Microbial methanogenesis in sediment plays a crucial role in CH₄ emission from freshwater lake ecosystems. However, knowledge of the layer-depth-related changes of methanogen community structure and activities in freshwater lake sediment is still limited. The present study was conducted to characterize the methanogenesis potential in different sediment-layer depths and the vertical distribution of microbial communities in two freshwater lakes of different trophic status on the Yunnan Plateau (China). Incubation experiments and inhibitor studies were carried out to determine the methanogenesis potential and pathways. 16S rRNA and mcrA genes were used to investigate the abundance and structure of methanogen and archaeal communities, respectively. Hydrogenotrophic methanogenesis was mainly responsible for methane production in sediments of both freshwater lakes. The layer-depth-related changes of methanogenesis potential and the abundance and community structure of methanogens were observed in both Dianchi Lake and Erhai Lake. Archaeal 16S rRNA and mcrA genes displayed a similar abundance change pattern in both lakes, and the relative abundance of methanogens decreased with increasing sediment-layer depth. Archaeal communities differed considerably in Dianchi Lake and Erhai Lake, but methanogen communities showed a slight difference between these two lakes. However, methanogen communities illustrated a remarkable layer-depth-related change. Order Methanomicrobiales was the dominant methanogen group in all sediments, while Methanobacteriales showed a high proportion only in upper layer sediments. The trophic status of the lake might have a notable influence on the depth-related change pattern of methanogenesis activity, while the methanogen community structure was mainly influenced by sediment depth.

Following heavy metal pollution caused by thallium in watersheds, people typically employ emergency treatment methods such as water sampling and transfer for dilution or in situ coagulation and precipitation. However, the thallium that is adsorbed by the precipitates in the sediment persists for a long time and is gradually released, posing a significant threat to the ecosystem. In this study, the 16S rRNA sequencing method was used to simulate the effects of water dilution or in situ coagulation and precipitation on microbial communities through thallium impact loading and thallium-containing iron floc shaking bottle experiments. The emendation of Fe(III) floc led to an increase in the relative abundance of Actinobacteriota. Meanwhile, Nitrospira and Proteobacteria exhibited distinct tolerances to Tl shock and Tl floc stress, respectively. Thallium pollution inhibited the reduction in nitric oxide and nitrogen fixation while increasing the relative abundance of the napA/B genes and decreasing the relative abundance of narG/H genes involved in nitrate reduction. This study offers new insights into the effects of various emergency treatment measures on river ecosystems following sudden thallium pollution, particularly from the perspective of microbial community composition and biogeochemical cycles.