The roles of biomolecules in corrosion induction and inhibition of corrosion: a possible insight

1 Introduction

Corrosion is a combination of chemical, physical and microbiological processes leading to the deterioration of materials such as steel and stone (Kip and Veen 2015). Corrosion is a worldwide issue that extensively affects areas such as shipping, oil refinery, construction, sewage and drinking water systems, as well as maintenance of historic buildings and monuments (Sanchez del Junco et al. 1992; Videla and Herrera 2005; Warscheid and Braams 2000) Figure 1. Metallic corrosion alone causes economic loss, estimated in the United States at 3.1% of global domestic product (Virmani 2002). Therefore, new strategies are needed to prevent corrosion and to make corrosion management techniques cheaper, more effective and less damaging to the environment.

Figure 1:

Corrosion in different places: (a) train bogie (outside); (b) iron door; (c) industrial tank; (d) oil/gas reservoir; (e) industrial wheel gears; (f) boat; (g) pipeline junction; (h) pipeline corrosion; (i) iron pillar; (j) medical scissor.

Most corrosion is the effect of electrochemical processes driven by microorganisms present as biofilms (Kip and Veen 2015). Such biocorrosion is a process in which microorganisms utilize insoluble products that accept electrons from the base metal. When microorganisms enter in contact with metallic structures, two main phenomena may happen: (1) The corrosivity of the environment may increase or (2) The corrosivity of the environment may decrease. In general, the corrosivity of the environment is connected to the type of organic species (biomolecules) in the environment. The arrangement of biofilms on materials submerged in an aquatic environment is a microbially organized activity involving: (i) adhesion of both organic and inorganic macromolecules, (ii) exopolymeric substance (EPS) production, (iii) microbial development, (iv) their metabolic production and (v) reaction to hydrodynamic erosion of the biofilm. In the first stage, a thin film is deposited, of inorganic ions and organic compounds of high relative molecular mass, which then becomes variously shaped in 3D. The underlying film that remains in contact with the metal substrate then adjusts the electrostatic charges and wettability of the metal surface, facilitating further colonization by bacteria. EPS can change the substratum surface properties by modifying surface charge, wettability and surface free energy, which can either enhance corrosion or inhibit it (Gunasekaran et al. 2004). It has furthermore been shown that organisms do not just aim to maximize consumption, but they produce structures and strategies to manage their environment, in particular the biofilm-metal interface through microbiologically influenced corrosion (MIC) and MIC inhibition (MICI) (Zuo 2007).

Microorganisms including bacteria, fungi (Geweely 2011) and algae (Javaherdashti et al. 2009) all enhance corrosion. The most broadly considered microorganisms in connection with biocorrosion, however, are the sulphur-reducing bacteria (SRB) (Enning et al. 2012), although SRB need not be present in abundance for MIC to take place (Roblero et al. 2004; Zhu et al. 2003). Other bacteria including sulphur-oxidizing bacteria, manganese-oxidizing bacteria (Rajasekar et al. 2005) iron-reducing/oxidizing bacteria (Herrera and Videla 2009), acid-producing bacteria and exopolymers or slime (Li et al. 2008) may also be playing an important role. These bacteria are commonly present in biofilm communities, often forming synergistic consortia, which influence electrochemical processes through their metabolic activities.

Many mechanisms have been proposed that produce anaerobic MIC through both SRB and thiosulphate-reducing bacteria (TRB). For example, precipitation of iron sulphide catalyses, electron reduction into molecular hydrogen and acts as a cathode in a copper-iron galvanic cell (Silva et al. 2007). Under anaerobic conditions, production of corrosive phosphides is possible, containing the substance PH₃ that enhances the dissolution of metal (Iverson et al. 1987). Metal particle complexation by additional cellular chemical compounds (Beech and Cheung 1995) and decrease of pH together with reduction of thiosulphate to sulphide (Miranda et al. 2006) are the most well-known mechanisms.

Microorganisms can use their biofilms to accelerate or even initiate surface degradation (Geesey 1994). They do this by promoting certain types of adhesion and growth of biofilm on surfaces, which leads to the formation of a hydrated gel matrix mainly of polysaccharides and proteins (Geesey 1994). Biofilm matrices frequently account for 90% of the structure, with the organisms constituting only 10%. This matrix includes a combination of various biopolymers collectively known as EPS and which includes macromolecules incorporating lipids, deoxyribonucleic acid and other metabolites. EPS forms a scaffold for the 3D structure of the biofilm and maintains biofilm consistency and the shape of the surfaces (Flemming and Wingender 2010). Mechanical interactions between surface and biofilm/EPS must involve effective (i.e. nonzero) surface tension and nutrient concentration; this transient behaviour may play an important role in shaping the structure of a biofilm. Several mechanisms are attributed to biocorrosion reflecting the wide range of physiological activities carried out by totally different microorganisms. A single mechanism cannot explain all MIC or nor can a single species. The mechanisms by which a biofilm will influence MIC may involve the following: (a) the creation of oxygen heterogeneities; (b) generation of corrosive substances such as acids; (c) removal of protecting films by detachment, thereby producing an ever-changing structure of passive protecting layers or increasing their dissolution from the surface of the metal; (d) increasing mass transport resistance at the metal surface and (e) ever-changing redox conditions at the interface between the metal and the bulk solution may thus act to produce functional cathodes (Lewandowski and Beyenal 2008). Recently, Karn et al. (2017) investigated the role of five different bacterial species as corrosion inducers and corrosion inhibitors with carbon steel (CS), and these authors determined both peroxidase, catalase enzyme activity in the biofilm having detached from the CS surface. Their results clearly showed that over a specific range both these enzymes can induce corrosion.

Carbohydrates are generally the most abundant constituents of this matrix accounting for 40–95%, while proteins typically contribute 1–60%, nucleic acids 1–10% and lipids 1–40%. The matrix, in addition, acts as a recycling centre by preventing the elements of lysed cells from dispersing and becoming lost to the consortium (Flemming and Wingender 2010). This includes DNA that could represent a reservoir of genes for horizontal gene transfer. Proteins might play a role as electron donors or acceptors by forming pili, nanowires and probably humic substances. Activities such as binding, stabilization and retention of enzymes may take place mainly by interactions with polysaccharides and proteins in the biofilm.

