Effect of Fall Turnover on Terminal Carbon Metabolism in Lake Mendota Sediments

Abstract

The carbon and electron flow pathways and the bacterial populations responsible for the transformation of H(2)-CO(2), formate, methanol, methylamine, acetate, ethanol, and lactate were examined in eutrophic sediments collected during summer stratification and fall turnover. The rate of methane formation averaged 1,130 mumol of CH(4) per liter of sediment per day during late-summer stratification versus 433 mumol of CH(4) per liter of sediment per day during the early portion of fall turnover, whereas the rate of sulfate reduction was 280 mumol of sulfate per liter of sediment per day versus 1,840 mumol of sulfate per liter of sediment per day during the same time periods, respectively. The sulfate-reducing population remained constant while the methanogenic population decreased by one to two orders of magnitude during turnover. The acetate concentration increased from 32 to 81 mumol per liter of sediment while the acetate transformation rate constant decreased from 3.22 to 0.70 per h, respectively, during stratification versus turnover. Acetate accounted for nearly 100% of total sedimentary methanogenesis during turnover versus 70% during stratification. The fraction of CO(2) produced from all C-labeled substrates examined was 10 to 40% higher during fall turnover than during stratification. The addition of sulfate, thiosulfate, or sulfur to stratified sediments mimicked fall turnover in that more CO(2) and CH(4) were produced. The addition of Desulfovibrio vulgaris to sulfate-amended sediments greatly enhanced the amount of CO(2) produced from either [C]methanol or [2-C]acetate, suggesting that H(2) consumption by sulfate reducers can alter methanol or acetate transformation by sedimentary methanogens. These data imply that turnover dynamically altered carbon transformation in eutrophic sediments such that sulfate reduction dominated over methanogenesis principally as a consequence of altering hydrogen metabolism.