The impact of fermentative organisms on carbon flow in methanogenic systems under constant low-substrate conditions

We compared carbon flow under constant low-substrate conditions (below 20 m uM glucose in situ) in laboratory-scale glucose-fed methanogenic bioreactor s containing two very different microbial communities that removed chemical oxygen demand at similar rates. One community contained approximately equa l proportions of spiral and cocci morphologies, while the other community w as dominated by cocci. In the former bioreactor, over 50% of the cloned SSU rRNA genes and the most common SSU rDNA terminal restriction fragment corr esponded to Spirochaetaceae-related sequences, while in the latter bioreact or over 50% of the cloned SSU rRNA genes and the most common SSU rDNA termi nal restriction fragment corresponded to Streptococcus-related sequences. C arbon flow was assessed by measuring C-14-Iabeled metabolites derived from a feeding of [U-C-14]glucose that did not alter the concentration of glucos e in the bioreactors. Acetate and ethanol were detected in the Spirochaetac eae-dominated reactor, whereas acetate and propionate were detected in the Streptococcus-dominated reactor. A spirochete isolated from a Spirochaetace ae-dominated reactor fermented glucose to acetate, ethanol, and small amoun ts of lactate. Maximum substrate utilization assays carried out on fluid fr om the same reactor indicated that acetate and ethanol were rapidly utilize d by this community. These data indicate that an acetate- and ethanol-based food chain was present in the Spirochaetaceae-dominated bioreactor, while the typical acetate- and propionate-based food chain was prevalent in the S treptococcus-dominated bioreactor.