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 susceptible to washout, with relative abundances declining from 60% to 47%, and recovering to 52% after the shock.