57Fe as a Tracer of Bacterially Mediated Iron Mobilization in Lake Sediments

Abstract

In simulation experiments the stable isotope, 57 Fe, was used as a novel tracer of iron transport to investigate the role of microbial activity on the distribution/redistribution of metals in lake sediments. A series of microcosm sediment columns was set up, containing a layer of 57 Fe-labelled ferrihydrite to track iron mobility temporally. Three series of columns were prepared, one set with unamended homogenized lake sediments, one set amended with 57 Fe-labelled ferrihydrite in the bottom layer of the column and one abiotic control set amended with 57 Fe-labelled ferrihydrite which was sterilized. During the experiments the iron concentrations increased in the upper sediment layers, pore water and overlying lake water and decreased in the lower amended sediment layer. The distribution of 57 Fe in the abiotic cores did not change during the experiment, implying that physical disturbance during sampling and chemical iron reduction and remobilization can be excluded as causes for the changes in iron distribution in the biotic cores. Thus, the remobilization and deposition of iron in the columns was initiated by microbial redox reactions. The implications of this work are significant for the interpretation of metal profiles in fresh water sediments as historical records of metal loadings, particularly because redox-sensitive amorphous Fe and Mn oxides and oxyhydroxides can serve as sites for metal adsorption or co-precipitation.