The temperature shock also resulted in a severe loss of microorganisms from the Syntrophomonadaceae group in the intermittent reactors (Fig. 12b). The temperature shock generated conditions that led to a reduction of the relative abundance of the Syntrophomonadaceae group from an initial value of 20% to values undetected by the FISH methodology used. Remarkably, after the mesophile operation was resumed, the relative abundance of this microbial group reached detectable levels (9%). This reduction of the Syntrophomonadaceae group did not impact the biogas production of the intermittent reactor. These results suggest that methane production from substrates other than lipids/LCFA and/or via metabolic pathways other than β-oxidation were affected by the temperature shock at a more limited extent more limited than the microorganisms from the Syntrophomonadaceae group. In the continuous systems the Archaea microbial group was the most