Competition between Fe(III)-reducing and methanogenic bacteria for acetate in iron-rich freshwater sediments

The kinetics of acetate uptake and the depth distribution of [2-C-14]acetate metabolism were examined in iron-rich sediments from a beaver impoundment in northcentral Alabama. The half-saturation constant (K-m) determined for acetate uptake in slurries of Fe(III)-reducing sediment (0.8 muM) was more than 10-fold lower than that measured in methanogenic slurries (12 muM) which supported comparable rates of bulk organic carbon metabolism and V-max values for acetate uptake. The endogenous acetate concentration (S-n) was also substantially lower (1.7 muM) in Fe(III)-reducing vs methanogenic (9.0 muM) slurries. The proportion of [2-C-14]acetate converted to (CH4)-C-14 increased with depth from ca 0.1 in the upper 0.5 cm to ca 0.8 below 2 cm and was inversely correlated (r(2) = 0.99) to a decline in amorphous Fe(III) oxide concentration. The results of the acetate uptake kinetics experiments suggest that differences in the affinity of Fe(III)-reducing bacteria vs methanogens for acetate can account for the preferential conversion of [2-C-14] acetate to (CO2)-C-14 in Fe(Ill) oxide-rich surface sediments, and that the downcore increase in conversion of [2-C-14]acetate to (CH4)-C-14 can be attributed to progressive liberation of methanogens from competition with Fe(III) reducers as Fe(III) oxides are depleted with depth.