Microbial Acidification as a Hidden Driver of Subsea Tunnel Concrete Deterioration

Concrete remains the backbone of modern infrastructure, yet beneath its perceived durability lies a growing vulnerability often overlooked: biological degradation. While concrete is engineered to withstand immense physical and chemical stresses, its exposure to microbial activity introduces a more insidious form of deterioration that is difficult to detect until significant damage has occurred. In marine and subterranean environments where concrete structures encounter persistent moisture and saline groundwater, these risks escalate. One notable environment where this vulnerability is pronounced is in subsea tunnels—complex constructions that endure not only the mechanical pressures of surrounding geology but also the quiet, steady assault of microbial life. Despite an increasing awareness of microbially induced concrete corrosion (MICC), much of the scientific understanding has been rooted in terrestrial settings such as sewer systems, where sulfate-reducing bacteria flourish under anaerobic conditions, producing sulfuric acid that attacks the cement matrix. However, marine-influenced, oxygen-rich environments like subsea tunnels present an entirely different set of microbial dynamics. Here, the availability of oxygen, iron, manganese, nitrogen, and sulfur creates fertile ground for aerobic microbial communities whose metabolic processes can slowly acidify the concrete environment. Yet, the specific biological players and the exact mechanisms fueling the observed deterioration in such contexts have remained poorly characterized. The Oslofjord subsea tunnel in Norway offered an intriguing natural laboratory for investigating these processes. Reports of concrete degradation and corrosion of embedded steel fibers in this tunnel raised serious concerns about the long-term stability of sprayed concrete linings. Observations of biofilm growth on concrete surfaces, coupled with mineral deposits characteristic of microbial activity, hinted that conventional models of MICC might not fully capture the phenomena occurring in these marine-influenced settings. The question loomed large: which microorganisms were driving these changes, and how exactly were they undermining the integrity of the concrete?

In response to this critical knowledge gap, new research paper published in Scientific Reports and conducted by Dr. Sabina Karačić, Dr.  Carolina Suarez, Dr.  Per Hagelia, Dr.  Frank Persson, Dr.  Oskar Modin, Assistant Professor Paula Dalcin Martins & led by Professor Britt-Marie Wilén from the Department of Architecture and Civil Engineering at Chalmers University of Technology, researchers dissected the biological, chemical, and structural underpinnings of sprayed concrete deterioration in the Oslofjord tunnel. Their goal was not merely to catalog the microbial inhabitants but to decode the specific metabolic pathways—such as nitrogen, sulfur, iron, and manganese oxidation—that could link biofilm formation to measurable structural damage. By pairing cutting-edge genomic tools with detailed mineralogical and chemical analyses, they sought to untangle the complex web of interactions that lead from microbial colonization to concrete failure. Their work aimed to lay a scientific foundation for better predicting, mitigating, and ultimately preventing biological degradation in vital underwater infrastructure.

Over a span of five years, the authors sampled biofilms from three distinct tunnel locations, capturing a timeline of microbial colonization and its tangible impacts on the concrete beneath. Using amplicon sequencing, they first profiled the microbial communities living within these biofilms, revealing a surprising richness dominated by bacteria capable of oxidizing nitrogen, sulfur, iron, and manganese. This biological profile alone hinted that the biofilms were far from passive—they were dynamic, chemically aggressive communities shaping their environment. Complementing the biological analyses, the team employed shotgun metagenomics to dig deeper, reconstructing hundreds of microbial genomes and decoding their functional potential. They discovered an overwhelming presence of genes linked to sulfur oxidation, iron cycling, and ammonia oxidation—metabolic activities known to generate acidic byproducts. This genetic evidence matched the chemical fingerprints observed in the field: mild but persistent acidification at the biofilm–concrete interface and increased calcium leaching from the cement matrix, confirmed by changes in water chemistry collected directly from tunnel seepage sites.

Concrete samples taken beneath these biofilms underwent polarized light microscopy, scanning electron microscopy, and X-ray diffraction analyses. These physical examinations uncovered profound mineralogical transformations. The researchers detected manganese and iron oxides deposited within the biofilms, secondary minerals such as magnesium-silicate hydrates, and the telltale presence of thaumasite—a mineral associated with severe sulfate attack. SEM imagery revealed friable, porous zones within the concrete, a clear sign that the matrix was losing its cohesive strength under microbial assault. The most striking correlation between experimental findings emerged when the researchers compared sites with differing water flow rates. Locations with low flow exhibited deeper, more aggressive deterioration, while areas with higher flow maintained relatively intact concrete structures. This elegantly tied the microbial processes to physical outcomes: stagnant water nurtured the acid-producing biofilms, while faster flows diluted their metabolic products. Together, the experiments painted a vivid, interwoven narrative—biofilm-driven acidification, fueled by complex microbial metabolisms, was directly responsible for the weakening and corrosion of subsea tunnel concrete, a slow-motion collapse orchestrated not by nature’s fury but by microscopic life.

In conclusion, the research work of Professor Britt-Marie Wilén  and her colleagues shows, with precise evidence, that biodeterioration in subsea tunnels is fundamentally distinct from traditional corrosion mechanisms observed in sewers and urban systems. Rather than depending on sulfur-reducing bacteria and high organic loads, the Oslofjord tunnel’s decay was driven by oxygen-fueled microbial metabolisms, using nitrogen, sulfur, iron, and manganese as biochemical levers to acidify the concrete’s surface and quietly unravel its internal strength. This discovery challenges long-standing assumptions about where and how microbial threats manifest in marine infrastructure.

The implications are far-reaching. Engineers and infrastructure managers can no longer rely solely on traditional models of concrete durability that overlook biological factors. Designs for sprayed concrete linings, particularly in saline, oxygen-rich environments, must now consider microbial colonization as a primary risk factor. Moreover, strategies to protect vital structures must expand beyond chemical additives and stronger concrete mixes to include microbial monitoring and perhaps even biofilm-resistant surface treatments. This study also opens new avenues for research into novel microbial communities that thrive in extreme engineered environments, revealing both a threat and a potential resource for future biotechnological innovations. By capturing the evolutionary dynamics of biofilm communities over time, the study underscores the critical importance of long-term monitoring, not merely at a structural level, but at the microbial level. Small shifts in microbial composition or water flow can dramatically alter the rate of biodeterioration, suggesting that early detection of biofilm changes could serve as a predictive tool for infrastructure maintenance. Fundamentally, the work transforms the perception of sprayed concrete in subsea tunnels from inert material to a dynamic interface where biology and engineering collide—with consequences that unfold over years but can ultimately threaten the stability of critical transportation arteries.