Activity and structure of methanogenic microbial communities in sediments of cascade hydropower reservoirs, Southwest China.

Freshwater reservoirs are an important source of the greenhouse gas methane (CH). However, little is known about the activity and structure of microbial communities involved in methanogenic decomposition of sediment organic matter (SOM) in cascade hydropower reservoirs. In this study, we targeted on sediments of three cascade reservoirs in Wujiang River, Southwest China. Our results showed that the content of sediment organic carbon (SOC) was between 3% and 11%, and it's positively correlated with both C/N ratio and recalcitrant organic carbon content of SOM. Meanwhile, SOC content was positively correlated with CH production rates but had no significant correlation with total CO production rates of the sediments, when rates were normalized to sediment volume. Resultantly, the sediment anaerobic decomposition rates hardly significantly increase along with the SOC content. These results suggested that the terrestrial organic matter accumulated after damming stimulated CH production from the reservoir sediments even though its decomposition rate was limited. Meantime, high throughput sequencing of 16S rRNA genes indicated that not only the hydrogenotrophic and acetoclastic, but also the methylotrophic methanogens (Methanomassiliicoccus) are abundant in the reservoir sediments. Moreover, metagenomic sequencing also suggested that methylotrophic methanogenesis are potentially important in the sediment of cascade reservoirs. Finally, the hydraulic residence time of the reservoir could be the key controlling factor of the structures of bacterial and archaeal communities as well as the CH production rates of the reservoir sediments.