Furthermore, excess cellular material discharged as a result of metabolic turnover by membrane vesicles (a membrane vesicle commonly called an outer membrane vesicle is a kind of spherical lipid bilayer nanostructure naturally secreted by bacteria) contains enzymes, lipopolysaccharides, nucleic acids, phospholipids, amongst other metabolites (Figure 2). The precise molecular interactions of the varied secretions in the biofilm matrix are still far from clear at the molecular level, as are their contributions to matrix integrity (Karatan and Watnik 2009) and function. It is thus imperative to combine knowledge established between the fields of corrosion engineering and biological science. Consequently, this review aims to list the type of existing biomolecules and describe how they affect the corrosion of metals and alloys in MIC and MICI mechanisms, pulling in information from studies on totally different materials and from different scientific fields to cross-pollinate understanding of the current state of knowledge, whilst identifying the promising roles of biomolecules in corrosion induction and prevention.

Figure 2:

Showing the role of biomolecules in corrosion.

2 Extracellular polymeric substances

Microbial EPSs are a complex mixture of macromolecules, including proteins, polysaccharides, lipids and nucleic acids (Hall-Stoodley et al. 2004). Their composition varies with microbial species, physiological state of the cells and environmental factors (Wingender et al. 1999).

EPSs may be classified into two fractions, tightly bound (TB) and loosely bound (LB). In relation to bacteria, TB EPSs comprise capsules and sheaths of cells, as well as more amorphous material, which all associate as an integral part of the biofilm matrix. LB EPSs, on the other hand, are mostly discharged or diffused into the ambient liquid medium. The roles of exopolysaccharides in corrosion induction and corrosion inhibition are summarized in Table 1.

Beech (2003), Kinzler et al. (2003), Sand (2003) and Rohwerder et al. (2003) reported the importance of binding capability in EPS to metal surfaces in MIC (Table 1, role 1). While this depends both on microorganism species and on the type of metal, binding to metals by EPS involves interaction between metal ions and anions including carboxyl, glycerate, phosphate, pyruvate, sulphate and succinate. Such ions are commonly bound to supermolecular backbones consisting mainly of polysaccharides (Sutherland 2001). Specifically, the affinity of multidentate anionic ligands for multivalent ions, such as Ca²⁺, Cu²⁺, Fe³⁺, Mg²⁺, is very strong. The presence of metal ions in numerous oxidization states within the biofilm matrix and the standard reduction potentials may end up in substantial shifts, even though the presence of metal ions at intervals in the biofilm matrix has been recognized as relevant to MIC. The possible involvement of EPS-bound metal ions in direct electron transfer from the base metal to an acceptable electron acceptor involves ferrous metal. Matrices furthermore have a significant influence on the activity levels of biofilm organisms present within them. The oxidation of metal ions adsorbable on the surfaces of oxyhydroxide iron that assembles on biofilm polymers probably further contributes to cathodic reactions.

It is exciting to think about such biominerals having electron-conducting fibres distributed at intervals in the biofilm matrix (Beech 2004) (Table 1, role 2). Beech and Sunner (2004) reported metal ions, bound inside the biofilm matrix, to be coordinated by a spread of ligands, thereby forming complexes presenting a spread of oxidoreduction potentials. Such complexes might participate within the electron transfer processes that drive corrosion reactions. The importance of such a mechanism within the overall corrosion method powerfully depends on the chemistry of the metallic surface associated with each bacterium and is specific to each bacterium species (Busalmen et al. 2002) (Table 1, role 3). Charge on polysaccharides may be associated with either carboxyl groups of uronic acids or with noncarbohydrate substituents (Sutherland 2001). Proteins rich in acidic amino acids, as well as in aspartic and aminoalkanoic acid, bear carboxyl groups that additionally contribute to the anionic properties of EPS. Nucleic acids are polyanionic to the phosphate residues within the ester moiety. Thus, charged elements of acidic amino acids, EPS, uronic acids and phosphate-containing nucleotides are likely to be involved in electrostatic interactions with multivalent cations.

Dong et al. (2011) studied the effects of extracted EPS on steel corrosion. They discovered that some types of EPSs delayed the corrosion by inhibiting oxygen ion mass transfer. However, corrosion inhibition decreased at a high EPS concentration because of the binding of additional iron ions to EPS. Once microorganisms colonize a metal surface, they tend to produce EPS that contains very different functional groups. These changed functional groups now have totally different affinities for the metal ions, which leads to zones of changed metal concentration. The area found below the surface exopolymers showing high affinities for metal act as anodes, whereas the areas underneath exopolymers with low metal affinities act as cathodes (Table 1, role 4).

Bautista et al. (2015) determined that EPS produced by bacteria plays a primary role in the different stages of microbial biofilm formation, maturation and maintenance. The different influences were found in LB and TB EPSs, respectively, isolated from the marine bacterium Pseudomonas (NCIMB 2021) (Table 1, role 5). This role may be compared with that played in the presence of the model protein bovine serum albumin (BSA). A thick duplex chemical compound layer deposited over a Cu₂O layer and an inner oxidized nickel layer of thickness of order 10 nm (Conlisk 2013; Jenkinson 2014) showed the presence of BSA, TB EPS and LB EPS, forming a mixed chemical compound layer (oxidized copper and nickel) of layer thicker than that of the underlying layer of 70Cu-30Ni alloy in static artificial seawater (ASW) but not thinner than a layer of biomolecules. In solutions of biomolecules, this oxide layer is covered by an adsorbed organic layer that is composed principally of proteins. Wikieł (2013) used a model to research impedance values obtained at the corrosion potential and showed a slowdown of the anodic reaction in the presence of biomolecules (BSA, TB EPS and LB EPS), as well as corrosion inhibition by LB -EPS and to a lesser extent by BSA. No harmful effect was shown by TB EPS. Wikieł’s (2013) results were obtained using St37-2 steel in static ASW containing EPS extracted from Desulfovibrio alaskensis. AL1 and protein concentrations of 20 mg/L showed adequate corrosion inhibition by LB EPS but insignificant or poor corrosion inhibition by TB EPS (Table 1, role 6).

Mohan et al. (2004) investigated corrosion potential in 316 stainless steel in natural pond water impacted by fresh water biofilm. Less corrosion-related current and higher resistance values were found for 316 stainless steel in the presence of fresh water biofilm. The broad phase obtained from measurements of electrical phenomena indicated the presence of intact passive film on 316 stainless steel in the presence of natural biofilm (Table 1, role 7).

