Microbial Ecology of Pectin Decomposition in Anoxic Lake Sediments

Abstract

Anaerobic digestion of pectin by bacteria was examined in two freshwater lakes in Wisconsin and in defined laboratory cultures of species prevalent in the lake sediment. The turnover times for pectin biodegradation to methane in sediments incubated at in situ temperature were much longer (100 h in Lake Mendota and 185 h in Knaack Lake) than either that observed for glucose (12 h in Lake Mendota) or previously reported for acetate (0.22 h in Lake Mendota). The numbers of pectinolytic anaerobes varied seasonally in both sediments (10^2-10^5 and 10^3-10^5 ml^-1 in Knaack Lake and Lake Mendota, respectively), and were highest during the fall after sedimentation of algal blooms and/or leaf detritus. Clostridium butyricum was identified as a prevalent pectinolytic anaerobe in both lakes. In mono-culture pectin fermentations, C. butyricum produced methanol, H2/CO2, acetate, ethanol and butyrate; growth stopped in the presence of excess energy source when the pH fell to 4.3. In co-culture pectin fermentations of C. butyricum/Methanosarcina barkeri, H2/CO2, methanol and acetate were detected as intermediary metabolites, and pectin was completely degraded to CH4 and CO2, the pH remaining neutral. 14C-radiotracer analysis substantiated the simultaneous conversion of H2/CO2, methanol and acetate to CH4 by M. barkeri as these metabolites were generated from pectin hydrolysis by C. butyricum.