Structure of Bacterial Communities Associated with Accelerated Corrosion of Port Transportation Infrastructure

Steel sheet piling material used for docks, bridges and bulkheads in the Duluth-Superior harbor has been reported to be corroding at an accelerated rate. Corroded areas are characterized by blister-like, raised orange tubercles, a pattern consistent with microbiologically influenced corrosion (MIC). We demonstrated that bacterial communities on corroded steel in the most affected part of this harbor were different from bacterial communities at a less affected area just outside the harbor using T-RFLP community DNA fingerprinting. Siderooxidans lithoautotrophicus, a microaerophilic chemotrophic bacterium, which oxidizes Fe²⁺ to Fe³⁺, was repeatedly isolated from the corroding structures. Sequencing the 16S rDNA gene of bacterial clones indicated that the majority of bacteria on the corroded surfaces were from the β- and α-Proteobacteria, and Cyanobacteria phyla. This clonal analysis also indicated the presence of a bacterium most similar to the iron-reducing bacterium Rhodoferax ferrireducens, which grows well at low temperatures (4°C) seasonally encountered in this harbor. Although we cannot provide conclusive evidence that these iron bacteria are the causative agents of the accelerated corrosion, our preliminary results indicate that the corroding steel structures are covered by complex microbial biofilms that contain bacteria of the type responsible for corrosion of steel in other environments.