Dexter and Zhang (1991) ascertained that biofilm from fresh pond water can ennoble the corrosion potential of alloys as happens in seawater. These authors also found that the open-circuit potential (OCP) values gradually accumulated with time in the presence of natural biofilm (Table 1, role 8). This observation is in agreement with the findings of Dexter and Zhang (1990), Maruthamuthu et al. (1996), Eashwar and Maruthamuthu (1995) (Table 1, role 9). On the contrary, ennoblement was noted by Little et al. (1997) in Gulf of Mexico water. But Little et al. (1997), performing gas measurements with microprobes, found that the biofilm-substrate interface remained virtually oxygen free. Furthermore, the sharp influence on corrosion potential was found to coincide with a rise in the number of Desulfovibrio and Desulphotomaculum species.

EPS-induced mineral precipitation at the scale of a biofilm matrix will have an important influence on the activity levels within such a biofilm. Iron oxyhydroxide surfaces assembled on biofilm polymers would take up ferrous iron resulting in its oxidization, thus further contributing to the cathodic reaction (Beech 2004). EPS production by SRB containing 84–92% proteins and 8–16% polysaccharides resulted in a rise in the corrosion of mild steel coupons (Videla and Herrera 2005) (see also Table 1, role 6).

Bacterial cells found on metallic and nonmetallic surfaces typically increase to form a hydrous polysaccharide-protein matrix, thus creating a biofilm that may enhance or otherwise influence corrosion. Biofilm sometimes occurs as a slimy gel, typically incorporating about 95% water, and is created from microorganism-produced metabolic products such as extracellular chemical compounds, organic and inorganic acids, enzymes and volatile compounds (Rani and Basu 2012). Biocorrosion and associated biofouling negatively impact oil and gas installations in areas including human and environmental safety, as well as resource security and economic return on investment (Stanley et al. 2016).

3 Proteins/enzymes

Bacteria produce a large variety of hydrolytic and proteolytic enzymes as lyases that react with substrates beyond the cell wall and cell membrane. Such enzymes are generally categorized as exoenzymes. Enzymes produced by the cell but acting outside it are specifically termed free-form enzymes. The latter include polysaccharides, proteases, lipases, esterases, peptidases, glycosidases, phosphatases and oxidoreductases. The discharge of enzymes by microorganisms into their ambient environment enables certain types of interaction between cells and substrates. Enzymes are of concern, particularly, as their influence needs to be detected in some marine biofilms (Basséguy et al. 2004; Rusling 1998). These enzymes are proteins possessing a haeme group (also known as haemins) that is concerned with the transfer of electrons throughout the accelerator chemical process (Lai et al. 1999). In aerobic corrosion, different protein-driven mechanisms are involved, including interaction with catalase and porphyrin compounds, while in anaerobic corrosion with SRB, FeS and hydrogenases are also involved. The roles of proteins and enzymes in corrosion induction and corrosion inhibition by various researchers are given in Table 2.

Many authors believe that oxidase-type enzymes are involved in the ennoblement of stainless steel (SS) (Amaya and Miyuki 1995, 1999; Dupont et al. 1998; L’Hostis et al. 2003). Normally, these enzymes use oxygen as the acceptor to catalyse the oxidation of organic compounds. Reactive oxygen species (ROS) are generated within the biofilm as a consequence of such accelerator activities. Hydrogen peroxide (H₂O₂) is one of such ROS which plays a significant role in several accelerator reactions occurring in aerobic biocorrosion (Amaya and Miyuki 1999; Lai et al. 1999). A number of studies have connected the necessary involvement of H₂O₂ in SS ennoblement in natural waters due to its redox potential over the oxygen molecule (Chandrasekaran and Dexeter 1994; Washizu et al. 2004).

Enzymatic activity within the biofilm is furthermore thought to be involved in corrosion (Beech 2003; L’Hostis et al. 2003), while recently some researchers have published the mechanism that increases the free corrosion potential (Ecorr) of SS in oxygenated natural waters, where it is considered that the ennoblement enzyme involved may be either extracellular, intracellular or both. Landoulsi et al. (2008) showed how the oxygen consumption through metabolic pathways within the biofilm influences its availability at the metal-biofilm interface. Therefore, oxygen gradients result from its diffusion within the biofilm and from its consumption in the metabolic pathways. Oxygen acts as the final electron acceptor in microorganisms. Most of this oxygen undergoes a four-electron pathway reduction in microorganisms, in agreement to reaction Eq. (1). This reaction is catalysed by a cytochrome c enzyme.

The enzyme is typically intracellular, but some secreted forms are inducible in response to oxidative stress (Naclerio et al. 1995). The activity of catalase enzyme generates O₂ which may be used to produce H₂O₂ by reduction on the surface of SS. This autocatalytic mechanism was first considered by Busalmen et al. (1998, 2002). They discovered rise in the cathodic current by using cultures of Pseudomonas sp. that secrete catalase enzymes. Landoulsi et al. (2008) developed an enzyme immobilization technique based primarily on a modified SS electrode. They immobilized glucose oxidase (Gox) in a polymeric film spread on the surface of SS to concentrate the enzymatic activity close to the metal and polymer film interface. Electrochemical tests were performed to check the impact of Gox with immobilized or free forms within the electrolyte. These experiments showed the reduction of O₂ and H₂O₂; each oxidant was involved within the Gox-catalysed reaction. The cathodic processes showed large variations between freely associated and immobilized proteins, whereas the OCP ennoblement was found to be due to an electrochemical effect of H₂O₂ in each case. The immobilized Gox resulted in a strong depletion of oxygen near the interface of a metal or polymer. It indicates that the reduction current recorded was mainly due to the H₂O₂ reduction.

Many researchers have ascribed ennoblement to the oxidation-reduction reaction catalysed by microbial enzymes (Scotto et al. 1985), organometallic complexes (Johnson and Bardal 1985) and further cellular chemicals (Mollica et al. 1990, 1992). Chandra Sekaran and Dexter (1993) considered that peroxide generated from microorganism activity might also contribute to ennoblement at low hydrogen ion concentrations. The biofilms increased the passivity of SS alloys by each increasing the passive region and therefore the vital breakdown potential. On the basis of photoelectrochemical studies, Maruthamuthu et al. (1996) and Dexter and Zhang (1991) showed that only alloys covered by a layer of n-type semiconducting chemical compound film exhibited a considerable positive shift of corrosion potential but whether the biofilm improves passivity is still not known.

