Shifts in the community composition of methane-cycling microorganisms during lake shrinkage

Intensification of anthropogenic activities has resulted in shrinkage of lakes, which are important sources and sinks of the greenhouse gas methane. Given the increasing interest in the potential importance of CH4 flux, we employed Illumina Miseq pyrosequencing to examine the effect of lake shrinkage (in a lake that had decreased from 3.7 km2 to 3.0 km2 in area during the last 20 years) on the community abundance, activity and structure of methane-cycling microorganisms under four types of soils during lake recession: Sediment (the sediment of the lake, at the maximum depth of 6 m), Slope (soil at a lake water depth of 0.6 m), Marsh (the seasonally flooded marsh and wetland formed after recession of the lake) and Meadow (the meadow formed after recession of the lake, about 200 m from the lake). The results showed that lake shrinkage decreased the relative abundance and transcripts of soil methanogens and methanotrophs along the gradient from Sediment to Slope to Marsh. Meadow, having the longest drying time, had both higher soil nutrient concentrations (including dissolved organic carbon, total carbon and total nitrogen), and higher relative abundance of 16S rRNA genes and transcripts of methanogens and methanotrophs. Results of methanogen community structure analysis indicated 'resistance' to change under lake shrinkage, with all communities dominated by Methanosaetaceae except the Slope community, which was dominated by Methanoregulaceae, while the dominant methanotroph changed from Methylomicrobium (a Type I methanotroph) (in Sediment, Slope and Marsh) to Methylocaldum (Type I) and Methylosinus (Type II) (in Meadow) under lake shrinkage. Taken together, the microbial community data indicate that CH4 production and consumption potentially recedes under lake shrinkage, whereas the formation of Meadow may increase CH4 emission and oxidization potential.