Microbial corrosion

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Introduction

Microbial corrosion is a form of biodeterioration, and is frequently referred to as biocorrosion or microbially influenced corrosion (MIC)¹. This degradative process primarily acts on metals, metalloids, minerals, and other rock-based materials. Aside from bacteria, fungi, microalgae, and naturally occurring organic/inorganic chemicals contribute to biocorrosion.

Associated taxa

Many different bacterial genera are associated with MIC and are often categorized by their respiration techniques. Some common examples of these are:

•Sulfate-reducing bacteria (SRB), including Desulfuromonas sp., Desulfobacter sp., and Desulfococcus sp., etc. The SRB are strictly anaerobic.²

•Sulfur-oxidizing bacteria, primarily of the genus Thiobacillus. These organisms notably form sulfuric acid during oxidation, and are capable of both oxidizing sulfur and ferrous iron.²

•“Iron” bacteria, of which there are two types: ‘stalked’ (notably Gallionella sp.) and ‘filamentous’ (Leptothrix sp., Clonothrix sp., Sphaerotilus sp., etc.). These bacteria primarily oxidize or reduce iron species during respiration.²

There are many other miscellaneous groups of bacteria that use or respire metallic substances and contribute to biocorrosion. Some Pseudomonas species can reduce iron, while other bacterial species can oxidize/reduce less abundant metals like manganese, palladium, nickel, etc. There are also biocorrosive bacteria that do not utilize metals but secrete organic acids which can break down surrounding materials.¹ ² It is important to note that not all species within a genus induce MIC and not all perform the same types of respiration.

Mechanisms of action

Despite a lot of experimentation and investigation into the detailed mechanisms of microbial corrosion, many elements of these processes are still unknown. In general, the process involves such mechanisms as: acidic degradation, electron movement, metal depolarization, polymerization and attachment of biofilms, and mineral formation.³

Biofilms

Biofilms are a primary tool used by bacteria to adhere to a substrate’s surface and facilitate metabolism/respiration optimization. In the case of MIC, biofilms are an essential part of the degradation process. Bacteria create biofilms by producing extracellular polymeric substances (EPS) such as lipids, polysaccharides, nucleic acids, and proteins. These substances form complex matrices in which the bacteria can reside and carry out essential functions. Fungal species can also create and reside within biofilms, and are often found within close vicinity of bacterial biofilms. The ability of a biofilm to bind to a metal substrate is dependent on the bonding affinity of the metallic ions and the anionic functional groups of the EPS’s.¹ Biofilms can be very diverse, and within one small area of biofilm, multiple species can be found and in very close proximity. Therefore, the actions of some species can affect the others. Scientific evidence suggests that within biofilms certain bacterial species might actually inhibit corrosion. Instead of physically blocking adhesion to the metal substrate, as originally hypothesized, the removal of nutrient sources and metabolites necessary for the functioning of other corrosive species has been observed.⁵ The close proximity of biofilms to metal substrates establishes electrochemical gradients that allow the generation of free electron flow at anodic sites (site where metal is dissociated by bacteria).³ The cathodic sites are found where oxygen is present and where the corrosion is visualized (rusting, fouling, etc).

Chemical degradation

Microbes can either fully corrode substances or partially degrade them, the latter allowing the substance to be further broken down by other organisms or chemicals. In most cases the microbes secrete exoenzymes that solubilize or dissimilate components of the substrate like complex compounds or large molecules. This action facilitates more rapid degradation of the substrate. Such enzymes include hydrogenases, catalases, and oxioreductases, which are found within biofilm matrices or the surrounding environment. The complete biocorrosion of metals is the coupled action of exoenzyme secretion, causing initial degradation, followed by oxidation either by free oxygenic chemicals or microbial oxidation.⁶ Many of these chemicals secreted by the microbes that solubilize metals and are not degraded by the microbes are known as chelators.³ Chelators are often coupled with ‘shuttle compounds’ that are reduced by the microbe with electrons from metabolic oxidation, which in turn reduce the metal ions of the substrate. Humic acids are a major naturally-occurring group of chelators that include quinoids, such as 2,6-anthraquinone disulphonate (AQDS), and other acidic compounds.⁷

Microbial uses

Bacterial species that perform MIC and form biofilms on metallic substrates use metal ions as terminal electron acceptors (TEA) during respiration. Metal respiring bacteria require extracellular reduction of metal compounds, often in a biofilm matrix, as they cannot uptake these compounds into the cell.⁸ This is termed “dissimilatory metal reduction” because metals are used solely as TEA’s and cannot be incorporated into bacterial biomass.⁹ The use of chelators and shuttle compounds allows for free electrons to be released and subsequently used by the bacteria for respiration. Many fungi are also capable of metal reduction in order to gain access to substrate-bound electrons. Some bacteria, like Shewanella sp. use nanowires, which are electrically conductive filamentous appendages that transfer electrons directly to metal surfaces. Most microbes that use metal substrates do so because their environments are nutrient poor, anaerobic, or they have few other choices.

Effects on materials

File:Corrodedpipe.jpeg

Extensive corrosion of an industrial pipe by sulfur bacteria.

References

(1) Lindow, S. Brandl, M. 2003. "Microbiology of the Phyllosphere." Applied and Environmental Microbiology. Vol. 69, No.4. 0099-2240

(2) Delmonte, N., Knief, C., Chaffron, S., 2009. "Community proteogenomics reveals insights into the physiology of phyllosphere." National Academy of Sciences.

(3) Machowicz-Stefaniak, Z., Krol, E. 2006. "Biotic effect of caraway phyllosphere fungi on the pathogenic fungus." Department of Plant Pathology University of Life Science. 2-8

(4)Suslow, T. 2005. "Microbial Food Safety IS Your Responsibility." Vegetable Research Information Center. 1-6.

(5) Howplantswork. 2009. "Life in the Phyllosphere: What Microbes Commonly Dwell on the Surface of Leaves?."

(6) Whipps, J.M. Hand, P. Pink, D. 2008. "Phyllosphere microbiology with special reference to diversity and plant genotype." 2-34

(7) Gardener, B., Frafel, D. 2002. "Biological Control of Plant Pathogens: Research, Commercialization, and Application in the USA." Plant Health Progress. 1-18

(8) Abell, G., Richter, A. 2008. "Nitrogen fixation by phyllosphere bacteria associated with higher plants and their colonizing epiphytes of a tropical lowland rainforest of Costa Rica." The IMSE journal. 2, 561–570

Edited by student of Angela Kent at the University of Illinois at Urbana-Champaign.