Anne Saari

Hydrology is reflected in the functioning and community composition of methanotrophs in the littoral wetland of a boreal lake

In lake ecosystems a major proportion of methane (CH(4) ) emissions originate from the littoral zone, which can have a great spatial variability in hydrology, soil quality and vegetation. Hitherto, spatial heterogeneity and the effects it has on functioning and diversity of methanotrophs in littoral wetlands have been poorly understood. A diagnostic microarray based on the particulate methane monooxygenase gene coupled with geostatistics was used to analyse spatial patterns of methanotrophs in the littoral wetland of a eutrophic boreal lake (Lake Kevätön, Eastern Finland). The wetland had a hydrology gradient with a mean water table varying from -8 to -25 cm. The wettest area, comprising the highest CH(4) oxidation, had the highest abundance and species richness of methanotrophs. A high water table favoured the occurrence of type Ib methanotrophs, whereas types Ia and II were found under all moisture conditions. Thus the spatial heterogeneity in functioning and diversity of methanotrophs in littoral wetlands is highly dependent on the water table, which in turn varies spatially in relation to the geomorphology of the wetland. We suggest that changes in water levels resulting from regulation of lakes and/or global change will affect the abundance, activity and diversity of methanotrophs, and consequently CH(4) emissions from such systems.

Seasonal variation in the function and diversity of methanotrophs in the littoral wetland of a boreal eutrophic lake

Littoral wetlands are responsible for most of the total methane (CH(4) ) emissions from lake ecosystems. We show that seasonally variable hydrological and temperature conditions in the littoral wetland of a eutrophic boreal lake affect the community composition and gene transcription of methanotrophs measured by a particulate methane monooxygenase (pmoA) gene-targeted microarray. Type Ib freshwater-cluster methanotrophs were favoured by the high water level, and CH(4) oxidation was positively correlated with their pmoA gene transcripts. In the dry subsite of the wetland, the more stagnant hydrological conditions in summer and autumn induced the dominance of type II methanotrophs over type I methanotrophs (community composition and pmoA gene transcripts). The relative abundance of type II methanotrophs increased in winter. The results provide new insight into the variation of methanotroph communities across seasons in littoral wetlands.

Effects of Nitrogen Load on the Function and Diversity of Methanotrophs in the Littoral Wetland of a Boreal Lake

Methane is the second most abundant greenhouse gas in the atmosphere. A major part of the total methane emissions from lake ecosystems is emitted from littoral wetlands. Methane emissions are significantly reduced by methanotrophs, as they use methane as their sole energy and carbon source. Methanotrophic activity can be either activated or inhibited by nitrogen. However, the effects of nitrogen on methanotrophs in littoral wetlands are unknown. Here we report how nitrogen loading in situ affected the function and diversity of methanotrophs in a boreal littoral wetland. Methanotrophic community composition and functional diversity were analyzed with a particulate methane monooxygenase (pmoA) gene targeted microarray. Nitrogen load had no effects on methane oxidation potential and methane fluxes. Nitrogen load activated pmoA gene transcription of type I (Methylobacter, Methylomonas, and LW21-freshwater phylotypes) methanotrophs, but decreased the relative abundance of type II (Methylocystis, Methylosinus trichosporium, and Methylosinus phylotypes) methanotrophs. Hence, the overall activity of a methanotroph community in littoral wetlands is not affected by nitrogen leached from the catchment area.