Response and resilience of the microbial methane filter to ecosystem changes in Arctic peatlands

Abstract

<p>Climate change is a major concern in the Arctic region, as large amounts of organic carbon (C) are stored in permafrost soils and sediments. Increasing average temperatures have the potential to release that C and making it available to biologic activity. Carbon-rich, anoxic soils such as peatlands are inhabited by methanogenic archaea that can metabolize by-products of microbial C decomposition and consequently release methane (CH₄). Methane oxidizing bacteria (MOB) comprise a major biological filter for CH₄ in terrestrial and aquatic ecosystems and thereby regulate CH₄ emissions to the atmosphere. The genus <i>Methylobacter</i> has been detected in many CH₄ rich ecosystems and several circumpolar locations. Climate change in the Arctic includes changes in both temperature and precipitation as well as ecosystem changes related to plant cover and herbivory. All of these changes have the potential to influence soil structure and soil biological processes. Thus, they are important factors controlling the soil C cycle and eventually the activity of MOB. The aim of this thesis was to investigate the ability of the Arctic biological CH₄ filter to adapt to changes in vegetation, CH₄ concentrations and temperature. Further, to gain insights in the resilience and resistance of the MOB community to environmental changes. <p>We have shown that herbivory by geese changes the soil structure and thus the vertical distribution of CH₄ and oxygen (O₂) concentrations in a high Arctic peatland. These differences are accompanied by changes in the potential rates of CH₄ oxidation. The highest activity was detected in shallower parts of the peatland in grazed sites compared to sites protected from grazing. Different MOB communities are responsible for the CH₄ oxidation, depending on the above ground grazing and these communities are composed of closely related <i>Methylobacter</i> OTUs. Exposing peat soils from both grazed and protected sites to increased CH₄ concentrations and temperature revealed that MOB respond strongly to changing CH₄ concentrations, but apparently not to temperature. The response to changing CH₄ concentrations involved different members of the <i>Methylobacter</i> community depending on the CH₄ concentration and their previous exposure to herbivory. Temperature had minor effects on the CH₄ oxidation activity and the MOB community <i>pmoA</i> transcription in the soils. However, temperature clearly influences growth, CH₄ oxidation and CO₂ production in the native peat soil isolate <i>Methylobacter tundripaludum</i> SV96. The temperature adaptation of <i>M. tundripaludum</i> SV96 is modulated by fine tuning transcription and translation to achieve the optimal balance between substrate availability, growth, and energy generation at different temperatures. These findings show that identical CH₄ oxidation rates can be produced from very different physiological states. This also explains how the apparent lack of temperature responses in soil MOB communities is a result of physiological acclimation. <p>Our results show that MOB in high Arctic peatlands and particular the genus <i>Methylobacter</i> is both physiologically and ecologically flexible in adapting to changes in plant cover, O₂ distribution, CH₄ concentrations and temperature. Thus, the MOB community comprises an efficient and resilient CH₄ filter in high Arctic peat soils.