Methanogenic pathway and archaeal community structure in the sediment of eutrophic Lake Dagow: Effect of temperature

Methanogenic degradation of organic matter is an important microbial process in lake sediments. Temperature may affect not only the rate but also the pathway of CH, production by changing the activity and the abundance of individual microorganisms. Therefore, we studied the function and structure of a methanogenic community in anoxic sediment of Lake Dagow, a eutrophic lake in north-eastern Germany. Incubation of sediment samples (in situ 7.5degreesC) at increasing temperatures (4, 10, 15, 25, 30degreesC) resulted in increasing production rates of CH4 and CO2 and in increasing steady-state concentrations of H-2. Thermodynamic conditions for H-2/CO2-dependent methanogenesis were only exergonic at 25 and 30degreesC. Inhibition of methanogenesis with chloroform resulted in the accumulation of methanogenic precursors, i.e., acetate, propionate, and isobutyrate. Mass balance calculations indicated that less CH4 was formed via H-2 at 4degreesC than at 30degreesC. Conversion of (CO2)-C-14 to (CH4)-C-14 also showed that H-2/CO2-dependent methanogenesis contributed less to total CH4 production at 4degreesC than at 30degreesC. [2-C-14]Acetate turnover rates at 4degreesC accounted for a higher percentage of total CH4 production than at 30degreesC. Collectively, these results showed a higher contribution of H-2-dependent methanogenesis and a lower contribution of acetate-dependent methanogenesis at high versus low temperature. The archaeal community was characterized by cloning, sequencing, and phylogenetic analysis of the 16S rRNA genes retrieved from the sediment. Sequences were affiliated with Methanosaetaceae, Methanomicrobiaceae, and three deeply branching euryarchaeotal clusters, i.e., group III, Rice cluster V, and a novel euryarchaeotal cluster, the LDS cluster. Terminal restriction fragment length polymorphism (T-RFLP) analysis showed that 16S rRNA genes affiliated to Methanosaetaceae (20-30%), Methanomicrobiaceae (35-55%), and group III (10-25%) contributed most to the archaeal community. Incubation of the sediment at different temperatures (4-30degreesC) did not result in a systematic change of the archaeal community composition, indicating that change of temperature primarily affected the activity rather than the structure of the methanogenic community.