is an anaerobic genus that draws energy from the transfer of electrons from iron, forming magnetite in the process. The underlying mechanism of how Geobacter corrodes iron has remained a mystery for a long time.

In an effort to uncover the details of electrobiocorrosion, a team of researchers from Northeastern University in Shenyang, China, led by Dake Xu, focused on investigating the role of electrically conductive pili. These pili are thin filaments that extend from the bacteria and exhibit conductivity. The researchers suspected that the pili could directly withdraw electrons from metal surfaces, contributing to the corrosion process.

To test their hypothesis, two strains of Geobacter were allowed to grow on a stainless-steel surface until biofilms formed. One strain had the ability to form conductive pili, while the other strain had genetically modified pili with lower conductivity. The researchers observed that the strain with conductive pili outperformed the other strain, showing more growth and causing deeper pits in the metal. They also measured a corrosion current, which directly indicated iron oxidation.

Based on their observations, the researchers concluded that the bacteria with conductive pili established an “electrical connection” with the metal. Furthermore, bacteria located further away in the biofilm, not in direct contact with the metal, were still able to obtain electrons through the conductive pili.

Considering that magnetite, a common product of iron corrosion, conducts electricity, the researchers also investigated its influence on microbial corrosion. They found that adding magnetite to the biofilm not only increased the growth of Geobacter but also intensified the corrosion current measured on the metal surface.

The researchers emphasize the significant corrosion implications of their finding that magnetite facilitates electrobiocorrosion. In future efforts to improve corrosion protection, they recommend considering the propensity of materials to form magnetite.

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