Mineralogical and microbial constraints of iron bioreduction and associated trace element cycling in riverbank sediments submitted to variable redox conditions

Fengfeng Zhang ab, F. Remy a, P. Gautret a, C. Grosbois c, J. Hellal b, F. Battaglia-Brunet ab, M. Motelica-Heinoab*

a ISTO, UMR 7327, CNRS, Université d’Orléans, BRGM, France

b BRGM, France

c E.A. 6293 GéoHydrosystèemes continentaux, Université de Tours, France

f.zhang@brgm.fr

Sediments in riverbanks and nearby lands submitted to flooding events undergo variations of redox conditions linked to water saturation. During the periods of anaerobic conditions, the bio-reduction of minerals containing ferric iron may induce the release of iron and several trace elements (TE), such as metals or metalloids that are associated to iron. The objective of the present study is to determine the mineralogical and microbial factors that constraint the kinetics of iron and TE release. It is also particularly interesting to determine if the nature of iron minerals influences the bacterial communities developed in contact with iron minerals, as biofilms. Complementary laboratory and on-site experiments were implemented to understand the interactions between iron oxides and iron-reducing microbial communities. The first phase of this experimental program included a characterization of biogeochemical parameters linked to iron reduction in riverbank sediments. The study site is located in Decize (Burgundy, France) near a Loire river channel. Three bulk samples submitted to different regimes of redox conditions were taken: soil from the dry river bank, flooded soil and sediment under water. First, cores were characterized using magnetic susceptibility, analysis of total carbon, total iron and sulfur, Raman and SEM-EDS. The iron-reducing bacteria Shewanella and Geobater were then searched in the different sediment samples using specific molecular primers. Samples from the three types of redox environments were inoculated in specific medium and three cultured enrichments of iron reducing bacteria were obtained that totally reduced FeIII-NTA in 1-2 days. Results showed that iron in sediments presented distinct oxidation states according to the water level. Some ferrihydrite was identified in an oxidized level by Raman spectroscopy, whereas iron sulfide globules (1-2 µm) were observed using SEM-EDS probe in a reducing level, suggesting dissolutive reduction of iron oxides concomitant to oxidation of organic matter. Additionally, Geobacter was detected in all soil samples but not the enrichments, whereas Shewanella was present in the soil, flooded soil and enrichments but not in the permanent aquatic sediment. In these iron reducing bacterial communities, Shewanella was present in higher proportion than Geobacter in enrichments, during the iron reducing process. The abundance of Geobacter may be linked to the type of iron oxide mineral.

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