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

The oxidation of methane by methane-oxidising microorganisms is an important link in the global methane budget. Oxic soils are a net sink while wetland soils are a net source of atmospheric methane. It has generally been accepted that the consumption of methane in upland as well as lowland systems is inhibited by nitrogenous fertiliser additions. Hence, mineral nitrogen (i.e. ammonium/nitrate) has conceptually been treated as a component with the potential to enhance emission of methane from soils and sediments to the atmosphere, and results from numerous studies have been interpreted as such. Recently, ammonium-based fertilisation was demonstrated to stimulate methane consumption in rice paddies. Growth and activity of methane-consuming bacteria in microcosms as well as in natural rice paddies was N limited. Analysing the available literature revealed that indications for N limitation of methane consumption have been reported in a variety of lowland soils, upland soils, and sediments. Obviously, depriving methane-oxidising bacteria of a suitable source of N hampers their growth and activity. However, an almost instantaneous link between the presence of mineral nitrogen (i.e. ammonium, nitrate) and methane-oxidising activity, as found in rice soils and culture experiments, requires an alternative explanation. We propose that switching from mineral N assimilation to the fixation of molecular nitrogen may explain this phenomenon. However, there is as yet no experimental evidence for any mechanism of instantaneous stimulation, since most studies have assumed that nitrogenous fertiliser is inhibitory of methane oxidation in soils and have focused only on this aspect. Nitrogen as essential factor on the sink side of the global methane budget has been neglected, leading to erroneous interpretation of methane emission dynamics, especially from wetland environments. The purpose of this minireview is to summarise and balance the data on the regulatory role of nitrogen in the consumption of methane by soils and sediments, and thereby stimulate the scientific community to embark on experiments to close the existing gap in knowledge.

Methanotrophs, cells that consume methane (CH(4)) as their sole source of carbon and energy, play key roles in the global carbon cycle, including controlling anthropogenic and natural emissions of CH(4), the second-most important greenhouse gas after carbon dioxide. These cells have also been widely used for bioremediation of chlorinated solvents, and help sustain diverse microbial communities as well as higher organisms through the conversion of CH(4) to complex organic compounds (e.g. in deep ocean and subterranean environments with substantial CH(4) fluxes). It has been well-known for over 30 years that copper (Cu) plays a key role in the physiology and activity of methanotrophs, but it is only recently that we have begun to understand how these cells collect Cu, the role Cu plays in CH(4) oxidation by the particulate CH(4) monooxygenase, the effect of Cu on the proteome, and how Cu affects the ability of methanotrophs to oxidize different substrates. Here we summarize the current state of knowledge of the phylogeny, environmental distribution, and potential applications of methanotrophs for regional and global issues, as well as the role of Cu in regulating gene expression and proteome in these cells, its effects on enzymatic and whole-cell activity, and the novel Cu uptake system used by methanotrophs.