Methanogenic pathways, ~(13)C isotope fractionation, and archaeal community composition in lake sediments and wetland soils on the Tibetan Plateau

摘要

Lakes are a significant source of atmospheric CH_4 and play an important role in the global carbon cycle. Little information on methane production in high-altitude lakes is available. We determined the methanogenic archaeal community composition, the methanogenic pathways, and carbon isotope fractionation in lake sediments and wetland soils on the Tibetan Plateau at about 4000 m above sea level (asl). We measured CH_4 production rates and the δ~(13)C of CO_2, CH_4, and acetate in the presence and absence of methyl fluoride, an inhibitor of acetotrophic methanogenesis, in sediments of five lakes (Awong Co, Bangong Co, Gongzhu Co, Daze Co, and Ranwu) and wetland soils adjacent to Bangong Co and Ranwu. Methane in Bangong Co sediment and in the wetland soil near Ranwu was mainly produced by acetotrophic methanogenesis, whereas methane in the sediments of the two saline lakes Awong Co and Gongzhu Co was mainly generated by hydrogenotrophic methanogensis; chemolithotrophic acetogenesis and methanol-dependent methanogenesis may also have played a role. The stable carbon isotope fractionation during CH_4 production from CO_2 was relatively large (average ε = -78‰). The methanogenic communities were similar to those found in lowland lake sediments, but those of saline and nonsaline Tibetan lakes differed. Hydrogenotrophic methanogens were dominant in lake sediments, while acetotrophic methanogens were dominant in wetland soils. Our results revealed diversity in the methanogenic communities and their methanogenic pathways and indicated that they are affected by sediment characteristics, such as salinity. However, the CH_4 production rates ranging from 6 to 122 nmol day~(-1) g dry weight~(-1) showed no relationship to environmental characteristics and were not limited by microbial abundance.