In natural settings, anaerobic digestion can take place in a wide temperature range, but industrial digesters are usually operated under either mesophilic (similar to 35 degrees C) or thermophilic (similar to 55 degrees C) conditions. The ability of anaerobic digestion microbiota to switch from one operating temperature to the other remains poorly documented. We therefore studied the effect of sudden temperature changes (35 degrees C/55 degrees C) in labscale bioreactors degrading C-13-labelled cellulose. An asymmetric behaviour was observed. In terms of methane production, after an adaptation period, mesophilic inoculum exhibited a functional resistance to temperature increase but no functional resilience when temperature was reset to 35 degrees C, while thermophilic inoculum methanogenic activity strongly decreased under mesophilic conditions but partially recovered when temperature was reset to 55 degrees C. Automated ribosomal intergenic spacer analysis community fingerprints evidenced a strong influence of temperature on microbial diversity, particularly pronounced and persistent for Archaea. Key phylotypes involved in C-13-cellulose degradation were identified with a coupled stable isotope probing (SIP)-16S rDNA pyrotag sequencing approach, suggesting that the hydrolytic and fermentative metabolic functions could be maintained thanks to functional redundancy between members of the class Clostridia, whereas methanogenic activity primarily relied on specialized groups affiliated either to genus Methanosarcina (mesophilicconditions),Methanothermobacter or Methanoculleus (thermophilic conditions) that were irreversibly modified by temperature increase.