## 3 Results and Discussion
### Quantification of mcrA Gene  
Monitoring of AD parameters, especially the yield of CH4, makes it possible to assess the efficiency of anaerobic decomposition of organic matter. However, to achieve a complete picture of the activity of methanogens, which are the key group of microorganisms involved in this process, it is necessary to evaluate the quantities of CH4 yield and to analyze quantitative changes in methanogenic populations on a molecular level. Back in the last years of the 20 th century, it was suggested to use the _mcrA_ gene as a phylogenetic tool to analyze the activity of _Archaea_ methanogenic populations [48; 49]. This gene, encoding the methyl-coenzyme M reductase, is characteristic of the metabolism of methanogens [25], and was therefore considered to be a molecular marker for this group of microorganisms [50]. Analysis of the occurrence of the _mcrA_ gene became a potential tool to evaluate the presence of methanogens in samples originating from different environments [50]. A great advantage of the _mcrA_ gene lies in the fact that only one or two copies of this gene have been found in sequenced methanogen genomes, which makes it a more precise instrument than the 16S _r_RNA gene for evaluation of the number of these groups of microorganisms, as the latter can reach, on average, from four to over a dozen copies per genome [51]. The statistical analysis showed significant differences in the concentration of _the mcrA_ gene relative to the control in all digestate samples exposed to antibiotics. The lowest _mcrA_ concentrations, in both CS and SS digestate samples, were determined in the presence of MET. In the case of SS, the presence of _mcrA_ gene copies in 1 g of digestate from the process and control bioreactors remained on a high level of 10\({}^{6}\)-10\({}^{7}\). A decrease in the number of _mcr_A gene copies by one order relative to the control (10\({}^{7}\) of copies in 1 g of digestate) was recorded only in a sample of the digestate with MET (number of copies in the order of 10\({}^{6}\) w 1 g\({}_{\text{D}}\)\({}^{-1}\)) (Figure 2A). Although the presence of AMO and OXY led to a significant decrease in the yield of CH4, lower by nearly 75 and 92 L kg\({}^{-1}\) VS, respectively (Table 2), the concentration of the _mcr_A gene in digestate samples including these antibiotics remained on the level in the same order of magnitude as in the control sample. In this case, the concentrations of the _mcr_A gene, which is a functional gene of methanogenic microorganisms, do not reflect the actual production of methane in bioreactors. With respect to CS as a substrate, the number of copies of the _mcr_A gene in 1 g of digestate of the control sample and of experimental samples exposed to OXY or SMX oscillated on a high level, in the order of 10\({}^{8}\). Importantly, the presence of OXY in CS resulted in an over twofold decline in biogas yield relative to the control. Paradoxically, the analyzed samples of digestate from both SS and CS exposed to the presence of OXY were observed to generate a significantly lower CH4 yield with the simultaneous increase in the _mcr_A concentration in comparison with the control sample. In turn, exposure of CS to MET and AMO resulted in a decreased concentration of the _mcr_A gene by two orders of magnitude (10\({}^{6}\) copies in 1 g of digestate) (Figure 2B). The research results therefore implicate that a decrease in the frequency of presence of _mcr_A gene copies was associated with the efficiency of biogas production, content of CH4 in biogas and concentration of VFAs only when samples were exposed to MET in the case of the SS substrate and AMO when CS was the substrate. These relationships were illustrated in a PCA diagram (Figure 3), which shows a positive correlation between the exposure of SS and CS to metronidazolezole (MET SS) and amoxicillin (AMO CS), respectively, and the VFAs accumulation. Moreover, a negative correlation with the production of CH4 and concentration of the _mcr_A gene is noticeable.