COMPETITION AND COEXISTENCE OF SULFATE-REDUCING AND METHANOGENIC POPULATIONS IN ANAEROBIC BIOFILMS

The microbial population structure and function of natural anaerobic c ommunities maintained in laboratory fixed-bed biofilm reactors were tr acked before and after a major perturbation, which involved the additi on of sulfate to the influent of a reactor that had previously been fe d only glucose (methanogenic), while sulfate was withheld from a react or that had been fed both glucose and sulfate (sulfidogenic), The popu lation structure, determined by using phylogenetically based oligonucl eotide probes for methanogens and sulfate-reducing bacteria, was linke d to the functional performance of the biofilm reactors, Before the pe rturbation, the methanogenic reactor contained up to 25% methanogens a s well as 15% sulfate-reducing bacteria, even though sulfate was not p resent in the influent of this reactor, Methanobacteriales and Desulfo vibrio spp, were the most abundant methanogens and sulfate-reducing ba cteria, respectively, The presence of sulfate-reducing bacteria (prima rily Desulfovibrio spp, and Desulfobacterium spp,) in the absence of s ulfate may be explained by their ability to function as proton-reducin g acetogens and/or fermenters, Sulfate reduction began immediately fol lowing the addition of sulfate consistent with the presence of signifi cant levels of sulfate-reducing bacteria in the methanogenic reactor, and levels of sulfate-reducing bacteria increased to a new steady-stat e level of 30 to 40%; coincidentally, effluent acetate concentrations decreased, Notably, some sulfate-reducing bacteria (Desulfococcus/Desu lfosarcina/Desulfobotulus group) were more competitive without sulfate . Methane production decreased immediately following the addition of s ulfate; this was later followed by a decrease in the relative concentr ation of methanogens, which reached a new steady-state level of approx imately 8%, The changeover to sulfate-free medium in the sulfidogenic reactor did not cause a rapid shift to methanogenesis, Methane product ion and a substantial increase in the levels of methanogens were obser ved only after approximately 50 days following the perturbation.