Additional file 11: Figure S6. Maximum likelihood phylogenetic tree of the most abundant operational taxonomic units in the switchgrass digesters (bold numbers), which when summed together account for greater than 80% of the average relative abundance. Numbers given to OTUs only allude to the abundance of the OTUs in one of the 61 switchgrass digester samples. Numbers adjacent to branches show bootstrap supports that were greater than 50%. The scale bar and numbers below the branch-breaks indicate the number of nucleotide substitutions per position in the V4 region of the 16S rDNA gene sequences.