Nitrate is a common contaminant causing eutrophication, particularly in agricultural areas, such as in Brittany (France). In this region, natural green-rust phases (fougerite) significantly contribute to the bluish color of hydromorphic soils. These minerals are thought to form upon reduction of Fe(III)-minerals by bacterial activity under anoxic conditions[1,2]. Since synthetic green rusts phases are known to reduce NO3-to NH4+ [3], potential for in situ denitrification involving bacterial metabolism and abiotic reduction by biogenic green-rust has been raised. In the present study, biogenic hydroxycarbonate green-rusts, GR(CO3), were produced in the laboratory from the reduction of either lepidocrocite or ferric oxyhydroxycarbonate [4] by Shewanellaputrefaciens strain ATCC 12099. These biogenic Fe(II,III)-containing minerals were characterized by XRD, TEM and transmission Mössbauer spectroscopy, and their interactions with nitrite were studied. Results show that kinetics of reduction by biogenic GR(CO3) differ from those reported for synthetic analogues [5]. In addition, XRD and EXAFS analysis of time-series samples help to elucidate the chemical mechanisms involved when green rusts get oxidized.