Methanosarcina spp., the key to relieve the start-up of a thermophilic anaerobic digestion suffering from high acetic acid loads

•Inocula were developed to improve the start-up under high acetate loads.•Gradually adapted inocula showed increased numbers of total methanogens.•Similar courses of the DNA content and CH4 production rates were determined.•Methanosarcina spp. was an essential driving factor for acetate degradation.•Representatives of Clostridium spp. were the predominant bacteria. This paper investigates if it is possible to produce inocula to counteract high acetic acid (CH3COO−) concentrations during thermophilic anaerobic digestion. To this end, fermenter sludge was exposed for different durations to either gradually increasing CH3COO− concentrations or directly exposed to a high concentration (150mM). Altogether, these enrichments led to inocula with a distinct decrease of representatives of Methanobacteriales, while those of Methanoculleus spp. were hardly affected by any treatment. After the inoculation, good agreements of the abundance of Methanosarcinales and Methanoculleus spp. with total DNA content and methane production rate were apparent. In addition, a gradual adaptation of the inoculum for at least 4weeks led to a significant increase of Methanosarcina spp. during the subsequent fermentation. These results demonstrate the potential of bioaugmentation to relieve the start-up of an anaerobic digestion suffering from high CH3COO− loads, especially pointing to the robust acetoclastic genus Methanosarcina.