Abstract
BackgroundThe currently accepted thesis on nitrogenous fertilizer additions on methane oxidation activity assumes niche partitioning among methanotrophic species, with activity responses to changes in nitrogen content being dependent on the in situ methanotrophic community structure Unfortunately, widely applied tools for microbial community assessment only have a limited phylogenetic resolution mostly restricted to genus level diversity, and not to species level as often mistakenly assumed. As a consequence, intragenus or intraspecies metabolic versatility in nitrogen metabolism was never evaluated nor considered among methanotrophic bacteria as a source of differential responses of methane oxidation to nitrogen amendments.ResultsWe demonstrated that fourteen genotypically different Methylomonas strains, thus distinct below the level at which most techniques assign operational taxonomic units (OTU), show a versatile physiology in their nitrogen metabolism. Differential responses, even among strains with identical 16S rRNA or pmoA gene sequences, were observed for production of nitrite and nitrous oxide from nitrate or ammonium, nitrogen fixation and tolerance to high levels of ammonium, nitrate, and hydroxylamine. Overall, reduction of nitrate to nitrite, nitrogen fixation, higher tolerance to ammonium than nitrate and tolerance and assimilation of nitrite were general features.ConclusionsDifferential responses among closely related methanotrophic strains to overcome inhibition and toxicity from high nitrogen loads and assimilation of various nitrogen sources yield competitive fitness advantages to individual methane-oxidizing bacteria. Our observations proved that community structure at the deepest phylogenetic resolution potentially influences in situ functioning.
Highlights
The currently accepted thesis on nitrogenous fertilizer additions on methane oxidation activity assumes niche partitioning among methanotrophic species, with activity responses to changes in nitrogen content being dependent on the in situ methanotrophic community structure widely applied tools for microbial community assessment only have a limited phylogenetic resolution mostly restricted to genus level diversity, and not to species level as often mistakenly assumed
We explored the differential response of various aspects of the nitrogen metabolism within fourteen genotypically distinct members of the genus Methylomonas (Type Ia), including the species M. methanica and M. koyamae
No growth or activity was observed for the strains cultivated in parallel in nitrogen-free medium under high oxygen tension, confirming the absence of nitrogenous compounds in the growth medium, which was demonstrated by colorimetric analysis of nitrate, nitrite and ammonium
Summary
The currently accepted thesis on nitrogenous fertilizer additions on methane oxidation activity assumes niche partitioning among methanotrophic species, with activity responses to changes in nitrogen content being dependent on the in situ methanotrophic community structure widely applied tools for microbial community assessment only have a limited phylogenetic resolution mostly restricted to genus level diversity, and not to species level as often mistakenly assumed. Aerobic methanotrophic bacteria constitute the main microbial methane sink through their ability to derive energy from its oxidation to carbon dioxide using the key enzyme methane monooxygenase [2] Cultured representatives of this functional guild are phylogenetically positioned within the Gammaproteobacteria (Type I methanotrophs, forming two clades, i.e. Type Ia and Ib), the Alphaproteobacteria (Type II methanotrophs) [3,4], and the Verrucomicrobia [5]. The currently accepted thesis assumes niche partitioning among methanotrophic species, with methane oxidation activity responses to changes in nitrogen content being dependent on the in situ methanotrophic community structure [19,21]
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