The iron sulphide (FeS) formed during this process works as a catalyst, promoting the reduction of protons on the material surface, which increases electron transfer, resulting in fast dissolution of the metal. Gu et al. proposed another theory called the bio-catalytic cathodic sulphate reduction (BCSR) theory
[40][22]. The BCSR theory postulates that in MIC caused by SRB, sulphate works as a cathode while iron works as an anode. With the help of a biocatalyst, sulphate consumes the electrons released during iron oxidation and becomes reduced
[40,41][22][23].
3. Fungi
Fungi are eukaryotic organisms, such as yeast, moulds, and some well-known mushrooms. Contrary to bacteria, fungi are heterotrophs that secrete digestive enzymes into the surrounding environment and absorb dissolved molecules for their nutrition. Fungi can survive and grow in an environment highly deficient in water. Two species of this group,
Aspergillus and
Penicillium, have been found to tolerate very high and low pHs (above 12 and below 2) in their environment
[53][24].
The involvement of fungi in MIC has been reported. Alekhova et al. found degradation of aluminium alloys by fungi-induced MIC on the Mir Space Station
[54][25]. Carbon steel and aluminium alloys exposed to hydrocarbon fuel have been found to have an increased corrosion rate caused by fungi
[55][26]. Videla et al. clarified the MIC mechanism in a fuel/water system as follows: (1) organic acidic metabolites locally increase the proton concentration, (2) metabolites of microorganisms decrease the surface energy of the interface passive oxides film to electrolytes, (3) the presence of microorganisms increases the oxidizing properties of the medium, thus increasing the chances of pitting corrosion, (4) microorganisms utilize corrosion inhibitors from the medium, and (5) adhesion of microbes speeds up the dissolution of metals
[55][26].
Metabolically, fungi are highly diverse microorganisms that are able to obtain nutrition from the degradation of various organic materials, such as polymeric organic compounds and hydrocarbons. Some fungi produce organic compounds, including organic acids and complexants, which affect the properties of metals
[56,57][27][28]. Fungus-induced degradation of the coatings and underlying metals has been reported in several studies
[58,59][29][30].
It has been stated that fungal-induced corrosion of metallic materials and coatings is associated with the production of organic acids
[61][31]. Fungal degradation of aluminium has attracted more attention from researchers due to its impact on the integrity of aircraft
[62][32]. Damage to aircraft integrity caused by fungal biofilms has been reported on several occasions
[62][32]. Fungi degrade organic materials, such as lubricants, cladding, and jet fuel, and generate organic acids
[62,63,64,65][32][33][34][35].
4. Algae
Algae are unicellular aquatic microorganisms and are able to produce their own food by the process of photosynthesis. Algae are not closely related to each other in an evolutionary sense. For instance, they can live as single-cell microscopic algae or can be found in macroscopic and multicellular forms, which exist in the form of a colony.
Besides their adverse effects on water quality, these microorganisms considerably influence the corrosion process of engineering materials in marine environments
[68][36]. The attachment of single-cell algae (diatoms) to stainless steel surfaces has been observed by different researchers
[69,70,71,72,73][37][38][39][40][41]. It was found that the colonization of diatoms on a stainless steel surface was more active and fast in the light compared to the dark
[74][42]. The process of photosynthesis plays a key role in enhancing corrosion by changing the surface state (such as dissolved oxygen and pH) of metallic materials
[75,76][43][44]. Degradation due to algal biofilms has been reported for metallic materials.
Furthermore, through ecological studies, it has been specified that algae and bacteria living together in a fouled part of the material maintain an extremely close association with each other
[77,78][45][46]. A study on symbiosis-induced biofouling in a marine micro-fouling system, where bacterial biofilms form the underlying layer and microalgae work as the elementary biofouling layer, has been reported
[79][47]. It was found that the corrosion rate of carbon steel (Q235) immersed in a culture medium inoculated with the bacterium
Bacillus altitudinis was 2.2 times higher than that in a sterile medium. Meanwhile, the corrosion rate of Q235 steel in the presence of both
Phaeodactylum tricornutum and its symbiotic bacterium
B. altitudinis was about 7 times higher compared to the effects of the individual bacterial strain.
Microalgae have been found to secrete EPS, which further triggers the corrosion process by complexation with metals
[68][36]. Multispecies biofilms are able to form stable micro-consortia that strengthen the three-dimensional structure of the adhesive layer and accelerate biofouling.
