Natural humic acids accelerate microbiologically influenced corrosion of X80 pipeline steel by Desulfovibrio vulgaris via extracellular electron transfer

Microbiologically influenced corrosion (MIC) caused by sulfate-reducing bacteria (SRB) poses a significant threat to the integrity of buried steel pipelines in soil and sediment environments. Natural humic acids (HAs), which are ubiquitous components of natural organic matter, possess redox-active functional groups that may influence microbial electron transfer processes, yet their role in MIC remains poorly understood. In this study, the effects of two representative natural humic acids, Leonardite humic acid (LHA) and Pahokee peat humic acid (PPHA), on the corrosion behavior of X80 pipeline steel mediated by Desulfovibrio vulgaris Hildenborough were systematically investigated. Electrochemical analyses, surface characterization, and weight loss measurements demonstrated that the presence of HAs significantly intensified corrosion, increasing the corrosion rate from 0.089 mm/y to approximately 0.17 mm/y and aggravating localized pitting damage. Transcriptomic and metabolic analyses revealed that HA exposure stimulated the expression of genes involved in dissimilatory sulfate reduction and promoted sulfate consumption and sulfide production. Additional experiments under carbon-starvation conditions indicated that HAs facilitate electron transfer from steel to microbial cells. Combined with FTIR characterization and comparative tests using the model quinone shuttle anthraquinone-2,6-disulfonate (AQDS), the results suggest that quinone moieties within HAs act as extracellular electron shuttles that accelerate the corrosion process. These findings highlight the active role of natural organic matter in MIC and suggest that the electron-shuttling capacity of humic substances should be considered when assessing corrosion risks in organic-rich environments.