Engineers and corrosionists often attribute a big role to Iron oxidizers in MIC corrosion processes. It’s probably the name that creates these high expectations of Iron oxidizers in Microbially influenced corrosion. Another thing is dat Fe-oxidizing microbes are less studied than other physiological groups such as sulfate-reducing bacteria, which explains the lack of understanding of their role in MIC. Nevertheless, the last decade has seen a surge in interest in the environmental role of Fe-oxidizing bacteria, particularly in biocorrosion.
David Emerson has published a great review article that summarizes the role of Iron oxidizers and the current scientific progress. This review article focuses on two points:
- Firstly, to understand their current knowledge about the role they play in microbially influenced corrosion (MIC).
- Secondly, to comprehend their contribution to the larger microbiome responsible for MIC.
These microbes have the ability to oxidize iron, and many can use the captured electrons as their primary energy source. The abundance of iron present in mild steel, which is the most commonly used type of steel in large-scale infrastructure, supports their growth. However, the chemical behavior of iron is intricate, and both biological and abiological processes contribute to its oxidation, depending on the environmental conditions. This complexity can sometimes lead to confusion regarding the specific role of Fe-oxidizers in MIC.
This review will focus on bacteria that can grow on ferrous iron (Fe(II)) through lithoautotrophic or chemosynthetic processes. It is important to consider other reactions between microbes and iron as well, as they can be important and also a source of confusion.
Lithoautotrophic Iron oxidizers
Lithoautotrophic bacteria gain energy from the oxidation of Fe(II) to Fe(III) and use this energy to fix carbon dioxide (CO2) as their primary source of carbon. On the other side of the spectrum, heterotrophic Fe-oxidation refers to bacteria that catalyze the oxidation of Fe(II) but do not gain energy from the process or fix CO2, instead using organic matter as a carbon and energy source. Leptothrix discophora and Sphaerotilus natans are examples of such bacteria.
It is important to note that Fe(II) oxidizes in the presence of O2 resulting in the spontaneous precipitation of Fe-oxyhydroxides (rust) at circumneutral pH. These oxides can passively adsorb bacteria, so the mere association of a bacterium with Fe-oxyhydroxides does not prove whether it is catalytically oxidizing Fe(II) or playing a more passive role. Sediminibacterium spp. is a genus within the Bacteroidetes bacterial phylum that can grow on complex organic matter, and although it is often referred to as a member of iron-oxidizing bacteria in corrosion literature.
Lithoautotrophy is a process used by certain bacteria to obtain energy from the oxidation of ferrous iron to ferric iron. They use this energy to fix carbon dioxide as their primary source of carbon.
Heterotrophic Fe-oxidation is a process where microbes catalyze the oxidation of Fe(II) but do not gain energy from it. Instead, they use organic matter as a carbon and energy source. Examples of this process include Leptothrix discophora and Sphaerotilus natans that produce proteins or enzyme systems that actively catalyze Fe-oxidation, or Mn-oxidation, yet derive no energetic benefit from it.
It’s essential to note that at neutral pH, Fe(II) quickly oxidizes in the presence of O2, resulting in the spontaneous precipitation of Fe-oxyhydroxides (rust). These oxides can passively adsorb bacteria. Hence, the mere association of a bacterium with Fe-oxyhydroxides does not prove whether it is catalytically oxidizing Fe(II) or playing a more passive role.
One potential example of this is the genus Sediminibacterium spp., a member of the Bacteroidetes bacterial phylum, which is known for its capacity to grow on complex organic matter. In the corrosion literature, some papers refer to Sediminibacterium as a member of the iron-oxidizing bacteria due to the finding of 16S rRNA genes related to this organism being found in DNA extracted from corrosion products.
As an engineer, you may encounter the problem of microbial iron oxidation, specifically with nitrate-dependent Fe-oxidation. Nitrate reduction, often coupled with organic matter oxidation, is a common microbial metabolism that can favorably support microbial Fe(II) oxidation. However, the challenge lies in demonstrating that microbes grow via this process, using energy gained from nitrate-dependent Fe-oxidation to fix CO2 instead of using organic matter as their energy and carbon source.
One issue is that nitrate reduction produces nitrite (NO2), which can chemically oxidize Fe(II). Therefore, there are no reports of pure cultures of bacteria that can be readily sustained growing solely on Fe(II) and nitrate as either obligate, or facultative nitrate-dependent Fe-oxidizers. The distinction between anaerobic iron-oxidation being linked to a product of heterotrophic nitrate reduction, versus being a lithotrophic process, is important in determining if a nitrate-dependent biocorrosion community can sustain itself via lithoautotrophy, rather than requiring a significant input of organic matter.
Another group of organisms that may be involved in anaerobic Fe-oxidation are the photoferrotrophs that are capable of carrying out anoxygenic photosynthesis by using Fe(II) as an electron donor with light. Although photoferrotrophy has been documented in several groups of photosynthetic microbes, it is often an ancillary metabolism for cell growth, and it is difficult to find well-documented reports of photoferrotrophs being involved in MIC. This limitation is because these organisms require light, although some can grow at very low light levels, which restricts the possible MIC habitats to external surfaces where light is present.
