Microbial acidification by N, S, Fe and Mn oxidation as a key mechanism for deterioration of subsea tunnel sprayed concrete

Abstract

The deterioration of fibre-reinforced sprayed concrete was studied in the Oslofjord subsea tunnel (Norway). At sites with intrusion of saline groundwater resulting in biofilm growth, the concrete exhibited significant concrete deterioration and steel fibre corrosion. Using amplicon sequencing and shotgun metagenomics, the microbial taxa and surveyed potential microbial mechanisms of concrete degradation at two sites over five years were identified. The concrete beneath the biofilm was investigated with polarised light microscopy, scanning electron microscopy and X-ray diffraction. The oxic environment in the tunnel favoured aerobic oxidation processes in nitrogen, sulfur and metal biogeochemical cycling as evidenced by large abundances of metagenome-assembled genomes (MAGs) with potential for oxidation of nitrogen, sulfur, manganese and iron, observed mild acidification of the concrete, and the presence of manganese- and iron oxides. These results suggest that autotrophic microbial populations involved in the cycling of several elements contributed to the corrosion of steel fibres and acidification causing concrete deterioration.

Keywords: Amplicon sequencing; Biodeterioration; Biofilm community; Fibre-reinforced sprayed concrete; subsea tunnels; Metagenomics.

Fig. 1

SEM and X-ray element map from thin section at location P2 (A). Mn-oxide (blue) formed on outermost sprayed concrete underneath slimy Fe-rich biofilm, (not preserved in the thin section, located at the lower part of the image). Notice successive layers of calcium carbonate (pink) derived from leaching of the cement paste and magnesium silicate hydrate (M-S-H) formation (green) at the expense of calcium silicate hydrate (C-S-H) further inside, scale bar = 600 μm; and selected SEM images of outer biofilms at site P1: Mn-rich biofilm (globules) on a substrate of pitted NaCl with minor presence of Fe-rich filaments (B); site P2: X-ray map of Mn-rich bacterial cells (light yellow) within Fe-rich stalked bacterial cells (blue), partly twisted (C); and site T: Fibrous Fe-rich stalks and smaller, round bacterial cells (D). Scale bars = 40 μm.

Fig. 2

Schematic development of the concrete degradation. (A) initial biofilm formation, (B) degradation at a low flow rate, (C) continued degradation at an elevated flow rate, (D) effects of long-term degradation. The cross section shows sprayed concrete (grey) in contact with jointed rock mass (pink) with the development of deterioration underneath the biofilm (brown and black). The reaction zones with characteristic pore fluid pH and inward increasing pH gradient are indicated (light grey, beige). Friable degraded concrete contains carbonates and Mn-oxide (grey stippled). The different pH values in the reaction zones are based on the stability of minerals, See text for an explanation. W water, CEM cement, CARB carbonate, TSA thaumasite sulfate attack, Mn-ox manganese oxide, BW  biofilm water.

Fig. 3

Alpha diversity of biofilm communities over time (A,B) and dissimilarity between biofilm communities were analysed at different time points (C,D). Both alpha diversity and dissimilarities were analysed using Hill-based indices with a diversity order of 0 (A,C) and 1 (B,D). Average values for each year and time difference are shown. The error bars represent the standard error of the mean.

Fig. 4

Relative abundance of taxa in Oslofjord tunnel biofilms: top five phyla among MAGs (A); top five phyla among ASVs (B); top 20 MAGs by median relative abundance (C). The boxplots (A,B) show median (vertical bold line), 25th and 75th percentiles (box delimiters), minimum and maximum values excluding outliers (horizontal lines), and outliers (dots) with n = 8 (A) and n = 129 (B).

Fig. 5

Genes encoding proteins for sulfur reduction or oxidation: numbers of MAGs that had these genes (A); relative abundances of MAGs that had these genes (B); phylogenetic tree of DsrA sequences retrieved from MAGs in this study (in bold) as well as reference genomes (NCBI accession numbers are provided) (C).

Fig. 6

Potential for iron oxidation and reduction in microbial community members of Oslofjord tunnel biofilms. Number of MAGs with genes for iron oxidation and reduction (A); relative abundance of MAGs with genes for iron oxidation and reduction (B); and relative abundance of Zetaproteobacteria in the 16 S dataset showing median (horizontal bold line), 25th and 75th percentiles (box delimiters), minimum and maximum values excluding outliers (vertical lines), and outliers (dots).

Fig. 7

Phylogenetic tree of MAGs affiliated to the order SBBL01 based on 92 single copy genes. Nitrospira genomes are included as reference. Thermodesulfovibrio genomes were used as outgroup. Genes for enzymes involved in oxidation of nitrogen (AMO, NXR) and manganese (Cyc2, PCC1, PCCR) are indicated.

Fig. 8

Conceptual model of the mechanisms underlying concrete biodeterioration. The colours represent: dark grey = concrete, light grey = steel fibre, orange = biofilm, green arrow = reduction, red arrow = oxidation.