Summary: Microbial activity was determined along electrode potential gradients within sediments, and along sediment surfaces in a stratified eutrophic lake. There was evidence of a change from a tricarboxylic acid cycle-based metabolism to a fermentative one with decreasing electrode potential. A more detailed examination of the stratification of the microbial community showed that the activities of enzymes associated with energy metabolism (in this case electron transport) were highest on electrode potential gradients. This was observed within a sediment core on a millimetredepth scale, as well as over several metres at the surface of sediments, on a transect which ran from oxic littoral muds to the anoxic profundal zone. In contrast, microbial hydrolytic enzymes, such as protease and amylase, were most active at the sediment surface, where the highest concentrations of substrates might be expected.


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