Fig. 7

Confocal scanning laser micrographs (CSLM, top) and transmission electron micrographs (TEM, bottom) of residual solids. Switchgrass tissue samples remaining in the residues of C. thermocellum fermentations with and without ball milling were imaged. Some disruption at the tissue scale can be seen as dislocations among regions of fiber cells even in the initial substrate without any ball milling (a, a’). This is likely caused by the knife milling. However, the impact of the knife milling alone does not seem to propagate into the cell walls as almost no delamination or surface ablation was observed in these cell walls (b, b’). The impact of the ball milling on the initial switchgrass was evident at both the micron (c, c’) and nanometer scale (d, d’). More dislocation of the cells is apparent, but now it appears to coincide with fracturing and delamination of the cell walls. All of the inoculation residues share common features of altered cell wall textures evidenced by a hazy (e’, g’) or mottled staining density (f’, j’), and irregular, sometimes scalloped surfaces (asterisks, f, f’, h, j’). Clearly the most dramatic change in tissue and cell wall structure was seen in the sample that was ball milled between the two inoculations (i, j). Almost no intact tissue is visible with single cells and cell wall fragments dominant (i, i’). Also, there was more frequent evidence of void space opened up deep within the cell walls (white arrows, j’)