Methane-cycling microbial communities are spatially structured, seasonally dynamic, and functionally coupled in sediments of two nearby eutrophic hydroelectric reservoirs

Hydroelectric reservoirs, which are expanding worldwide, are freshwater bodies that, despite being considered environmentally friendly, are important sources of methane emissions. Methane emissions vary with the availability of organic matter, especially with season in subtropical reservoirs, as well as with hydrology and reservoir age, watershed properties, and the interactions among methane-producing and consuming microbial populations. This study examined the activity, spatial distribution, and seasonal dynamics of methanogenic and methanotrophic communities in the sediments of two nearby eutrophic hydroelectric reservoirs in South America, whose physicochemical characterization and methane emissions were previously reported. To elucidate the structure of both populations, high-throughput amplicon sequencing was used, while potential activity was quantified by determining the maximum methane production or consumption rates in substrate-amended sediment incubations. The potential methanogenic activity varied depending on the sampling site and substrate but was always surpassed by the potential aerobic methanotrophic activity. The abundance of methanogens and methanotrophs, as well as their activity, were positively correlated, indicating that both processes were coupled year-round. Although Methanoregula and Methanosaeta , among methanogens, and Methylocystis and Methylomicrobium, among methanotrophs, prevailed in all three sampled sites of each reservoir, the structure of the entire methanogenic and methanotrophic communities significantly differentiated the reservoirs. Methanotrophs accounted for 2.5 to 7.5% of the total Bacteria in the Rincón del Bonete and Palmar reservoirs. Methanogens were more dynamic and diverse, comprising between 5 and 65% of the archaeal sequences, and decreased towards spring as the summer algal bloom accumulated in the sediments decayed. Contrastingly, the distribution of anaerobic methanotrophs, Ca. Methylomirabilis and ANME (anaerobic methanotrophic archaea), was site-dependent rather than reservoir-dependent. These findings indicate that the activity of methanogenic and aerobic methanotrophic communities was influenced by local site-specific factors, whereas their structure seems to be shaped by features shared among all sites within the same reservoir.