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 (CH4). However, little is known about the activity and structure of microbial communities involved in methanogenic decomposition of sediment organicmatter (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 correlatedwith both C/N ratio and recalcitrant organic carbon content of SOM. Meanwhile, SOC content was positively correlated with CH4 production rates but had no significant correlation with total CO2 production rates of the sediments, when rateswere 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 CH4 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 CH4 production rates of the reservoir sediments. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study found that sediment organic carbon content is positively correlated with methane production rates in cascade hydropower reservoirs, but not significantly correlated with total CO2 production rates. The anaerobic decomposition rates of sediments do not significantly increase along with the increase of SOC content.