Anaerobic Microbial Iron Corrosion Due to Conductive Pili
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
Iron is well-known for rusting, but this doesn’t just happen on
contact with oxygen and water. Some bacteria are also able to able to
decompose iron anaerobically in a process referred to as
electrobiocorrosion. The sediment-dwelling bacterium Geobacter sulfurreducens uses electrically conductive protein threads for this purpose, as a team of researchers reports in the journal Angewandte Chemie. They produce magnetite from the iron, which promotes further corrosion in a positive feedback loop.
Bacterial biofilms are the cause of microbial metal corrosion, a
destroyer of metals which causes more costly damage than all other
biofilm-related damage put together. Electrobiocorrosion is often caused
by bacteria such as those found in river sediments, for example, the
anaerobic genus Geobacter. Geobacter does not use
atmospheric oxygen for respiration; instead, it draws energy from the
transfer of electrons from iron, forming magnetite in the process. Thus
far, the way in which Geobacter corrodes iron metal has been something of a mystery.
The exact mechanism of action of electrobiocorrosion has now been
investigated more closely by Dake Xu and colleagues from Northeastern
University in Shenyang, China. The team worked on the assumption that
electrically conductive pili, thin filaments which grow out of the
bacteria, could play an important role in this mechanism. Geobacter
forms “e-pili” from conductive proteins, and these e-pili act like
electric wires, conducting electricity. Before this study, it was
unclear whether the e-pili could withdraw electrons directly from metal
surfaces.
In order to prove the team’s suspicions, namely direct electron withdrawal, they left two strains of Geobacter
to grow on a stainless-steel surface until biofilms formed. One of the
two strains formed conductive e-pili, while the other still produced
pili, but had been genetically modified so that the pili were formed
from less conductive proteins. The researchers observed that the
bacterial strain that grew e-pili fared significantly better on the
steel plate. It grew more and made deeper pits in the metal,
demonstrating how much metal it was consuming. The team also measured a
corrosion current, a direct sign of the oxidation of iron.
The team concluded that the bacteria with the e-pili formed a sort of
“electrical connection” to the metal. Bacteria located further away in
the biofilm, not in direct contact with the metal, were also able to
supply themselves with electrons using e-pili.
Because magnetite is formed during the corrosion of iron, and this
mineral also conducts electricity, the team also investigated its
influence on microbial corrosion. They noted that not only did adding
magnetite to the biofilm increase the growth of Geobacter, it
also led to a stronger corrosion current measured at the surface of the
metal: “The finding that magnetite, a common corrosion product,
facilitates electrobiocorrosion has significant corrosion implications,”
the team emphasize. For future attempts to improve corrosion
protection, therefore, they recommend taking the propensity of materials
to form magnetite into consideration.
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About the Author
Dr. Dake Xu is a
professor at the School of Materials Science and Engineering,
Northeastern University, China. His research interests are microbial
corrosion and antibacterial and antibiofouling materials. He is
currently working in the Electrobiomaterials Institute, Key Laboratory
for Anisotropy and Texture of Materials, Northeastern University, which
is supervised by Prof. Derek R. Lovley.
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