Microbially induced concrete degradation in subsea tunnels - Community structure of biofilms from sprayed reinforced concrete in the Oslofjord tunnel

Abstract: Microbially induced concrete deterioration has been discovered in several Norwegian subsea tunnels with significant disintegration of the calcium silicate hydrate (C-S-H) and steel fibre corrosion in areas with leakages of saline groundwater. Complex microbial biofilm activity together with abiotic attack from saline ground water, with high concentrations of Cl−, SO42−, Mg2+ and HCO3−, are responsible for the degradation in the specific subsea tunnel environment. While general mechanisms and microbial processes involved in Microbial Induced Deterioration (MID) are well known in sewer tunnels, knowledge about MID problems related to subsea road tunnels with reinforced sprayed concrete is limited.  In order to understand and ultimately prevent sprayed concrete deterioration and corrosion processes caused by microbial activity, different molecular methods and chemical measurements were preformed to assess the microbial diversity and community composition as well as the micro-environmental conditions within the biofilm. High throughput amplicon sequencing of 16S rRNA genes was used to survey the microbial composition, and microsensors were used to measure profiles of oxygen and pH within the biofilm. The combination of techniques was used to provide linkage between microbial diversity and chemical micro-environment within the biofilm. High abundance of iron-oxidising bacteria within genus Mariprofundus and autotrophic nitrogen converting microorganisms were detected in the biofilms from three different tunnel sites. Furthermore, a number of the numerically important contributors to the biofilm communities could not be assigned to any function. Microsensor measurements showed relatively stable pH around 7-8 towards the bottom of the biofilm up to the biofilm/electrode interface. The measured dissolved oxygen profiles revealed differences among local spots which varied in the superficial flow rate of water across the biofilm. With high superficial flow rate, dissolved oxygen concentrations decreased less with biofilm depth than at lower superficial flow rate. Furthermore, measurements alpha- and beta-diversity of the biofilm communities showed significant differences in richness and community structure between the tunnel sites. Pairwise microbial community comparisons at the three sites suggested that deterministic factors were important for the assembly of the microbial communities. Apparently, the biofilm transformations were more complex than initially anticipated and a combination of advanced molecular approaches are needed to trace down the role of these bacteria for the degradation of sprayed concrete in subsea tunnels.