In aquatic environments, the presence of microorganisms often leads to severe deterioration of metallic materials. While sessile cell (biofilm)-driven microbiologically-influenced corrosion (MIC), or biocorrosion, is recognized as a contributor to metal failures in marine systems, uncertainty still exists whether and to what extent products resulting from metabolic activity of the planktonic cells impact corrosion.

Electrochemical measurements, molecular ecology techniques, comparative untargeted metabolomics, light and electron microscopy and surface analyses were employed to determine whether there is a difference in corrosion of 1020 carbon steel electrodes exposed to a native North Pacific seawater bacterial population that was either allowed to or prevented from developing a biofilm on the electrode surface. Laboratory studies were carried out using batch reactors. The investigation re-addressed the following issues (a) whether  physical contact between cells and the metal surface is essential to enhance corrosion of 1020 carbon steel; (b) what, if any, is the impact of metabolic products secreted by  cells kept in the planktonic phase, through confinement within a dialysis tube, on the severity of 1020 carbon steel deterioration; (c) are there any differences between the mineralogy of corrosion products formed on biofilm–free and biofilmed electrodes and (d) whether and to what extent does the biofilm population differ from that of the initial planktonic inoculum and the implication of this difference for diagnosing and mitigating MIC.  

Increased corrosion rates and extensive pitting damage were observed on the surface of biofilmed electrode (BE) compared to the biofilm-free (BFE) electrode which served as a “sterile” control. Corrosion products recovered from the BE surface had a significantly higher content (% w/w) of CaCO₃ (aragonite) than those recovered from BFE surface (24.6% and 1.3% respectively). A reverse trend was observed for goethite (α-FeOOH); the latter dominated the BFE surface (43.8 wt %) and was relatively scarce (6.4 wt %) on BE surface. A vast disparity, mainly in putative lipids content, was found between metabolomes of the bacterial population allowed to form a biofilm and the same population kept in planktonic phase.  DNA profiles of planktonic and biofilm bacterial populations, obtained through Illumina sequencing, were also considerably different. Extracellular DNA (e-DNA) comprising 16S rRNA sequences (up to 590 OTU), some of which were characteristic of bacteria implicated in MIC, was observed in the corrosion products recovered from the BFE surface.

These results confirmed other reports that, in an oxygenated marine environment, the physical presence of an actively metabolizing bacterial biofilm leads to a pronounced pitting damage and increases corrosion rates of carbon steel. The study revealed that when cells are prevented from interacting with the surface, i.e. confined within a planktonic phase, their metabolic products released into the planktonic phase do not cause significant pitting corrosion. The corrosion in this case, is similar to that measured in filter sterilized seawater. Importantly, the investigation demonstrated that DNA profile of a biofilm differed significantly from that of planktonic seawater population, thus confirming that MIC risk assessment should not be carried out using sampling of planktonic cells alone. The presence of eDNA within corrosion products recovered from the biofilm-free electrode showed that identifying biocorrosion exclusively based on DNA profiling can be misleading and should not be used as a sole MIC diagnostic method.