Resum

Most high mountain lakes across the planet are covered by ice and snow for several months a year. A phytocentric view of the lake ecosystem could consider the ice-free period as the most relevant for the microbial lake communities. However, heterotrophic and chemoautotrophic prokaryotes could experience more favourable conditions under-ice. We used 16S rRNA gene sequencing to monthly monitor the bacterioplankton community of a deep high mountain lake in the Pyrenees. The results indicate richer bacterioplankton assemblages under the ice than during the ice-off period. The winter assemblages were resilient to the warm period persisting in the hypolimnion. On the other hand, a bacterial background assemblage, which was resistant to all depths and season conditions, was identified, mainly consisting of Actinobacteria ACK-M1. The summer epilimnetic waters, submitted to damaging high UV radiation, were the poorest in OTUs and showed a characteristic opportunistic assemblage. The bacterioplankton paradigm for long seasonally ice-covered lakes should consider the primary role of under-ice highly diverse resistant and resilient assemblages. Much reduced ice-cover periods due to global warming might drastically affect the bacterial communities of these lakes.