# Salinity Inhibition in Thermophilic Anaerobic Digestion of Organic Waste
## 4 Discussion  
The results of T-RFLP analysis (Figure 2) showed that bacteria and archaea in the experiment in which salt addition was gradual responded rapidly to salt addition, as indicated by the fact that the samples taken at the chloride concentration of less than 22.5 g/L (31.6 g/L NaCl) were clustered in one group, while samples taken at higher chloride concentrations (above 22.5 g/L and up to 60 g/L NaCl) were clustered separately. Changes in the composition of both bacterial and archaeal microbial communities and lower diversity of both communities as a consequence of high salinity have already been noted by several authors [1; 9; 12; 23]. The increase in NaCl concentration above 31.6 g/L correlates with higher acetic and propionic acid concentrations and lower methane production, too. As reported by [PERSON] et al. [40], propionate is an important precursor for methane production, and its accumulation which means hindered conversion to acetate, results in lower SBP. Propionate accumulation is the result of salt stress for propionate-oxidizing bacteria at NaCl concentrations above 31.6 g/L. Figure 4: UPGMA dendrogram of T-RFLP profiles of bacterial (**left**) and archaeal (**right**) communities at shock exposure to salinity. The numbers above the dendrogram represent the percent of similarity. As for the changes in the composition of organic acids, it is noted that higher concentrations of acetic acid, butyric acid, valeric acid, and tSCFA were obtained under continuous salt exposure than under salt shock conditions. Higher concentrations of propionic acid and isobutyric acid were obtained under salt shock conditions than under continuous salt exposure. Acclimation of the bacterial and archaeal microbiome to increasing salinity occurs gradually and successfully up to the NaCl concentration of 31.6 g/L so that SBP is almost the same level as at a NaCl concentration of 1.4 g/L. Higher salinity does not reduce effective microbial acclimation (Figure 1). Although the bacterial community at the end of the experiment differs by about 35% and the archaeal community by about 25% from the initial one, and the diversity of both microbial communities is reduced, they still allow the resumption of the methane production process after the salinity decrease at the end of the experiment. The archaeal community is usually considered to be less diverse than that of the bacterial community [9]. The TRFLP electrophergrams in the experiment with gradual salt addition showed three common peaks from the beginning to the end of the experiment, one of which became dominant with increasing salinity, while the other two became less distinct, suggesting that one of the archaeal species (presumably halotolerant) was enriched by the high salinity (Figure 2, right). [PERSON] et al. [41] explained the maintenance of biogas production under saline conditions by acclimation of the microbial community to elevated salt concentrations. [PERSON] et al. [42] concluded from their experiments with hypersalinity in AD, where they analyzed the microbial community composition that the reason for the process recovery after exposure to salinity could be the redundancy of the microbial community. [PERSON] et al. [23] conducted the only relatively comparable experiment to the presented one. They tested the anaerobic degradation of slaughterhouse wastewater with the enriched (adapted) anaerobic cultures under mesophilic and thermophilic conditions. They found that the bacterial community was strongly affected by the high salinity of 20 and 40 g/L, and bacterial diversity decreased with increasing salinity, which is consistent with the present experiment. The effects of high salt concentration were more evident in thermophilic bacterial cultures than in mesophilic ones. The archaeal community in the experiment in which salt shocks were applied was more uneven than the bacterial community and less diverse (Figure 4, right). The archaeal community responded rapidly to the salinity shocks.