Microbial Methanogenesis and Acetate Metabolism in a Meromictic Lake

Abstract

Methanogenesis and anaerobic acetate metabolism were examined in the sediment and water column of Knaack Lake, a small biogenic meromictic lake located in central Wisconsin [USA]. The lake was sharply stratified during the summer and was anaerobic below a depth of 3 m. Large concentrations (4000 .mu.mol/l) of dissolved CH4 were detected in the bottom waters. A CH4 concentration maximum occurred at 4 m above the sediment. The production of 14CH4 from 14C-labeled HCOOH, HCO3- and CH3OH and [2-14C]acetate demonstrated microbial methanogenesis in the water column of the lake. The maximum rate of methanogenesis calculated from reduction of H14CO3- by endogenous electron donors in the surface sediment (depth, 22 m) was 7.6 nmol/h per 10 ml and in the water column (depth, 21 m) was 0.6 nmol/h per 10 ml. The methyl group of acetate was simultaneously metabolized to CH4 and CO2 in the anaerobic portions of the lake. Acetate oxidation was greatest in surface waters and decreased with water depth. Acetate was metabolized primarily to methane in the sediments and water immediately above the sediment. Sulfide inhibition studies and temperature activity profiles demonstrated that acetate metabolism was performed by several microbial populations. Sulfide additions (less than 5 .mu.g/ml) to water from 21.5 m stimulated methanogenesis from acetate, but inhibited CO2 production. Sulfate addition (1 mM) had no significant effect on acetate metabolism in water from 21.5 m, while nitrate additions (10-14,000 .mu.g/l) completely inhibited methanogenesis and stimulated CO2 formation.