Comparing the quality of natural fresh waters to seawaters, it is well established that correlation exists between Ecorr ennoblement and microbial deposition of Mn oxides/hydroxides (Dickinson and Lewandowski 1996). This Mn biomineralization is liable to produce pitting corrosion of SS samples exposed in low chloride media (Geiser et al. 2002; Shi et al. 2003). Marconnet et al. (2008) studied natural exposure of SS samples in the River Seine (France) and reported that both H₂O₂ and Mn biomineralization mechanisms occurred, depending on the exposure site. Ecorr ennobled appreciably, whereas no pitting corrosion was observed under these conditions.

Landoulsi et al. (2009) reported that the ennoblement of AISI 316L SS was elicited when Gox catalysed a reaction or by adding H₂O₂ in seawater. The corrosion behaviour of the sample was studied using potentiodynamic and galvanostatic polarization tests. Once the ennoblement occurs, the pitting potential also becomes nobler. The involvement of H₂O₂ in reinforcing pit repassivation appears to be an ever-present key component of these processes.

The use of refined enzymes in MIC studies is increasingly identified as an indispensable tool to uncover chemical aspects of surface science processes, particularly the ennoblement of SS in natural waters. Such ennoblement has attracted the interest of many researchers because pitting corrosion has economically important consequences, but it is still poorly understood. Various studies have reported the catalytic effect of biofilms, involving enzymes and their influence on the native physicochemical conditions that are associated with their heterogeneous structure. Several previous studies revealed the necessary involvement of H₂O₂ in SS ennoblement in natural waters through its oxidoreduction potential beyond that of oxygen (Amaya and Miyuki 1995; Chandershekhran and Dexter 1993; Washizu et al. 2004). The chemical properties of the biofilm matrix include the presence of various binding sites among macromolecules for matrix formation. They are likely to promote close association between EPS enzymes and exogenous substrates, thus enabling enzymatic reactions.

De la Rosa and Yu (2005) used 3D mapping of oxygen element distribution in waste material biofilms that showed pockets with a dissolved oxygen element in deep parts of biofilms. This will tend to enable the development of biofilms. The presence of certain dissolved oxygen deep within biofilms is likely to be important in the formation and maintenance of massive biofilms, partly by facilitating at least some of the metabolic activities taking place deep in biofilms of large mass.

Most reports on watching accelerator expression in biofilms specialize in cell-associated activities and barely address noncellular areas of the biofilm matrix. Enzymes, like the hydrogenases of the anaerobic SRB, are known to remain active inside the biofilm matrix, regardless of the absence of living cells, and may play a major role in the biocorrosion of iron and metal alloys (Beech 2002). Additionally, the activity of enzymes, like catalases, phosphatases, lipases and esterases, may be detected in aqueous oxygenated solutions of freeze-dried SRB exopolymer (Beech 2002; Beech and Coutinho 2003). The impact of such enzymes on steel corrosion is not completely understood. These studies indicate the wide physiological versatility of various microorganism species and have pointed to considerable species specificity, which may lead to the apparently paradoxical diversity of deterioration processes observed in biofilms even under apparently identical environmental conditions (Dinh et al. 2004). Close observation of microorganism processes on surfaces at the single-cell level is needed to resolve this apparent paradox of unexplained biodiversity, as was presented by Hutchinson (1961) for plankton and later largely resolved by Tilman (1982). The activities of enzymes play an important role in the dissolution of oxide/hydroxide films coating and protecting metal surfaces. Thus, passive layers on steel surfaces will be lost or replaced by less stable and reduced metal films that facilitate corrosion. One of the most effective examples of a microorganism producing biocorrosion because of dissimilarity in iron reduction is the gram-negative bacterium Shewanella oneidensis (formerly known as S. putrefaciens). This bacterium oxidizes various carbon substrates by reductively dissolving Fe(III) contained in minerals like ferrihydrite, goethite and haematite. The biocorrosion of steel in the presence of S. oneidensis was documented by Lee and Newman (2003). The corrosion rate was measured and found to depend on the type of oxide film under attack (Little et al. 1997).

Many mechanisms have been projected to explain anaerobic biocorrosion by SRB and TRB. After the precipitation of iron compound, catalysis occurs of proton reduction to molecular hydrogen. The H₂ acts as a cathode for a galvanic couple with bronze, iron and other anodic materials, with depolarization promoting local acidification at the anode (De-Silva Muñoz et al. 2007); possible production of corrosive phosphides releasing matter including PH₃ enhances the dissolution of the metal under anaerobic conditions (Iverson and Olson 1983). Metal ion complexation by further cellular polymer substances (Beech and Cheung 1995) decreases the pH, allowing the metabolic reduction of thiosulphate to sulfide (Miranda et al. 2006).

In the classical theory of SRB-influenced corrosion of metallic iron, the flux of electrons (unit area)⁻¹ is projected to be transported from the metal surface to the salt reduction pathway of microorganisms through a hydrogen intermediate. This method requires the activity of hydrogenase enzymes. Clusters of hydrogenase-type enzymes associated with SRB-covered areas in MIC were extensively studied by Beech (2002). The role of hydrogenases was confirmed by De-Romero et al. (2003) in anaerobic MIC. Furthermore, direct electron transfer between hydrogenase purified from Ralfstonia eutropha and the surface of SS has been shown by Da Silva et al. (2004). These results support early reports that hydrogenases (found to be expressed intermittently in the biofilm matrix) have an instantaneous direct influence on the cathodic reaction even in the absence of viable bacterial cells (Beech 2004).

