Abstract
Methane emissions from peat bogs are mitigated by methanotrophs, which live in symbiosis with peat moss (e.g. Sphagnum). Here, we investigate the influence of temperature and resultant changes in methane fluxes on Sphagnum and methanotroph‐related biomarkers, evaluating their potential as proxies in ancient bogs. A pulse‐chase experiment using 13C‐labelled methane in the field clearly showed label uptake in diploptene, a biomarker for methanotrophs, demonstrating in situ methanotrophic activity in Sphagnum under natural conditions. Peat cores containing live Sphagnum were incubated at 5, 10, 15, 20 and 25°C for two months, causing differences in net methane fluxes. The natural δ13C values of diploptene extracted from Sphagnum showed a strong correlation with temperature and methane production. The δ13C values ranged from −34‰ at 5°C to −41‰ at 25°C. These results are best explained by enhanced expression of the methanotrophic enzymatic isotope effect at higher methane concentrations. Hence, δ13C values of diploptene, or its diagenetic products, potentially provide a useful tool to assess methanotrophic activity in past environments. Increased methane fluxes towards Sphagnum did not affect δ13C values of bulk Sphagnum and its specific marker, the C23 n‐alkane. The concentration of methanotroph‐specific bacteriohopanepolyols (BHPs), aminobacteriohopanetetrol (aminotetrol, characteristic for type II and to a lesser extent type I methanotrophs) and aminobacteriohopanepentol (aminopentol, a marker for type I methanotrophs) showed a non‐linear response to increased methane fluxes, with relatively high abundances at 25°C compared to those at 20°C or below. Aminotetrol was more abundant than aminopentol, in contrast to similar abundances of aminotetrol and aminopentol in fresh Sphagnum. This probably indicates that type II methanotrophs became prevalent under the experimental conditions relative to type I methanotrophs. Even though BHP concentrations may not directly reflect bacterial activity, they may provide insight into the presence of different types of methanotrophs.
Highlights
Peat bogs play an important role in the global carbon cycle since they sequester one-third of the Earth's terrestrial carbon (Smith et al, 2004)
The lack of a clear temperature response of the stable carbon isotope values in both bulk Sphagnum and Sphagnum-derived biomarkers implies that reconstruction of methane oxidation and methanecycling in peat bogs will have to rely on biomarkers specific for methanotrophs, such as δ13C values of hopenes and specific BHPs
Mesocosm experiments with Sphagnum peat cores incubated at different temperatures resulted in large variations in the δ13C values of diploptene extracted from Sphagnum, with values of −34‰ at 5°C and −41‰ at 25°C
Summary
Peat bogs play an important role in the global carbon cycle since they sequester one-third of the Earth's terrestrial carbon (Smith et al, 2004). We investigate the influence of temperature and the resultant changes in methane flux rates on Sphagnum and methanotroph-related hopanoids, evaluating their potential as proxies for methanotrophs and methane fluxes in ancient bogs To this end, Sphagnum mosses, incubated at different temperatures in a previously described mesocosm experiment (van Winden, Reichart, et al, 2012), were analysed for their compound-specific carbon isotopic composition and BHP composition. Sphagnum mosses, incubated at different temperatures in a previously described mesocosm experiment (van Winden, Reichart, et al, 2012), were analysed for their compound-specific carbon isotopic composition and BHP composition These parameters were subsequently compared to methane flux rates, to evaluate their potential as proxies for methane fluxes
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