5. Archaea
Archaea are a group of microorganisms originally believed to be bacteria and called archaebacteria owing to their physical similarities. But later, through genetic analysis, it was found that archaea are different organisms from bacteria and eukaryotes. This analysis earned them their own domain in the three domain classification originally proposed by Woese in 1977, alongside the eukaryotes and the bacteria
[81][48]. In addition to bacteria, archaea are also an important part of the microbial system
[82][49].
Archaea are broadly distributed in the world. The majority of archaea have the ability to inhabit and thrive in some extreme environments, such as those with enormously low oxygen levels, high acidity, high salinity, and very high temperatures, which provide archaea with distinctive cell structures and metabolic characteristics
[83][50].
Archaea have been found to cause MIC of metallic materials
[84,85][51][52]. For instance, the presence of methanogenic and thermophilic archaea has been reported in high-temperature, anaerobic oil production fluids collected from the North Sea and North Slope of Alaskan oil fields
[86][53]. Both the methanogenic and thermophilic archaea found in the above-mentioned locations were reported to have corrosion triggering effects
[87,88][54][55]. It was stated that the methanogenic archaea (
Methanothermobacter sp.) used carbon steel as an energy source and accelerated its corrosion process, while the thermophilic archaea (
Thermococcales sp.) enhanced carbon steel degradation through its iron reduction ability as well as the secretion of fatty acid metabolites. Furthermore, Usher et al. observed the colonization and corrosive effects of methanogenic archaeal communities on a carbon steel surface
[89][56].
Methane-producing microbes trigger the corrosion process of iron-containing metals. H
2 has been considered as an electron shuttle between Fe(0) and methanogens. Some of the methanogens, such as the
Methanosarcina acetivorans, catalyse direct electron transfer from metal-to-microbe to support methane production
[92][57]. In
M. acetivorans, deletion of the gene for multiheme eliminated methane production from Fe(0) by the outer-surface c-type cytochrome
MmcA, which is consistent with the basic role of
MmcA in other forms of extracellular electron transfer.
6. Lichens
A lichen is actually two organisms working as a single stable unit. Lichens are plant-like organisms that consist of a symbiotic association of algae or cyanobacteria and fungi. Lichens have about 20,000 known species worldwide that have been found surviving in different environmental conditions. This is a diverse group of organisms, having the ability to colonize a wide range of surfaces, including tree bark, exposed rock, biological soil crust, and other metallic and non-metallic materials in various environments. Through metabolism, lichens discharge different kinds of organic molecules, such as oxalic acid and polyphenolic acids, indicated as “lichen acids”, that have been confirmed to play a vital role in weathering and neogenesis
[94][58].
The deterioration caused by lichens occurs at the interface between the lichen and metal substrate. This interface has been considered a place of significant physical and chemical activities, presenting a very complex heterogeneity in which both primary and secondary minerals, organic acids, and compounds, as well as all kinds of organisms, including lichens, free-living fungi, free-living algae, and bacteria, are involved
[95][59].
The deterioration of ceramics due to lichen development on their surfaces has been reported
[96][60]. It has been stated that the oxalic acid released by lichens was the main reason for ceramic deterioration and aging
[96][60]. The deteriorating effects of lichens on natural rocks and building stones have been recognized long before
[97,98][61][62]. The mycobiont of lichens, which is always in close contact with the substrate, makes them able to cause deterioration. Bio-deterioration by lichens is, in general, attributed to a combination of physical mechanisms (such as the pressure exerted by the expansion and contraction of thalli, rhizine adhesion, and hyphal penetration) and chemical factors, which include the interaction of carbon dioxide, organic acids, and lichen substances with complex properties
[99][63].