Scanning electron microscopy. (i) Granule surface. The
granules were irregular spheres (Fig. 1). A close examination
of the surfaces of 25 granules revealed the presence of a large
diversity of bacterial morphotypes. These included chainforming
coccoid organisms (approximately 0.7 p.m in diameter),
large rods (1.0 by 2.0 p.m), long thin filaments (approximately
0.27 p.m in width), and small rods and cocci (less
than 1.0 p.m in length or diameter) (Fig. 2). Other surface
areas (Fig. 3) contained large numbers of clustering coccoid
organisms (approximately 1.1 p.m in diameter) that resembled
members of the order Methanococcales. Still other
areas (Fig. 4) were covered with large numbers of small rods
(0.28 to 0.37 by 0.7 to 0.9 ,um) and small cocci (0.23 to 0.47 septa. This unique cell envelope structure is characteristic of
strains of the archaebacterial genus Methanothrix.