Bacteria primarily metabolize at the active layer/permafrost border in the peat core from a permafrost region in western Siberia

The microbial activity in the soils of the permafrost-affected zones is assumed to be one of the major factors that modify the organic carbon and nitrogen cycle under current climate change. In contrast to the extensive research centered on bacterial abundance, diversity, and metabolic activity in permanently and seasonally frozen mineral soils from high latitudes, frozen peat (organic) environments remain poorly characterized in terms of the physiological diversity and metabolic potential of bacteria. The evolution of soil heterotroph microbial number and metabolic activity across the “seasonally thawed (active)—permanently frozen layer” boundary was studied on 100-cm-thick cores from frozen peat mounds located in the discontinuous permafrost zone in western Siberia. There was a systematic decrease of metabolic activity in the upper 40 cm of the peat core from the surface layers of the mosses and lichens towards the beginning of the frozen horizon, followed by an abrupt increase in bacterial metabolism exactly at the border between the thawed layer and the permafrost table. The aerobic viable cell count and total bacterial number from the active layer were similar to those from the permafrost peat layer. The highest metabolic activity was observed at the beginning of the frozen peat layer and might correspond to the highest availability of amino substrates, which were depleted in the active layer but preserved in the deeper frozen horizons. The enhanced microbial activity at the frozen peat-active layer boundary in western Siberia may persist for another 50–100 years based on the current rate of increase in active layer thickness