De Bout (1976) reported that hydrogenase activity in the N₂ fixing, methane-oxidizing bacterium (strain 41 of methylosinus type) rose markedly once growth was dependent upon the fixed gaseous N₂. A direct relationship may exist between nitrogenase and hydrogenase enzyme activity during growth of this bacterium. Acetylene reduction was additionally supported by the presence of hydrogen gas. Little et al. (2001) demonstrated that fungal degradation of lubricating grease produced organic acids. It produced localized corrosion of carbon steel cables in polyvinyl chloride sheaths. Fusarium sp., Hormoconis sp. and Penicillium sp. were isolated from corroding steel tendons in an exceedingly posttensioned structure that was used in their experiment. In these cases, once fungal spores were deliberately introduced to covered steel tendons, localized corrosion was observed. There was a spatial relationship between fungal hyphae and corrosion. Prochnow et al. (2008) proposed that Pseudoalteromonas tunicata (a marine bacterium) produces an antibacterial autolytic protein, AlpP, which causes the death of a part of a population of cells during biofilm formation and mediates differentiation, dispersal and phenotypic variation among dispersed cells. A homologue of AlpP (LodA) in the marine bacterium Marinomonas mediterranea was recently identified as a source of lysine oxidase, which mediates cell death through the production of hydrogen peroxide. Prochnow et al. (2008) too confirmed that AlpP in P. tunicata acts as a source of lysine oxidase, as well as of hydrogen peroxide, and is liable to cause cell death in microcolonies within growing biofilms of M. mediterranea, as well as of P. tunicata. LodA-mediated varying distribution of cell death is shown to be related to the phenotypically determined pattern of growth and biofilm formation amongst M. mediterranea biofilm dispersal cells. Furthermore, AlpP homologues also occur in numerous different gram-negative bacteria from various environments.

Death of cell subpopulations in microcolonies was moreover confirmed throughout biofilm formations in Caulobacter crescentus and Chromobacterium violaceum. In these organisms, hydrogen peroxide was involved in biofilm death, as a result of which it might be detected at the moment the killing occurred. Therefore, the addition of catalase enzyme considerably reduced the killing of biofilm. The AlpP homologue was clearly connected with biofilm death events since isogenic mutant (CVMUR1) did not suffer biofilm death in C. violaceum. We suggest that biofilm killing through peroxide might be connected with AlpP homologue activity and that it plays a necessary role in the diffusion and constitution across a spread of gram-negative microorganisms.

Mehanna et al. (2008) found hydrogenase activity by Clostridium acetobutylicum throughout its anaerobic corrosion of mild steel. Two short-circuited mild steel ectrodes were exposed to the same solution, and hydrogenase was retained on the surface of one of these electrodes. The electrode potential and galvanic current were measured as a function of time so as to understand the distinction in electrochemical behaviour that is an iatrogenic effect produced by the presence of hydrogenase. A sharp decrease in potential of around 500 mV was produced by the deoxygenating part. Once hydrogenase was introduced and deoxygenation completed, vital heterogeneous corrosion was ascertained under the vivianite deposit on the electrode in contact with hydrogenase, whereas the other electrode showed only the vivianite deposit. The effect of the hydrogenase was then confirmed by the observation of free potential on single coupons, whether exposed or not to the enzyme, in an ideal cell when completely deoxygenated. In all phosphate and Tris-HCl buffers, the presence of hydrogenase increased the free potential around 60 mV, and iatrogenic effects marked general corrosion. The researchers concluded that Fe-hydrogenase acts in the absence of any final electron acceptor by catalysing direct electron action on the mild steel surface.

Two thermophilic archaea (strain PK and strain MG) were isolated from a culture enriched at 80 C from the inner surface material of a hot oil pipeline (Davidova et al. 2012). Strain PK can ferment complex organic nitrogen sources (e.g. yeast extracts, peptone, tryptone) and can reduce elemental sulphur S, Fe³⁺ and Mn⁴⁺. Phylogenetic analysis showed that the organism belongs to the order Thermococcales. During yeast extract fermentation, incubations of this strain with elemental iron (Fe) resulted in the abiotic formation of ferrous iron, and therefore, accumulation of volatile fatty acids sometimes takes place. The other strain (MG) was an H₂:CO₂-utilizing archaeobacterium that is phylogenetically part of the genus Methanothermobacter. Cocultures of the two strains grew as aggregates that produced CH₄ but not exogenous H₂. The coculture created similar results, but higher concentrations of fatty acids were obtained from yeast extract than from strain PK alone. Thus, the physiological characteristics of the strains, either alone or together, might contribute to the pipeline corrosion. The Thermococcus strain PK can reduce elemental sulphur to sulphide, produce fatty acids and reduce ferric iron. The hydrogenotrophic archaeobacterial strain MG increased carboxylic acid production by fermentative organisms, but on the other hand, they could not couple the dissolution of Fe with the consumption of water-derived H₂ like other methanogenic bacteria.

Ishii et al. (2014) worked on microorganism extracellular electron transfer (EET) to solid surfaces, an important reaction for metal reduction that occurs in many anoxic environments. However, it is difficult to accurately characterize the EET activity in microorganism communities and the contribution to EET reactions of each member because of changes in the concentrations and composition of electron donors and solid-phase acceptors. The authors used bioelectrochemical systems to consistently measure the synergistic effects of carbon sources and surface redox potential on EET-active microorganism community development, metabolic networks and overall electron transfer rates in several studies. Rapid biocatalytic rates were found to occur under electropositive electrode surface potential conditions and under fatty acid–fed conditions.

EET to solid surfaces is a very important reaction for metal reduction occurring in numerous hypoxic environments (Ishii et al. 2014). However, it is difficult to accurately characterize the EET-active microorganism communities as every member contributes to EET reactions and to changes in composition and concentrations of electron donors and solid-phase acceptors. Here, the authors used bioelectrochemical systems to consistently measure the synergistic effects of carbon supply and surface oxidation-reduction potential on EET-active microorganism community development, metabolic networks and overall rates of electron transfer. The results indicate that faster catalyst rates were discovered under positive conductor surface potential conditions and under fatty acid–fed conditions. Temporal microorganism community analyses showed that Geobacter phylotypes were extremely numerous and all dependent on the surface potential. These well-known electrogenic microbes are affiliated with G. metallireducens and are associated with rather low surface potential and low current generation. Collectively, these results suggest that surface potential gives a powerful selective pressure at the species and strain level for each solid surface for respirer and fermentative microbes throughout the EET-active community.

