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 <i>pmoA</i>. 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 <i>pmoA</i> sequences formed only 5 operational taxonomic units (OTUs) at the species level and, with one exception, could be assigned to either <i>Methylocystis</i> or the as-yet-uncultivated Upland Soil Cluster &gamma; (USC&gamma;). The latter dominated T-RFLP patterns of all siliceous and most calcareous samples, while <i>Methylocystis</i> dominated in 4 calcareous samples. Members of Upland Soil Cluster &alpha; (USC&alpha;) were not detected. Apparently, USC&gamma; adapted best to the oligotrophic cold climate conditions at the investigated pioneer sites.