Distribution patterns and environmental correlates of Thaumarchaeota abundance in six deep subalpine lakes

The presence of the mesophilic Thaumarchaeota (Thaum) in deep lakes seems to be restricted to oligotrophic lakes, where they have been found in the deep hypolimnion. We evaluated the vertical distribution of Thaum in six subalpine deep lakes, in spring and summer, to ascertain if there was a Thaum vertical gradient common to all the lakes in the region, possibly related to lake mixing and stratification. We examined nine environmental variables potentially correlated with Thaum abundance. Thaum abundance ranged from 0.61 9 103 to 236 9 103 cells mL-1, representing between 0.02 and 19 % of total prokaryotic cells, in the six lakes across different seasons. Although their absolute and relative abundance varied along the trophic gradient and with mixing conditions, a general pattern common to all the deep lakes was observed. In summer, the abundance of Thaum significantly increased with depth in all the lakes. Temperature emerged as the main environmental correlate of Thaum abundance in deep subalpine lakes. In spring, at low water temperature their abundance was significantly higher in warmer lakes and lower in colder lakes. Conversely, in summer, the correlation with temperature was inverse and Thaum thrived preferentially in the colder hypolimnion.