Attenuation of Fe(III)-reducing bacteria during table fluctuation of groundwater containing Fe.

Groundwater table fluctuation during natural and anthropogenic processes can facilitate the interaction between oxygen (O) from the unsaturated zone and ferrous iron (Fe) from the saturated zone. In light of previous findings that Fe(III)-reducing bacteria can be killed by the reactive oxidants produced from Fe oxidation under static oxic conditions, we hypothesize that Fe(III)-reducing bacteria will be attenuated during groundwater table fluctuations. To test this hypothesis, this study explored the variations of cell numbers of Shewanella oneidensis strain MR-1 (MR-1), a typical strain of Fe(III)-reducing bacteria, together with dissolved oxygen (DO) and Fe, at different points during controlled groundwater table fluctuations in a sand column. The results showed that, during the rise of the water table, O in the pore air was entrapped by the deoxygenated groundwater, and Fe in the groundwater was oxidized by the entrapped O. In this process, 1.40-2.42 orders of magnitude of viable MR-1 cells were killed at different points in the column. Further investigation proposed that the death of MR-1 is caused by the production of intracellular reactive oxidants, such as O• and OH•, from the oxidation of adsorbed/absorbed Fe instead of by bulk reactive oxidants, such as OH• and Fe(IV), produced from the oxidation of aqueous Fe. The findings here provide new insights for Fe biogeochemical cycling in the redox-dynamic zone.