Fig. 2

Representation of dominant microbial groups per cluster (a, b) and the diversity indices for microbial communities (c, d) analysed for the 20 monitored mesophilic AD reactors. The relative abundance of the dominant bacteria was shown at the order level (a) and archaea at the OTU level (b) and for each of the clusters. In the case of the richness (sobs index) and the diversity (invsimpson index), increased values indicate higher species richness and diversity, respectively, and correspond to the bacterial (c) and archaeal (d) clusters. For the evenness (invsimpsoneven index), values range from 0 (non-even) to 1 (even). The boxes represent the interquartile range and the error bars show the 95% confidence intervals; cluster _aI (n = 33), _aII (n = 105), _aIII (n = 113), _bI (n = 23), _bII (n = 80), _bIII (n = 89), _bIV (n = 36), _bV (n = 34). Statistical analyses were performed using Kruskal–Wallis test; boxplots holding the same label within a single panel do not differ significantly (p ≤ 0.05)