Hexagonal crystals of green rust (carbonate and/or sulfate) have also been obtained as a byproducts of bioreduction of ferric oxyhydroxides by dissimilatory iron-reducing bacteria, such as Shewanella putrefaciens, that couple the reduction of  with the oxidation of organic matter. This process has been conjectured to occur in soil solutions and aquifers.
In one experiment, a 160 mM suspension of orange lepidocrocite γ- in a solution containing formate (), incubated for 3 days with a culture of S. putrefaciens, turned dark green due to the conversion of the hydroxide to GR(), in the form of hexagonal platelets with diameter ~7 µm.  In this process, the formate was oxidized to bicarbonate  which provided the carbonate anions for the formation of the green rust.  The live bacteria were shown to be necessary for the formation of the green rust.