AbstractThe bacterial, archaeal, and fungal taxonomies of a semi-batch-fed, lab-scale cellulolytic enrichment reactor inoculated by the organic fraction of municipal solid waste were explored using 16SrRNA and internal transcribed spacer (ITS) sequencing. Biomass samples were partitioned into total, planktonic, and sessile (biofilm) fractions using a centrifugation separation technique developed to quantify sessile biomass growth on cellulose particles during anaerobic digestion. The relative abundances of bacteria and archaea taxa were determined over a 1-day period. The sessile and planktonic biomass fractions showed significantly different relative abundances for 13 taxa, with the sessile biomass having 10 times lower relative abundance of Unclassified Firmicutes, nearly 20 times higher relative abundance of Methanothrix spp., double the relative abundance of Hydrogenispora spp., and 44.3% higher relative abundance of an uncultured Leptospira sp. than the planktonic biomass. This study shows how a relatively simple physical separation method can be employed to add another dimension to taxonomic studies. Better understanding the microbial communities in these systems is a key first step toward achieving performance improvements such as enhanced cellulose solubilization rates and other microbial biotransformations through process, genetic, and/or other manipulations.