The formation of green rust induced by tropical river biofilm components

In the Sinnamary Estuary (French Guiana), a dense red biofilm grows on flooded surfaces. In order to characterize the iron oxides in this biofilm and to establish the nature of secondary minerals formed after anaerobic incubation, we conducted solid analysis and performed batch incubations. Elemental analysis indicated a major amount of iron as inorganic compartment along with organic matter. Solid analysis showed the presence of two ferric oxides ferrihydrite and lepidocrocite. Bacteria were abundant and represented more than 1011 cells g−1 of dry weight among which iron reducers were revealed. Optical and electronic microscopy analysis revealed than the bacteria were in close vicinity of the iron oxides. After anaerobic incubations with exogenous electron donors, the biofilm's ferric material was reduced into green rust, a FeII–FeIII layered double hydroxide. This green rust remained stable for several years. From this study and previous reports, we suggest that ferruginous biofilms should be considered as a favorable location for GR biomineralization when redox conditions and electron donors availability are gathered.

Research highlights
► Characterization of ferruginous biofilm components by solid analysis methods.
► Lepidocrocite and ferrihydrite were the main iron oxides.
► Anaerobic incubation of biofilm with electron donors produced green rust.
► Biofilm components promote the formation of the green rust.
► Ferruginous biofilm could contribute to the natural mercury attenuation.