The Influence of Above-Ground Herbivory on the Response of Arctic Soil Methanotrophs to Increasing CH4 Concentrations and Temperatures

Abstract

Rising temperatures in the Arctic affect soil microorganisms, herbivores, and peatland vegetation, thus directly and indirectly influencing microbial CH4 production. It is not currently known how methanotrophs in Arctic peat respond to combined changes in temperature, CH4 concentration, and vegetation. We studied methanotroph responses to temperature and CH4 concentration in peat exposed to herbivory and protected by exclosures. The methanotroph activity was assessed by CH4 oxidation rate measurements using peat soil microcosms and a pure culture of Methylobacter tundripaludum SV96, qPCR, and sequencing of pmoA transcripts. Elevated CH4 concentrations led to higher CH4 oxidation rates both in grazed and exclosed peat soils, but the strongest response was observed in grazed peat soils. Furthermore, the relative transcriptional activities of different methanotroph community members were affected by the CH4 concentrations. While transcriptional responses to low CH4 concentrations were more prevalent in grazed peat soils, responses to high CH4 concentrations were more prevalent in exclosed peat soils. We observed no significant methanotroph responses to increasing temperatures. We conclude that methanotroph communities in these peat soils respond to changes in the CH4 concentration depending on their previous exposure to grazing. This “conditioning” influences which strains will thrive and, therefore, determines the function of the methanotroph community.

Thermal acclimation of methanotrophs from the genusMethylobacter

“…This further indicates that the competitiveness of a strain does not necessarily correlate with its CH 4 consumption. Thus, while CH 4 production rates are often expected to increase with increasing temperatures [ 53 ], our data demonstrate that CH 4 oxidation rates might not always correlate, for physiological reasons, possibly explaining some of the divergent observations of temperature effects on CH 4 oxidation in environmental studies (e.g., [ 23 , 25 ]).…”

“…Soils experience a mix of stable temperatures and large temperature fluctuations driven by diurnal cycles, weather changes and season, and depending on soil type, depth, latitude, and altitude [17][18][19][20]. Highly variable temperature effects on soil CH 4 oxidation rates have been observed, ranging from, for example, strong temperature responses in landfills [21] and marine sediments [22] to variable responses in permafrost soils [23] and minor effects under atmospheric CH 4 concentrations in forest soils [24] or high CH 4 concentrations in peat [25]. Temperature responses in methanotrophs have been suggested to depend on the soil type and CH 4 concentration [26].…”

Herbivores in Arctic ecosystems: Effects of climate change and implications for carbon and nutrient cycling

“… 32 , 33 Recent work also showed that the structure and function of the methanotroph community within peat soils depended on previous exposure to goose grazing, which could have implications for C‐cycling. 92 In addition, invertebrate herbivores may have an underappreciated role in influencing C dynamics at the individual plant level. Moth outbreaks in Greenland were associated with lower production and reduced structural support of the common shrub Salix glauca due to changes in the lignin and carbohydrate contents of the plant fibers.…”

“…Reduced herbivory associated with the exclusion of small mammals similarly caused higher plant biomass and led to increased litter accumulation and slower decomposition 32,33 . Recent work also showed that the structure and function of the methanotroph community within peat soils depended on previous exposure to goose grazing, which could have implications for C‐cycling 92 . In addition, invertebrate herbivores may have an underappreciated role in influencing C dynamics at the individual plant level.…”