The microbial population structure and function of natural anaerobic communities maintained in laboratory fixed-bed biofilm reactors were tracked before and after a major perturbation, which involved the addition of sulfate to the influent of a reactor that had previously been fed only glucose (methanogenic), while sulfate was withheld from a reactor that had been fed both glucose and sulfate (sulfidogenic), The population structure, determined by using phylogenetically based oligonucleotide probes for methanogens and sulfate-reducing bacteria, was linked to the functional performance of the biofilm reactors, Before the perturbation, the methanogenic reactor contained up to 25% methanogens as well as 15% sulfate-reducing bacteria, even though sulfate was not present in the influent of this reactor, Methanobacteriales and Desulfovibrio spp, were the most abundant methanogens and sulfate-reducing bacteria, respectively, The presence of sulfate-reducing bacteria (primarily Desulfovibrio spp, and Desulfobacterium spp,) in the absence of sulfate may be explained by their ability to function as proton-reducing acetogens and/or fermenters, Sulfate reduction began immediately following the addition of sulfate consistent with the presence of significant levels of sulfate-reducing bacteria in the methanogenic reactor, and levels of sulfate-reducing bacteria increased to a new steady-state level of 30 to 40%; coincidentally, effluent acetate concentrations decreased, Notably, some sulfate-reducing bacteria (Desulfococcus/Desulfosarcina/Desulfobotulus group) were more competitive without sulfate. Methane production decreased immediately following the addition of sulfate; this was later followed by a decrease in the relative concentration of methanogens, which reached a new steady-state level of approximately 8%, The changeover to sulfate-free medium in the sulfidogenic reactor did not cause a rapid shift to methanogenesis, Methane production and a substantial increase in the levels of methanogens were observed only after approximately 50 days following the perturbation.
