Role of mixed species microbial community biofilms in microbially influenced corrosion

Microbially influenced corrosion (MIC) is a process where microorganisms are involved in the deterioration of materials. It is commonly associated with species rich microbial communities forming biofilms on surfaces, facilitating habitat remodeling through complex metabolic processes, such as the formation of anaerobic zones that facilitate the activity of key organisms such as sulfate reducers and iron oxidizers. MIC significantly impacts many industries such as oil and gas, transportation and logistics resulting in the annual loss of billions of dollars from structural damage and early replacement of infrastructure. Environmental factors such as nutrients, oxygen concentration, and convective fields affect the community composition and microstructure of biofilms, thus determining the corrosion rate of the material. Despite the general understanding that MIC results from community level metabolic activity, most MIC studies focus on single-species, axenic cultures or limited combinations of species. The aim of this study was to apply an interdisciplinary approach to assess the onset of MIC. Initial experiments to investigate the effect of a mixed community on MIC of stainless steel (SS) in equatorial seawater, showed a strong selection for biofilm-forming microorganisms from the seawater onto the metal surface. This selection happened rapidly within 1 h and the community did not change significantly over the subsequent 7 d of the experiment. Confocal Laser Scanning Microscopy (CLSM) analysis indicated that attachment and biofilm formation begin within a few hours after inoculation. At the same time, the corrosion potential was observed to increase, indicating that ennoblement of the metal had occurred. Due to challenges in controlling variability in the natural environment, it was essential to design laboratory systems to mimic environment under controlled conditions. Marine mixed microbial communities were enriched from seawater using different artificial seawater formulations and those communities were used as the inocula for subsequent experiments. Electrochemical analysis of coupons incubated with mixed species communities showed a rapid onset of MIC for stainless steel coupons that coincided with the initial stages of biofilm formation. In contrast to the results from the environmental study, no ennoblement of the stainless steel was observed in the laboratory experiments. Visualization of the metal surfaces and biofilms by confocal laser scanning microscopy (CLSM) and field emission scanning electron microscopy (FESEM) showed distinct changes in the topography of metal surfaces during the onset of MIC. It was additionally observed that the biofilms appeared to preferentially form at the inter grain boundaries and this may be a driving factor in pit formation. The community composition of the biofilms formed on the metal surface were found to depend on the composition of the type of stainless steel used and were significantly different from the planktonic community. The effect of several parameters (e.g., stirring, nutrient concentration, surface polishing and oxygen concentration) on MIC rate were investigated in a systematic manner. The carbon source determined the composition of the microbial community, thus the corrosion rate. Surface polishing affected the topography of the metal surface, which in turn changed the localization of biofilms on the surface as compared to unpolished coupons. A continuous flow cell system was designed to allow for continuous nutrient replenishment, which should more closely reflect the natural environment. As for the batch experiments, the results suggested that nutrient composition plays an important role in biofilm formation and determines MIC rate. The electrochemical impedance of the biofilm formed on stainless steel was consistent with the rapid MIC onset. This study has attempted to reproduce the environmental conditions in a reliable laboratory setup, while employing different techniques for the analysis of corrosion behaviour due to biofilms. Overall, the results from this study showed that marine communities rapidly colonize metal surfaces and such biofilms may be formed by organisms that specialize in growth as a biofilm. The results also showed that the early corrosion behaviour is primarily driven by nutrient concentration. However, to better understand the relationship between community members and MIC, further mechanistic studies are needed. Such studies will be possible through the combination of electrochemistry, image analysis and –omics methods to improve our understanding of the role of biofilms in MIC.Doctor of Philosophy (IGS