Microbial distribution throughout a cellobiose-supplemented three-stage laboratory-scale anaerobic digester

Substrate and intermediate metabolite degradation were monitored in a three‐stage anaerobic digester fed cellobiose as sole carbon source. The digester vessels were of different sizes and the resultant different dilution rates enabled separation of the trophic groups. Biofilms were allowed to develop on microscope slides suspended at different depths within each vessel and these were examined using phase contrast and fluorescence microscopy. Cell quantification was performed on these and planktonic samples using image analysis. All cellobiose degradation and sulphate reduction and most of the acidogenesis was performed in the first vessel, by an activated sludge‐like population (attached or in flocs). Eighty percent of the methanogenesis and some acetogenesis occurred in the second vessel where populations of attached fluorescent rods and Methanosarcina ‐like cells were abundant. The low population levels in the third vessel were responsible for 8% of the total methanogenesis.