## 1 Introduction  
Environmental problems associated with the increase in waste volumes and the depletion of natural resources, causing, as a result, the need for an active search for alternative energy sources, sparked a scientific interest in the process of the microbial anaerobic digestion (AD) of organic waste. AD technologies speed up the waste stabilization by hundreds of times, decrease emission of greenhouse gases from landfills and produce renewable energy in the form of biogas. Efficient biogas production depends on balanced microbial activity, which necessarily involves hydrolytic, acidogenic and syntrophic (acetogenic)bacteria, as well as methanogenic archaea. Complex trophic interactions exist between different microbial groups in methanogenic communities [1; 2]. Volatile fatty acids (VFAs) are the key intermediates of organic waste transformation into biogas. The degradation of VFAs (along with hydrolysis) determines the overall rate of AD process [3]. In the absence of inorganic electron acceptors, VFAs can be degraded only when an active and balanced consortium of syntrophic bacteria and methanogenic archaea develop in the reactor. Syntrophic bacteria oxidize VFAs anaerobically only when the concentration of the products (especially H\({}_{2}\) and formate) is kept low. This leads to the dependence of syntrophic bacteria on their hydrogen/ formate-utilizing methanogenic partners [1]. VFAs accumulate rapidly when syntrophic bacteria or methanogenic archaea are suppressed or in the case of the substrate overloading. In the latter case, hydrolytic and acidogenic bacteria develop faster than synthrophs and methanogens, which are unable to oxidize VFAs at the same rate as they are produced. VFA accumulation results in a gradual decrease in pH level, which negatively affects methanogenesis, and the AD process becomes unstable. An increased concentration of VFAs is therefore an important indicator of instability of the AD process [4; 5]. VFA (mainly acetate, propionate and butyrate) accumulation often happens during AD of a readily biodegradable feedstock, such as food waste [6], sewage sludge [7] or agroindustrial waste [8]. For example, the acidification and termination of methane production were observed during the thermophilic AD of food waste when the concentrations of VFAs reached 10-14 g/L, corresponding to 4-14 mM propionate, 54-70 mM butyrate and 75-120 mM acetate [9]. The investigation of adaptation mechanisms in microbial communities exposed to increased VFA concentration and searching for syntrophic bacteria and methanogenic archaea, which are resistant to high concentrations of acetate, propionate and butyrate, is of special importance for the practical application of these consortia in the AD process and for the stabilization of biogas production. Acetate is a pivotal intermediate in the anaerobic decomposition of organic matter, and it contributes to a major part of the produced methane [10]. It has been reported that syntrophic acetate oxidation (SAO) has become the dominant pathway in overcoming acetate accumulation from initially acetoclastic methanogenesis-dominant anaerobic systems under high ammonia stress [11; 12]. Additionally, the SAO pathway also played a significant role in the anaerobic digester suppressed through high concentrations of VFAs [13; 14] and high temperatures [15] due to the inhibition of acetoclastic methanogens. Propionate has the most pronounced and the most difficult-to-overcome inhibitory effect on the microorganisms in the methanogenic community. A number of works have therefore focused on syntrophic propionate degradation [16; 17]. In the work of [PERSON] et al. [18], propionate degrading consortia were enriched in sequential mode, from lower (1 g/L or 13.7 mM of propionate) to higher strength phase with 3 g/L (or 41.1 mM) propionate. Four syntrophic microbial groups, _Syntrophlocaeae_, _Syntrophomonadaceae_, _Methanobacterium_ and _Methanosaeta_ were dominant in the lower strength phase.