This study investigated granular characteristics and microbial profiles in granules from the upflow anaerobic sludge blanket (UASB) reactor treating palm oil mill effluent at a thermophilic temperature of 60o C in comparison with a mesophilic UASB. Phylogenetic analysis of microbial communities in sludge granules fromboth reactors based on bacterial 16S rRNA and archaeal mcrA genes revealed different bacterial and archaeal communities under the different operating conditions. The bacterial community under the mesophilic condition was composed mainly of Proteobacteria, Firmicutes and Chloroflexi while a less complex community comprising mainly Proteobacteria and Actinobacteria was present under the thermophilic condition. The mesophilic methanogen assemblage was found to be comprised mainly of those related to uncultured methanogens in class Methanomicrobiales and Methanobacteriales. The thermophilic methanogen community was dominated by those related to class Methanosarcinales in addition to some Methanobacteriales. Most were closely related to Methanosaeta thermophilia. The microbial community structures suggested that different methanogenesis pathways dominated under the mesophilic and thermophilic conditions and implied different capabilities of extracellular polymer production which affected reactor performance.
