Methanogenesis pathways in a stratified eutrophic alpine lake (Lake Bled, Slovenia)

The production pathway and carbon source of CH4 in the surface sediment of a eutrophic alpine lake (Lake Bled, northwest Slovenia), in which the hypolimnion is anoxic for most of the year, were determined from molecular and biogeochemical studies. The average delta C-13(CH4) value of -69.5% +/- 1.2%, associated with low acetate concentrations, suggested that CH4 should be formed, predominantly, hydrogenotrophically. The proportion of fresh autochthonous lipids in total extractable lipids in sediment decreased from 62% at the surface to 41% at a depth of 20 cm. The contribution of lipids of bacterial origin was more pronounced at the surface, comprising 13%. Terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S ribosomal ribonucleic acid (rRNA) sequences of bacterial and archaeal community members suggested that larger sediment-depth-dependent changes occurred in the latter. The majority of archaeal sequences belonged to Euryarchaeota. The methanogenic population accounted for 73% and 38% of the archaeal community at depths of 0-2 cm and 10-12 cm, respectively. In the upper 2 cm, hydrogenotrophs, mostly Methanomicrobiaceae, were dominant. In the deeper sediment, archaeal sequences were mostly those of unknown affiliation with Euryarchaeota, Thermoplasmatales, and related linkages, and only 21% of the hydrogenotrophic methanogenic archaea were detected. Somewhat lower percentages (< 18%) of sequences representing acetotrophic archaea (Methanosaetaceae) were present in the two layers. The biogeochemical processes and structure of the archaeal community support the hypothesis that hydrogenotrophic methanogenesis is the dominant pathway in the sediment of alpine Lake Bled, despite low temperature and the prevalence of fresh autochthonous-derived organic matter.