Scanning electron microscopy. The washed granules were placed in sealed 50-ml serum bottles which contained 5% glutaraldehyde in anaerobic cacodylate buffer, and fixation was executed overnight at 4°C. To obtain cleaved preparations of the granules, the fixed samples were quick-frozen in liquid nitrogen and cleaved with a mortar and pestle. Whole and cleaved granules were dehydrated through graded series of water-ethanol and ethanol-Freon 113. The samples were placed on aluminum specimen mounts, coated with goldpalladium, and examined with a Hitachi S-450 scanning ,um in diameter). A few chain-forming rods (approximately 0.6 by 1.6 ,um) which exhibited angular shapes similar to those of Methanothrix spp. were occasionally observed on
the surface. A collapsed extracellular polymer was observed to be associated with the bacteria on the surface, particularly with the large coccoid organisms (Fig. 4).