BACKGROUND Microbial consortia resistant to high ammonia concentrations may facilitate biogas production from high ammonia-containing wastes and manures during anaerobic digestion. Microbial communities were gradually enriched via sequential batch cultivations (stepwise exposure) at increasing ammonia concentrations up to the extremely high concentration of 9 g total ammonia nitrogen (TAN) L-1. This study examined the adaptation of anaerobic microbial consortia to high ammonia concentrations by the use of a complex substrate based on manure in order to generate tailor-made inocula for bioaugmentation purposes. RNA analysis was performed to determine the microbial community composition and activity of anaerobic bacteria and methanogens. RESULTS The enrichment series was successful in terms of methane production at ammonia concentrations as high as 5 g TAN L-1 in comparison with the control concentration of 2 g TAN L-1. Methanosarcina was the core active dominant methanogen genus in all enrichment cultures and its relative activity was sharply increased at 3 g and 9 g TAN L-1. Cellulolytic bacteria activity mostly decreased with ammonia increase. Moreover, syntrophic butyrate and long-chain fatty acid degraders along with hydrogenotrophic methanogens were activated at increased ammonia concentrations. CONCLUSIONS The results of the study demonstrated an efficient method to enrich ammonia-tolerant microbial consortia for bioaugmentation purposes in biogas reactors with concentrations <= 5 g TAN L-1. RNA analysis revealed high relative abundances of Methanosarcina and hydrogenotrophic methanogens at high concentrations of ammonia. (c) 2020 Society of Chemical Industry