DISCUSSIlOlN.
Our findings show that the granules obtained from a
mesophilic reactor, which was fed over a 1-month period
with a sucrose medium, exhibited a three-layered structure
and that each layer possessed a distinguishing morphology.
The exterior layer contained a variety of organisms, including
clusters of large coccoid organisms (Fig. 3) which
strongly resembled methanogenic cocci of the order Methanococcales
(30). Thin filaments similar to Methanospirillum
spp. were also present (31). The second layer consisted of a
large number of rod-shaped bacteria. Among these rods was
a very electron-dense organism (Fig. 12) which resembled a ; -Mmeitihanobrevibacter sp. and a larger short rod which resem-
-Li1a- bled a Syntrophobacter sp. as described by Dubourguier et
al. (10). The third layer contained large numbers of rods with
flat ends (Fig. 8-10). Scanning electron micrographs revealed
that these bacteria occupied large areas of the center
of the granules. These bacteria were the same size and shape
as previously described Methanothrix spp. (9). Transmission
electron microscopy provided further discriminating evidence
with respect to this morphotype. The presence of two
wall layers and the angular shape are very distinctive features
(Fig. 14). Similar morphological characteristics were
presented previously for Methanothrix concilii (25) and