ISOLATION AND MICROBIAL REDUCTION OF NATURAL FE(III)-BEARING PHYLLOSILICATES FROM SUBSURFACE SEDIMENTS

Fe-bearing phyllosilicates can be important sources of Fe(III) for dissimilatory microbial reduction in soils and sediments. However, the rate and extent of Fe-bearing phyllosilicate versus Fe(III) oxide reduction in natural environments has not yet been rigorously quantified. The goal of this research was to isolate phyllosilicate and Fe(III) oxide phases in two different subsurface sediments in order to permit experimentation with each phase in isolation. Physical partitioning by density gradient centrifugation did not adequately separate the phyllosilicate and Fe(III) oxide phases (primarily nanoparticulate goethite) present in a weathered shale saprolite. Hence we examined the ability of chemical extraction methods to remove Fe(III) oxides without significantly altering the properties of the phyllosilicates. Extraction with oxalate alone or oxalate in the presence of added Fe(II) altered the structure of Fe-bearing phyllosilicates, and in particular natural smectites present in the weathered shale saprolite. In contrast, room temperature and 80C citrate-bicarbonate-dithionite (CBD) extraction followed by reoxidation with hydrogen peroxide led to minimal alteration of smectite structure. Fe-stripped and pristine sediments were subjected to microbial Fe(III) reduction by Geobacter sulfurreducens. The extent of Fe(III) reduction in pristine shale saprolite was more than two times higher than that in CDB extracted sediment, suggesting a significant contribution of Fe(III) oxides to the overall Fe(III) reduction capacity of this material. In contrast, the extent of microbial Fe(III) reduction was similar in both pristine and CDB extracted Oyster, suggesting that phyllosilicate-Fe(III) is the dominant form of Fe(III) available as an electron acceptor this sediment.