In summary, nitrate-dependent Fe-oxidation and photoferrotrophs are two groups of organisms that may be involved in microbial iron oxidation. While nitrate-dependent Fe-oxidation has the potential to support microbial growth, it is challenging to demonstrate that microbes grow via this process, using energy gained from nitrate-dependent Fe-oxidation to fix CO2 instead of using organic matter as their energy and carbon source. Photoferrotrophs, on the other hand, require light, which restricts their possible MIC habitats to external surfaces where light is present.
Corrosion rates by Iron oxidizers
From an engineering perspective, the presence of iron-oxidizing bacteria (FeOB) in the microbiologically influenced corrosion (MIC) community is an important discovery. However, it is crucial to consider the practical implications of FeOB’s ability to accelerate corrosion through physiological and chemical reactions.
A study conducted by Lee et al. (2013) examined pure cultures of FeOB, including one isolated from a steel surface, which were incubated with MS coupons (steel samples). Surprisingly, the results did not show any evidence of pitting or a statistically significant increase in the corrosion rate compared to uninoculated controls. Another study by Mumford et al. (2016) used a reactor system to understand how Mariprofundus sp. DIS-1, an FeOB, colonizes steel surfaces. Despite rapid colonization, no pitting of the steel surfaces was observed.
However, when co-cultures of FeOB and Fe-reducing bacteria were grown on MS coupons, there was an increase in surface roughening. It is important to note that no active pitting or statistically significant increase in the corrosion rate was observed in this case.
In summary, while FeOB’s presence in MIC communities is significant, the practical implications of their ability to accelerate corrosion through physiological and chemical reactions seems thusfar to be limited. Studies have not proven a significant impact of FeOB on pitting and corrosion rates. The interaction between FeOB and other bacteria, such as Fe-reducing bacteria, may play a role in surface roughening but does not necessarily lead to active pitting or a significant increase in the corrosion rate.
Biofouling and clogging
Many studies on freshwater systems have focused on how FeOB contribute to clogging and biofouling in water wells. This process can lead to corrosion, but it’s challenging to tell if the corrosion is caused by biocorrosion (microbially influenced breakdown of steel infrastructure) or just the accumulation of biofilms on the interior surfaces of the pipes. Typically, the groundwater being pumped through these systems has relatively high concentrations of Fe(II), which can support the growth of FeOB.
Iron oxidizers and EMIC/CMIC principles
Most studies on microbially influenced corrosion (MIC) have focused on the chemical processes involved, such as the production or consumption of metabolites that accelerate corrosion. This is known as chemical MIC (CMIC). However, recent research has revealed another form of MIC called electrical MIC (EMIC), where anaerobic microbes can directly transfer electrons from the metallic surface to their cells, enabling energetically favorable redox reactions that provide energy for microbial growth.
EMIC is most well-documented in sulfate-reducing bacteria (SRB), but there is also evidence for methanogens and acetogenic bacteria to stimulate corrosion through EMIC. Fe-oxidizing bacteria (FeOB) oxidize Fe(II) at the cell exterior and transfer the electrons to the electron transport chain that energizes the cell on the inner cytoplasmic membrane of the cell. However, there is currently no direct evidence to support EMIC in FeOB, especially as a means of accelerating steel corrosion, as has been shown in SRB. To take advantage of EMIC, FeOB would need to adhere directly to the steel surface, which has not yet been observed in FeOB that grow well on MS. Thus, FeOB’s role in EMIC and its potential to accelerate steel corrosion remains an open question in corrosion research.
Cyc2 as Iron oxidizer biomarker
So far, researchers have found that FeOB that grow well on MS produce stalks that attach to the metal surface, moving the cell away from the surface. These stalks are made up of ferrihydrite-like ironoxyhydroxides, which are not good conductors of electricity. However, in mature corrosive biofilms where SRB are present and complex mineral crusts form, including iron sulfides, FeOB could take advantage of the conductive properties of these minerals. It is possible that under micro-oxic conditions, FeOB could capture electrons directly from these minerals as well, although more research is needed to confirm this.
Researchers are also trying to gain a better understanding of how FeOB conserve energy via Fe-oxidation. Recent progress has been made with the discovery of a porin-cytochrome c complex protein (Cyc2) found in all known freshwater and marine lithotrophic FeOB. This protein has iron oxidase activity and is highly expressed in environments where biogenic Fe-oxidation is active. As such, Cyc2 has utility as a functional marker for FeOB.
There are a few commercial kits in the market that actually offer this relative new biomarker.
Iron ozidizers, slowing down corrosion
Research on the Fe-reducing bacterium Shewanella oneidensis has shown that microbial-induced corrosion (MIC) can have both inhibitory and facilitative effects on corrosion. When Fe-oxyhydroxides are reduced, Fe(II) is produced and can lead to enhanced O2 consumption near the metal surface due to the re-oxidation of Fe(II), which inhibits corrosion. However, other studies have shown that H2 consumption coupled with Fe-reduction can actually facilitate biocorrosion. To better understand the role of microbial communities in MIC, co-cultures or synthetic consortia of Fe-oxidizers, Fe-reducers, sulfate-reducers, and hydrogenotrophs can be used. These studies can provide insight into the synergistic effects of microbial communities on corrosion and help to develop strategies to selectively inhibit the most aggressive microorganisms responsible for MIC. Overall, a better understanding of how the MIC microbiome functions can lead to the development of effective ecosystem management practices for treating biocorrosion.
NOTE: This article is derived from the work from David Emerson, The role of iron-oxidizing bacteria in biocorrosion:
a review. Link to the original article