Da Silva et al.’s (2004) discovery of fluctuation between positive and negative values suggests that the hydrogenase enzyme favours both anodic and cathodic sites on an equivalent conductor. Its global behaviour is controlled by the balance between the native anode and cathode sites. Such native anode/cathode sites on steel surfaces already have been induced in the presence of hydrogenase from Ralstoinia eutropha. Advanced carbon substrates are degraded by fermenting microorganisms to by-products of the fermentation methods. Such by-products include volatile fatty acids and H in an anaerobic system which are utilized as electron donors for sequential microbial reduction of nitrate and sulphate, as well as reduction of solid metals through microbial EET reactions (Lovley et al. 2004; Nealson and Saffarini 1994). The high rates of microbial electron uptake discovered through microbially influenced corrosion of iron Fe⁽⁰⁾and through microbial electrosynthesis have been thought to indicate immediate electron uptake in these microbial processes. However, the molecular mechanisms of direct electron uptake from Fe⁽⁰⁾ remain unknown.

Deutzmann et al. (2015) investigated the facilitation of electron uptake characteristics of Fe⁽⁰⁾ corrosion by extracellular enzymes through electromethanogenic Archaea and Methanococcus maripaludis. The authors’ results suggest that the free, surface-associated redox enzymes including hydrogenases and formate dehydrogenases mediate direct electron uptake. Rates of H₂ and formate formation by a cellular spent medium were used to elucidate rates of methane formation from Fe⁽⁰⁾ and cathode-derived electrons by wild-type M. maripaludis, as well as by a mutant strain carrying deletions for all catabolic hydrogenases.

Amino acids areas are characterized by eco-friendly properties such as water solubility, natural occurrence and biodegradability (Morad 2005). Amino acids have been reported as corrosion inhibitors for Cu, Al, Sn and Fe, with inhibition related to the nature of both the metal and the medium (Ece and Bilgic 2010), particularly amino acids containing sulphur and long organic chains (Mobin et al. 2016). Much research has targeted corrosion inhibition by amino acids on copper (Mobin et al. 2011) and iron (Fu et al. 2010). The degree of inhibition by the medium can be related to its composition of amino acids and in some cases is increased by addition of certain cations and anions (Migahed and Al-Sabagh 2009).

Recent enzymatic activities, measured per unit area in biofilms, have shown enzyme-mediated reactions to be more important than was previously thought and are thus likely to be highly relevant to biocorrosion. The mechanism, which increases the free corrosion potential (Ecorr) of SS, has been widely reported in oxygenated natural waters and cited as an ennoblement agent, as understanding improves (Beech 2003; Busalmen et al. 2002). In microorganisms, enzymes, including catalases, peroxidases and superoxide dismutases, are part of the respiratory chain and act as accelerators. They are involved in oxygen reduction. Thus, they may facilitate corrosion by accelerating oxygen reduction reactions. However, it is imperative to comprehend that the power of such enzymes to accelerate oxygen reduction depends strongly on the chemistry of surface films (Beech 2004).

Recently, Awad et al. (2017) examined the inhibition of the corrosion of mild steel in the presence of various amino acids, including cysteine, methionine and methyl-cysteine complex. The analyses were done with each amino acid alone and also in combination. The most marked inhibition was found in the presence of amino acid mixtures.

Many factors and processes in biofilm development can lead to reduction in corrosion (Jayaraman et al. 1999). Earlier Chongdar et al. (2005) discovered aerobic P. cichorii that had the potential to inhibit corrosion of mild steel because of a layer of passive oxide products formed by corrosion, as did Dubiel et al. (2002) who found that microorganism respiration led to oxygen removal by biofilms, which they surmised explained the inhibition of corrosion that they found. Similarly, Juzeliunas et al. (2006) found that biofilm produced by Bacillus mycoides magnified charge transfer resistance of the Al substrate and thereby decreased the corrosion rate.

4 DNA/environmental DNA

Table 3 reviews the role of DNA, lipids and various metabolites in the induction and inhibition of corrosion. Zhao et al. (2014) have proposed a model to elucidate DNA-induced corrosion behaviour experimentally consistent with their characterization of porous elements through fourier infrared spectrometry and prism coupling optical measurements. Increasing the concentration of either DNA probes or targets improves this corrosion method and masks binding events, whereas passivation of the porous silicon (PSi) surface by oxidation and silanization is shown to diminish the corrosion rate.

Zhao et al. (2014) devised a model in which corrosion in PSi waveguides depends on the surface charge density and passivation conditions of the PSi structures. Once the surface coverage of the immobilized DNA molecules increases, the strength of a negative charge related to DNA increases as well. This build-up of negative charges close to the PSi surface attracts the majority carriers of p-type PSi to migrate to the neighbourhood of the DNA binding event, which then reinforces the localized field of force close to the DNA-oxide interface. This electric field of force could promote further oxidization of the Si pore wall and end in blue shift of the PSi conductor reflection factor spectrum.

Lauw et al. (2010) used another method to investigate the mechanisms behind PSi corrosion. The electrical double layer with one polar element of the electrolyte becomes preferentially accumulated at the electrode surface and within the diffuse layer, whereas the opposite polar element is depleted within the same region. The electrical double layer was found to accelerate the wet etching of SiO₂ by locally increasing the concentration of hydroxide around a hydrophobic nanotube (Liu et al. 2009). However, formation of electrical double layers is unlikely to be the predominant reason for the corrosion impact found because a hydrophilic, thermally oxidized PSi surface functionalized with silane molecules and negatively charged DNA strands cannot be absorbed by the high effective concentration of hydroxide. Thus, it will accelerate SiO₂ etching.

The optical thickness of a porous PSi matrix was observed by Steinem et al. (2004) to decrease in close relation to DNA presence. The authors interpreted this thinning as resulting from enhanced corrosion (oxidation-hydrolysis) associated with DNA hybridization and suggested that accumulation of negatively charged DNA close to the PSi surface enhances the polarization of surface silicon bonds. It facilitates nucleophilic attack by water molecules at exposed Si–H bonds. Extracellular DNA is another element of the biofilm matrix, at first thought to be a product of cell lysis. Whatever its origin, it is currently understood to play an important role in biofilm formation and structural stability. The marine photosynthetic bacterium Rhodovulum sp. produces EPS that includes nucleic acid (Watanabe et al. 1995). These authors confirmed that this nucleic acid plays a structural role because nucleolytic enzyme treatment resulted in deflocculation of the bacterium, whereas the polysaccharide-degrading enzymes, such as amylase, trypsin and pectinase, had no effect.

