Insights into the influence of reinforcing iron bars corrosion on historical buildings: Towards optimizing limestone performance

Iron bar corrosion in ancient limestone buildings and sculptures causes severe damage, yet the underlying mechanisms and microbial contributions remain poorly understood. This study examined heavily deteriorated pinnacles from a 16th-century building in Granada, Spain, combining mineralogical, compositional, textural and metagenomic analyses. A highly diverse microbial community was identified, including iron-oxidizing [e.g., Massilia (4.59 %) and Ralstonia (<1 %)], sulfate-reducing [SRB, e.g., Desulfovibrio (<1 %) and Clostridium (1.43 %)], and nitrate-reducing [e.g., Pseudomonas (22.93 %) and Staphylococcus (2.53 %)] bacteria. The degradation process initiates by (bio)corrosion of embedded iron bars, followed by SRB-induced in situ (authigenic) framboidal pyrite formation under anoxic conditions, in the presence of sulfates from pollution-derived gypsum. Pyrite is then bacterially oxidized into goethite pseudomorphs under fluctuating aerobic and anoxic conditions, triggering a localized acid mine drainage-like process. This results in significant limestone dissolution and structural instability. These findings highlight the crucial role of microbial activity and reveal previously unrecognized pathways in stone degradation. Understanding such processes offer valuable insights for improving conservation strategies of cultural heritage.