Interior of granules. Quick-freezing and cleavage of 25 granules revealed an internal structure which consisted of a central core of bacterium-encased cavities surrounded by several distinct layers of bacteria (Fig. 5 and 6). The exterior layer was approximately 10 to 20 ,um thick (Fig.7). Not surprisingly, it contained the cell structures observed on the outside surfaces of whole granules. Large Methanococcaleslike organisms, chain-forming cocci, large long rods, thin
filaments, and smaller coccoid organisms and rods were present throughout this layer. Gas spaces often appeared to separate this layer from the underlying one. Thin filamentous organisms, approximately 0.25 ,um in width, were one of the predominant bridging species in these spaces. The second layer was a tightly packed structure of bacteria embedded in extracellular polymer (Fig. 7 and 8). Like the exterior layer, the second layer was approximately 10 to 20 ,um thick.
Various rods appeared to be the predominant structures. The third layer consisted of large microcolonies that were composed almost exclusively of angular-shaped rods (0.4 to 0.5 by 1.0 to 1.8 ,um) which possessed a structure similar to those of Methanothrix spp. (Fig. 8). These bacteria were electron microscope operating at an accelerating voltage of 20 kV.
Interior of granules. Quick-freezing and cleavage of 25 granules revealed an internal structure which consisted of a central core of bacterium-encased cavities surrounded by several distinct layers of bacteria (Fig. 5 and 6). The exterior layer was approximately 10 to 20 ,um thick (Fig.7). Not surprisingly, it contained the cell structures observed on the outside surfaces of whole granules. Large Methanococcaleslike organisms, chain-forming cocci, large long rods, thinfilaments, and smaller coccoid organisms and rods were present throughout this layer. Gas spaces often appeared to separate this layer from the underlying one. Thin filamentous organisms, approximately 0.25 ,um in width, were one of the predominant bridging species in these spaces. The second layer was a tightly packed structure of bacteria embedded in extracellular polymer (Fig. 7 and 8). Like the exterior layer, the second layer was approximately 10 to 20 ,um thick.Various rods appeared to be the predominant structures. The third layer consisted of large microcolonies that were composed almost exclusively of angular-shaped rods (0.4 to 0.5 by 1.0 to 1.8 ,um) which possessed a structure similar to those of Methanothrix spp. (Fig. 8). These bacteria were electron microscope operating at an accelerating voltage of 20 kV.
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