Extracellular DNA appears to be of diverse origins in several species. In Pseudomonas aeruginosa and Pseudomonas putida biofilm, extracellular DNA and genomic DNA were found to be identical (Steinberger and Holden 2005), whereas in Pseudomonas epidermidis, DNA was discovered to originate from lysis of a population of attached microorganisms by autolysin AtlE (Qin et al. 2007). In aquatic bacteria of strain F8, extracellular DNA was, on the contrary, distinct from genomic DNA, suggesting a source different from cell lysis (Bockelmann et al. 2006). DNA obtained from environmental samples including sediments, ice and water (environmental DNA [eDNA]) represents an important source of information on past and present biodiversity. eDNA is furthermore important for marine environmental and industrial research. The origins and behaviour of eDNA, while still poorly understood, are a very active research topic. eDNA is deposited through skin flakes (Bunce et al. 2005), hair (Higuchi et al. 1988; Taberlet et al. 1993), urine (Valiere and Taberlet 2000), faeces (Poinar 1998), saliva (Nichols et al. 2012), insect exuviae (Hofreiter et al. 2003), eggshells (Strausberger and Ashley 2001), regurgitation pellets (Taberlet and Fumagalli 1996), feathers (Taberlet and Bouvet 1991), root cap cells and leaves (Trevors 1996), pollen (Levy-Booth et al. 2007) and in living prokaryotes through plasmids and chromosomes included in secretions (Meier and Wackernagel 2003).

Microorganisms and plant studies have shown that dead and living cells coming into the atmosphere can quickly be lysed and their DNA can be released (Nielsen et al. 2007). DNA survival is helped through binding to environmental compounds including clay, larger organic molecules, minerals and different charged particles that shield adsorbed DNA from enzyme activity (Crecchio and Stotzky 1998). This shielding from nucleases additionally inhibits their ability to interact with the biota (Blum et al. 1997). As an example, clay minerals including montmorillonite will adsorb and absorb up to their own weight of DNA and incorporate it into to their large charged area (Huang et al. 2014; Pietramellara et al. 2007). Furthermore, humic acids (some of which are highly resistant to decay) conjointly bind DNA molecules because of negative surface charge and thus prolong survival of the DNA. Surface binding to sand is also feasible and markedly increases with concentrations of divalent cations such as Ca²⁺and Mg²⁺, which readily form sand-DNA bridges (Lorenz and Wackernagel 1987). Most eDNA studies have assumed that the age of the DNA molecule recovered is the same as that of their matrix; however, in certain conditions, DNA molecules can leach through the strata and contaminate lower layers (Haile et al. 2007). Whether DNA can also leach through frozen soil (permafrost) or in sediments recently frozen has not been determined with certainty (Hebsgaard et al. 2009). However, leaching has been reported in sediments from both temperate and desert environments (Andersen et al. 2011; Jenkins et al. 2012), so the possibility of leaching should always be taken into account and checked using independent methods (Haile et al. 2007). While the roles of DNA in corrosion have been treated above, analysis of eDNA in the oceans is also proving a powerful tool to discover the rich ocean biomes from viruses through bacteria to fish and mammals (Flaviani et al. 2017; Lacousière-Roussei et al. 2018).

5 Metabolite analysis

Bonifay et al. (2017) recently demonstrated corrosion processes in two marine oil production pipelines using metabolomic techniques. Metabolites were extracted and analysed by ultrahigh performance liquid chromatography or high-resolution mass spectrometry analysis with electrospray particle ionization in both positive and negative ion modes. Using standards indicative of aerobic and anaerobic organic compound metabolism, these results were compared to predicted masses for metabolites in Kyoto Encyclopedia of Genes and Genomes (KEGG). Peptides and lipids were the dominant classes of compound, comprising 671 dipeptides, tripeptides and tetrapeptides, as well as 590 lipids (sterol lipids, sphingolipids, prenols, polyketides, glycerophospholipids and fatty acyls) among the 2353 claimed metabolites. Peptides also were highly diverse, but lipids were four times more abundant in terms of mass. A large (247) group of putatively known metabolites was mapped as metabolically functional amino acids (168) and carbohydrates (37). Other metabolites were represented by 110 known compounds, while a complete list of 735 putatively known metabolites was not related to any specific metabolic pathway listed in the KEGG database. The high corrosion (HC) pipeline metabolome showed great richness and was also abundant in lipids. Different purported metabolites were preferentially present within the HC system that were involved in biogenesis and/or metabolism of steroid lipids, phenylpropanoids, porphyrins, arachidonic acid, unsaturated fatty acids (UFAs), taurine and isoprenoids. Further, ionic iron was detected in high abundance in all HC samples but was four times more abundant in HC11 than in HC3 (HC sample number 3 and 11).

6 Potential roles of omics and functional genes

Genomics, transcriptomics, proteomics and metabolomics are vital tools to investigate the biology of an organism and its response to environmental stimuli. Proteomics science provides helpful data on the various cellular processes by analysing extracellular enzyme activities, stress proteins and metabolic proteins and on the homologous and heterologous cell-to-cell interactions with analysis of proteins or peptides that are involved in quorum sensing and genetic exchange activities. Furthermore, understanding more about biofilms is necessary to closely monitor the expression of factor sequences at each transcription and translation. Although the extraction procedures of natural biofilm RNA and DNA are similar in principle, successful extraction and characterization of mRNA from biofilm has lagged behind those of DNA because of issues such as the activity of nucleases and the rapid turnover of prokaryotic template RNA. However, many strategies are currently available to characterize template RNA and measure gene expression level (Krsek et al. 2006). An RNA meta-transcriptome approach involves the extraction of both mRNA and rRNA with reverse transcription to cDNA and also with direct pyrosequencing made from cDNA and both rRNA tags and mRNA tags. This has allowed the quantification of microorganisms and provided information on the activity of the enzymes involved (Urich et al. 2008).

Multiple macromolecule isoforms are often synthesized by a single cistron as a result of messenger RNA molecules. They are often subjected to posttranscriptional management, including alternative splicing, polyadenylation and messenger RNA editing. The analysis of expressed proteins in pure culture is sensitive and rapid. It involves extraction of proteins, their separation by 2-dimensional gel electrophoresis, solubilisation of the excised protein band and sequential trypsin digestion followed by analysis of the digestible peptides by ionization mass spectrometry (Pandey and Mann 2000). Metabolomic studies capture global biochemical events by assaying thousands of small molecules in cells, tissues, organs and biological fluids followed by application of informatics tools to outline identities of the metabolomic entities. Metabolomic studies will result in increased understanding of biological mechanisms. It is used furthermore to produce a direct functional read-out of the physiological condition of the organism (Gieger et al. 2008). A variety of analytical techniques has been used to investigate metabolites in the tissues and the fluids of several organisms. Mass spectrometry analysis can be combined with different chromatographical separation techniques, like liquid or gas chromatography or nuclear magnetic resonance. These are the main tools to determine a large variety of metabolites at the same time.

