MICROBIAL SUBPOPULATIONS IN THE BIOFILM ATTACHED TO THE SUBSTRATUM AND IN THE FREE FLOCS OF A FIXED-BED ANAEROBIC BIOREACTOR

The microbial flora of a fixed-bed anaerobic methanogenic bioreactor f ed with acetate/propionate/butyrate was studied by direct, qualitative and quantitative methods avoiding culture isolation. The aims were to identify species, determine the distribution of microbes between the biofilm attached to the substratum and the free flocs, and define the acidogenic, acetogenic and methanogenic contingents. Optical and scann ing electron microscopies showed heterogeneous assemblies of microbes in the biofilm and flocs, which were confirmed by antigenic fingerprin ting. A diversity of species involved in the three phases of methanoge nesis was detected, and most of these species were antigenically diffe rent from the reference organisms. Some microbial subpopulations ident ified by antigenic fingerprinting changed in size within an interval o f 3 weeks, i.e. they either increased or decreased their concentration s by at least tenfold, while others remained relatively constant. The total cell concentration in the flocs was lower than in the biofilm, b ut at least one microbial subpopulation was more concentrated in the f ormer than in the latter, indicating a preference of location between the compartments available within the bioreactor.