Microbial biofilm communities associated with degradation of sprayed concrete in subsea tunnels

Sammanfattning: Deterioration of concrete leads to reduced structural strength implying high societal challenge with huge economic impact. In the Oslofjord subsea tunnel, complex microbial biofilm activity together with abiotic attack from saline groundwater are responsible for concrete matrix degradation and steel fiber corrosion. Previous research has revealed that microbial attack causes disintegration of cement paste matrix and thinning of sprayed concrete at rates varying from 0.5-10 mm/year in areas with leakages of saline groundwater. General knowledge about biodegradation of concrete infrastructures in marine environment is lacking and research in this area is therefore needed. A long-term study of biofilm microbial community composition and dynamics was performed between 2015-2020 in the Oslofjord subsea tunnel. Due to its complexity it was necessary to work multi-disciplinary, including studies of the microbial community structure by using advanced molecular techniques in combination with chemical analysis to get a comprehensive picture of the prevailing micro-environmental conditions within the biofilm. High throughput amplicon sequencing of 16S rRNA gene together with metagenomics shotgun sequencing revealed temporal dynamics in microbial community structure, and metabolic potential of biofilms in the Oslofjord tunnel. Water chemical analysis and microsensor measurements of oxygen and pH profiles within the biofilm were performed on-site to assess environmental conditions in the biofilms. Additionally, SEM microscopy together with XRD analyses were used to investigate concrete degradation beneath the biofilms over time. In parallel, a mesocosm experiment was performed over a period of 65 weeks to study the role of concrete material properties and fiber reinforcement for microbial colonization and composition. The long-term study preformed in the Oslofjord subsea tunnel revealed a complex microbial community involved in cement paste matrix degradation and steel fiber corrosion. The microbial communities at the different tunnel localities were composed of nitrogen converting bacteria, iron-oxidizing bacteria, sulfur oxidizing bacteria, heterotrophic aerobic bacteria, putative manganese-oxidizing bacteria and many microorganisms that could not be assigned to any function. Microsensor measurements showed relatively stable pH around 7-8 throughout the biofilm, whereas the dissolved oxygen profiles decreased with biofilm depth. Significant differences in community structure and richness between biofilms at the different tunnel locations were revealed with alpha and beta-diversity analysis. However, the microbial communities at the three sites shared many taxa. Pairwise comparisons suggested that deterministic factors were important for the assembly of the microbial communities of mature biofilms. Results from the mesocosms study indicate that stochastic factors were important during the initial colonization and time was the main factor which drives turnover in the biofilm communities on concrete material. Presence of steel fiber reinforcement was found to have a greater effect on the biofilm community composition than the surface roughness. The results obtained in this thesis help us to understand the complexity of microbial induced concrete deterioration and corrosion of steel fiber reinforcement observed in the subsea tunnel environments.