Although these technologies are extremely refined and sensitive, there are still some bottlenecks in metabolomics. Because of the large diversity of chemical structures, there is no single technology currently available that can analyse the whole metabolome. Therefore, a variety of complementary approaches has to be established for extraction, detection, quantification and identification of several metabolites (Villas-Bôas et al. 2007). Metabolomics will, therefore, be seen as bridging the gap between the genotype and phenotype. They are providing additional comprehensive examination of how cells work, as distinct from distinguishing novel or prominent changes in specific metabolites (Fiehn 2002).

7 Lipids

The functions and metabolism of fatty acids are already widely studied by biologists, but it is still difficult to extract and exactly pinpoint the positions of these molecules within the cells using normal techniques. Fatty acids and their derivatives are necessary for the cell to perform and are associated with many alternative metabolic activities. Fatty acids that are reproduced by lipid reactions conjointly feed the citric acid cycle at the extent of acetyl-CoA. The advanced method of extracting electrons from fat molecules for ATP production is named β-oxidation. Entry of fatty acyl-CoA into the mitochondrial matrix is prohibited unless the fatty acid is coupled via anester linkage to carnitine that facilitates uptake of acyl molecules into the matrix, catalysed by carnitine palmitoyltransferase1 (Cpt1).

Carnitine is instantly exchanged with CoASH by Cpt2, and acyl-CoA enters into β-oxidation once it is inside the matrix. Note that 1FADH₂ and 1NADH along with an acetyl-CoA are yielded from the reaction of two carbons on the fatty acyl chain. Acetyl-CoA then enters the citric acid cycle where it is oxidized further (Mailloux 2015). However, electron movement in mitochondria is much more difficult to characterize, given the completely different redox centres in mitochondrial enzymes. Metabolic process complexes are separated by peptide chains with most carriers buried deep inside the lipid bilayer (Nicholls and Ferguson 2002). Thus, electron transfer cannot be envisaged as an easy donation and acceptance of an electron between two completely different ions in an aqueous solution (Nicholls and Ferguson 2002). Rather, electrons move between prosthetic groups and proceed via electron tunnelling channels (Nicholls and Ferguson 2002). Basically, electron tunnelling determines the chance of whether or not an electron can move from a donor to an acceptor molecule. Tunnelling between donor and acceptor molecules is heavily influenced by the distance between the centres, distinction in oxidation-reduction potential and response of electron carriers to changes in charge on donor/acceptor molecules (Nicholls and Ferguson 2002). Discussing the principles of electron transfer reactions in mitochondria is a vital consideration in the formation of O₂, which is presumably influenced by constant factors such as oxidation-reduction difference and response to donor/acceptor molecules (Klinman 2007). Peterson (1991) previously showed that UFAs facilitate electron transfer between iron centres including ferrous iron and ferric element cytochrome c. This author discovered that electron transfer is additionally increased during this process and has proposed a more physiological model of fatty acids related to proteins. Further investigation is increasingly finding that superoxide dismutase (SOD) increases electron transfer, while investigation continues of whether or not free superoxide is involved in this electron transfer. Both UFA and SOD are taking part in membrane redox systems, yet the mechanism of electron transfer remains largely unknown.

Rate constants for photoinduced electron transfer reactions of UFAs with a series of singlet excited states of oxidants in acetonitrile at 298 K were examined by Kitaguchi et al. (2006), and the ensuing electron transfer rate constants [k(et)] were evaluated in the light of the free energy properties of electron transfer to determine the one-electron oxidation potentials [E(ox)] of UFAs and therefore the intrinsic barrier to electron transfer. The k(et) values of linoleic acid with a series of oxidants are similar because the corresponding k(et) values of arachidonic acid, methyl linoleate and linolenic acid resulted in a similar E(ox) value for linoleic acid, methyl linoleate, linolenic acid and arachidonic acid (1.76 V vs. Singlet Excited States [SCE]). This is considerably less than that of monoUFA/oleic acid (2.03 V vs. SCE), as indicated by the k(et) values for oleic acid smaller than those of other UFAs. The unconventional ion of linoleic acid created photoinduced electron transfer from linoleic acid to the singlet excited state of 10-methyl acridinium ion, as well as that of 9,10-dihydroanthracene. It was detected by laser flash photolysis experiments. The apparent rate constant of deprotonation of the radical cationion of linoleic acid was determined as 8.1 × (10³) s⁽⁻¹⁾. No thermal electron transfer or proton-coupled electron transfer occurred from linoleic acid to a robust one-electron oxidant, Ru(bpy)3⁽³⁺⁾ (bpy = 2,2′-bipyridine) or Fe(bpy)3⁽³⁺⁾ in the presence of oxygen, to the deprotonated radical produces the peroxyl radical. The above mentioned proton transfer and electron transfer properties of UFAs give valuable mechanistic insight into lipoxygenases to clarify the proton-coupled electron transfer method in the catalytic function.

8 Conclusions and future perspectives

Biocorrosion causes billions of dollars of losses every year. MIC is important in a number of key industries including maritime, chemical engineering and bioprocess engineering. Presently no environmentally friendly technology has been available in published literature to minimize the economic loss of biocorrosion. Currently available antibiocorrosion technology depends heavily on physical and chemical methods to regulate biofilm formation. Therefore, it is essential to know the fundamentals of MIC and also the roles of biomolecules within the whole process to develop detailed research capabilities and potential control and management strategies. This review targets the roles of EPS, proteins, lipids and different substances currently known to be involved in corrosion processes. The potential roles of lipids and enzymes are still poorly understood. There are still collective issues that need to be addressed, including the importance of microbial role in MIC. More species-specific questions concern the impact of enzyme activities inside the biofilm matrix on the dynamics of corrosion reactions. Also, there is involvement of organometallic complexes in electron transfer from zero-valent metals and from chemically and morphologically diverse metallic surface films to ultimate electron acceptors.