Spatial arrangement of Bacteria, Archaea, and D. hafniense in the anaerobic FF reactor. Biofilm samples were fixed, permeabilized, hybridized with fluorescently labeled oligonucleotides, and then counterstained with YOYO-1 before being examined by CLSM. FISH was performed with EUB338-Cy3 (Bacteria, panel A), ARC915-Cy3 (Archaea, panel B), and PCP-1-4-Cy3 and PCP-1-8-Cy5 (D. hafniense, panel C). Panels A, B, and C are biofilms taken after 56, 116, and 225 days of operation, respectively. Bars correspond to 20 μm.