Active methane oxidizing bacteria in a boreal peat bog ecosystem
Esson, Kaitlin Colleen
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Boreal peatlands are important ecosystems to the global carbon cycle. Although they cover only 3% of the earth's land surface area, boreal peatlands store roughly one third of the world's soil carbon. Peatlands also comprise a large natural source of methane emitted to the atmosphere. Some methane in peatlands is oxidized before escaping to the atmosphere by aerobic methane oxidizing bacteria. With changing climate conditions, the fate of the stored carbon and emitted methane from these systems is uncertain. One important step toward better understanding the effects of climate change on carbon cycling in peatlands is to ascertain the microorganisms actively involved in carbon cycling. To investigate the active aerobic methane oxidizing bacteria in a boreal peat bog, a combination of microcosm experiments, DNA-stable isotope probing, and next generation sequencing technologies were employed. Studies were conducted on samples from the S1 peat bog in the Marcell Experimental Forest (MEF). Potential rates of methane oxidation were determined to be in the range of 13.85 to 17.26 μmol CH₄ g dwt⁻¹ d⁻¹. After incubating with ¹³C-CH₄, DNA was extracted from these samples, separated into heavy and light fractions with cesium chloride gradient formation by ultracentrifugation and needle fractionation, and fractions were fingerprinted with automated ribosomal intergenic spacer analysis (ARISA) and further interrogated with qPCR. Based on ARISA, distinct banding patterns were observed in heavy fractions in comparison to the light fractions indicating an incorporation of ¹³C into the DNA of active methane oxidizers. This was further supported by a relative enrichment in the functional gene pmoA, which encodes a subunit of the particulate methane monooxygenase, in heavy fractions from samples incubated for fourteen days. Within heavy fractions for samples incubated for 8 and 14 days, the relative abundance of methanotrophs increased to 37% and 25%, respectively, from an in situ abundance of approximately 4%. Phylogenetic analysis revealed that the methanotrophic community was composed of both Alpha and Gammaproteobacterial methanotrophs of the genera Methylocystis, Methylomonas, and Methylovulum. Both Methylocystis and Methylomonas have been detected in peatlands before, however, none of the phylotypes in this study were closely related to any known cultivated members of these groups. These data are the first to implicate Methylovulum as an active methane oxidizer in peatlands, though this organism has been detected in another cold aquatic ecosystem with consistent methane emissions. The Methylovulum sequences from this study, like Methylocystis and Methylomonas, were not closely related to the only cultivated member of this genus. While Methylocystis was dominant in ¹³C-enriched fractions with a relative abundance of 30% of the microbial community after an eight-day incubation, Methylomonas became dominant with a relative abundance of approximately 16% after fourteen days of incubation. The relative abundance of Methylovulum was maintained at 2% in ¹³C- enriched fractions after eight and fourteen days.