Terrestrial leaf fossils often form through authigenic preservation in which the leaf surface is coated by a variety of minerals, especially iron oxides. The mechanism of this fossilization is unclear, because the largely hydrophobic leaf surfaces do not readily bind metal ions. Previously proposed mechanisms for mineral encrustation include precipitation of minerals in sediment pore space and precipitation of iron oxides at the surface by decay-produced CO2. Here we demonstrate that diverse bacterial species rapidly colonize leaf surfaces and form a biofilm within days of the leaf's entry into a stream environment. Experimental mineralization of fresh and biofilm-coated leaves indicates that leaves without biofilm do not mineralize, but leaves with biofilms rapidly adsorb metal ions such as Fe3+ onto the anionic biofilm surface where the ions form ferrihydrite. Once these mineralized leaves are buried by the sediment, they are more likely to be converted to fossils than non-mineralized leaves. Examination of a fossil leaf surface by scanning electron microscopy shows bacteria-sized structures resembling those found in biofilms. These experimental data imply that bacterial colonization of leaves may be an essential prerequisite for authigenic preservation.