Methanogenesis as an Important Terminal Electron Accepting Process in Estuarine Sediment at the Mouth of Orikasa River

Abstract

Environmental factors and rates of terminal electron-accepting (TEA) activities, such as methane production, sulfate reduction, and metal reduction, in estuarine sediment at the mouth of the Orikasa river (Yamada-cho, Iwate, Japan) were investigated over a period of two years. TEA processes in the surface sediment (~16 cm) were more active in summer (approx. 25°C) than winter (approx. 5°C). Comparisons of TEA rates also indicated that sulfate reduction served as the dominant TEA process in summer, while methane production was the most active in winter. The contribution of metal reduction to total carbon oxidation was shown to be relatively minor. Anaerobic incubation of the sediment with various substrates indicated that trimethylamine was the best substrate for methane production whereas acetate, formate, ethanol, and H₂ served as good substrates for sulfate reduction, suggesting the presence of substrate partitioning. To identify methanogens likely responsible for methane production in the estuarine sediment, 16S rRNA gene clone libraries were constructed by using Euryarchaeota-specific PCR and analyzed. The libraries contained some Methanosarcinales-related clones as well as many clones which were neither affiliated with known orders nor related (<90%) to isolated organisms. These results suggest that methanogenesis is an important TEA process in estuarine sediment, particularly in winter, which may be catalyzed by yet uncultured groups of the order Methanosarcinales.