Large-scale biogeography and environmental regulation of methanotrophic bacteria across boreal inland waters.

Aerobic methanotrophic bacteria (methanotrophs) use methane as a source of carbon and energy, thereby mitigating net methane emissions from natural sources. Methanotrophs represent a widespread and phylogenetically complex guild, yet the biogeography of this functional group and the factors that explain the taxonomic structure of the methanotrophic assemblage are still poorly understood. Here, we used high-throughput sequencing of the 16S rRNA gene of the bacterial community to study the methanotrophic community composition and the environmental factors that influence their distribution and relative abundance in a wide range of freshwater habitats, including lakes, streams and rivers across the boreal landscape. Within one region, soil and soil water samples were additionally taken from the surrounding watersheds in order to cover the full terrestrial-aquatic continuum. The composition of methanotrophic communities across the boreal landscape showed only a modest degree of regional differentiation but a strong structuring along the hydrologic continuum from soil to lake communities, regardless of regions. This pattern along the hydrologic continuum was mostly explained by a clear niche differentiation between type I and type II methanotrophs along environmental gradients in pH, and methane concentrations. Our results suggest very different roles of type I and type II methanotrophs within inland waters, the latter likely having a terrestrial source and reflecting passive transport and dilution along the aquatic networks, but this is an unresolved issue that requires further investigation.