Dynamics of methanogenic archaean populations in a FBR digester during the transition from mesophilic to thermophilic conditions applying control schemes

The purpose of this study was the molecular characterization of the methanogenic Archaea community in a fixed biofilm reactor (FBR) fed with tequila vinasses. This characterization was realized during the transition from mesophilic (37 °C) to thermophilic (55 °C) conditions applying cascade control schemes. Initially, the FBR was started-up and operated under mesophilic conditions applying a change from 5 to 6.6 g-COD L-1 d-1 in the organic loading rate to evaluate a control scheme where the reference variable was the alkalinity (alpha factor) and the controlled variable was the tequila vinasses (substrate) flow rates. Subsequently, the FBR was operated under thermophilic conditions increasing the temperature in a direct manner and after, in a stepwise manner by applying a control scheme which used the alkalinity (alpha factor) as a reference factor to regulate the temperature of the bioprocess, keeping the tequila vinasses flow rate constant. The Archaea community was characterized by mass sequencing and further phylogenetic analysis of the variable regions 4 and 5 of the 16S ribosomal RNA gene. The results on the characterization of the Archaea community indicated that the populations of methane-producing Archaea by the acetotrophic pathway (Methanosaeta harundinaceae, M. thermoacetophila, Methanosaeta sp., Methanosarcina thermophila and Methanometylovorans uponensis) showed a reduction in their relative abundances during the disturbances applied under mesophilic conditions. Whereas, the populations of methane-producing Archaea by the hydrogenotrophic pathway (Methanobacterium beijingense, M. subterraneum, Methanofollis liminatans, Methanoculleus marisnigri, Methanolinea tarda, and Methanomassiliicoccus sp.) increased their relative abundances, indicating a phenomenon of physiological adaptation to the environmental disturbances under mesophilic conditions, obtaining a hydrogenotrophic:acetotrophic Archaea ratio of 87:8. However, the population dynamics during the thermophilic stage indicated that the methanogenic acetotrophic archaea doubled their relative abundance reaching a hydrogenotrophic:acetotrophic Archaea ratio of 81:16, indicating a phenomenon of physiological adaptation to thermophilic conditions when the control scheme applied the increase of the temperature in a stepwise manner, since the direct increase to 55 °C destabilized the Archaea community and the alpha factor.