Morphology and spatial distribution of dominant populations. FISH micrographs are shown of the microbial community from AD3 on (a) day 88, (b) day 178 and (c) day 362. Universal probes targeting archaea (ARC951, green) and bacteria (EUB338 and EUB338+, blue) were used in combination with population-specific probes targeting Bacteroidales (Bac1080 and CF319a, red), Clostridiales (Clo549 and Ace731, magenta) and Fibrobacterales (Fibr225, cyan). The arrows point to possibly hydrolytic populations attached to cellulose particles (a, b), and clustering of archaeal rod-shaped cells into a sheathed filament (c).