Activity and diversity of methane-oxidizing bacteria in glacier forefields on siliceous and calcareous bedrock

Abstract

The global methane (CH₄ ) cycle is largely driven by methanogenic archaea and methane-oxidizing bacteria (MOB), but little is known about their activity and diversity in pioneer ecosystems. We conducted a field survey in forefields of 13 receding Swiss glaciers on both siliceous and calcareous bedrock to investigate and quantify CH₄ turnover based on soil-gas CH₄ concentration profiles, and to characterize the MOB community by sequencing and terminal restriction fragment length polymorphism (T-RFLP) analysis of pmoA. Methane turnover was fundamentally different in the two bedrock categories. Of the 36 CH₄ concentration profiles from siliceous locations, 11 showed atmospheric CH₄ consumption at concentrations of ~1-2 μL L^-1 with soil-atmosphere CH₄ fluxes of -0.14 to -1.1 mg m^-2 d^-1 . Another 11 profiles showed no apparent activity, while the remaining 14 exhibited slightly increased CH₄ concentrations of ~2-10 μL L^-1 , most likely due to microsite methanogenesis. In contrast, all profiles from calcareous sites suggested a substantial, yet unknown CH₄ source below our sampling zone, with soil-gas CH₄ concentrations reaching up to 1400 μL L^-1 . Remarkably, most soils oxidized ~90 % of the deep-soil CH₄ , resulting in soil-atmosphere fluxes of 0.12 to 31 mg m^-2 d^-1 . MOB showed limited diversity in both siliceous and calcareous forefields: all identified pmoA sequences formed only 5 operational taxonomic units (OTUs) at the species level and, with one exception, could be assigned to either Methylocystis or the as-yet-uncultivated Upland Soil Cluster γ (USCγ). The latter dominated T-RFLP patterns of all siliceous and most calcareous samples, while Methylocystis dominated in 4 calcareous samples. Members of Upland Soil Cluster α (USCα) were not detected. Apparently, USCγ adapted best to the oligotrophic cold climate conditions at the investigated pioneer sites.

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