Extremophilic microorganisms colonization of acidic historic ink with precipitation, dissolution and translocation of jarosite

Iron gall ink (IGI), used in Europe from the 12th to 19th centuries, was made from iron(II) sulfate, tannin-rich plant extracts and organic binders. Polyphenol reactions yield brown iron complexes that darken on oxygen exposure; IGI typically has pH 2-4. In some ancient documents, jarosite crystals formed in the ink, causing text to fade. Reproduction attempts reached no conclusive explanation; wetting-drying cycles are considered the main trigger. A 17th-century Italian manuscript with discolored IGI was examined by micro-Raman spectroscopy, SEM-EDS and metagenomics. The hydroxy-sulfate mineral jarosite (potassium-, sodium- and hydronium-variety) was identified as the cause: dendritic precipitates and pseudo-rhombohedral crystals showing dissolution, associated with fungi and bacteria. Jarosite was closely associated with fungi and bacteria. Acidophilic fungi ( Geotrichum, Coniochaeta ) mobilized elements inside and outside inked areas. Jarosite formation reflects interconnected inorganic-organic processes, iron redox cycling, organic-iron complexation and pH decrease below 3.0, mainly non-biological, with fungi driving dissolution and reprecipitation.