Abstract
Summary Cyanide or cyanide–metal complexes are frequent contaminants of soil or aquifers at industrial sites, which can be released from such sites by outgassing or transport with the groundwater. They form very stable complexes with iron, which may occur in the subsurface as an insoluble blue mineral, the so‐called Prussian Blue (Fe 4 [Fe(CN) 6 ] 3 ). In this study, we show that the insoluble and colloidal Fe(III)–cyanide complex Prussian Blue can be reduced and utilized as electron acceptor by the dissimilatory iron‐reducing bacteria Geobacter metallireducens and Shewanella alga strain BrY. The microbial reduction of the dark blue pigment Prussian Blue leads to the formation of a completely colourless solid mineral, presumably Prussian White (Fe 2 [Fe(CN) 6 ]), which could be reoxidized through exposure to air, regaining the dark blue colour. In addition, the microorganisms were able to grow with Prussian Blue, using it as the sole electron acceptor. Geobacter metallireducens could also reduce Prussian Blue coatings on sand, which was sampled from a contaminated site.