Impact of operating history on mixed culture fermentation microbial ecology and product mixture.

Mixed culture fermentation is an alternative to pure culture fermentation for production of biofuels and valuable products. A glucose-fed, continuous reactor was operated cyclically to a central pH of 5.5 from a number of precedent pHs, from 4.5 to 7.5. At each pH, stable chemical production was reached after 2 retention times and was held for least 2 further retention times prior to the next change. Bacterial groups were identified by phylogenetic analysis of 16S rRNA gene clones. Bacterial community dynamics were monitored by terminal-restriction fragment length polymorphism. More ethanol was produced at high pH, and more butyrate at lower pH. At pH 5.5, the product spectrum was not measurably influenced by precedent pH but showed seemingly random changes. The impact of precedent pH on community structure was more systematic, with clear indications that when the pH was returned to 5.5, the bacterial group that was dominant at the precedent pH remained at high abundance. This result is important, since it indicates a decoupling between microbial function (as indicated by product spectrum), and community structure. More work is needed to determine the longevity of this hysteresis effect. There was evidence that groups retained their ability to re-emerge even after times of low abundance.

Monitoring associations between clade-level variation, overall community structure and ecosystem function in enhanced biological phosphorus removal (EBPR) systems using terminal-restriction fragment length polymorphism (T-RFLP).

The role of Candidatus "Accumulibacter phosphatis" (Accumulibacter) in enhanced biological phosphorus removal (EBPR) is well established but the relevance of different Accumulibacter clades to the performance of EBPR systems is unknown. We developed a terminal-restriction fragment length polymorphism (T-RFLP) technique to monitor changes in the relative abundance of key members of the bacterial community, including Accumulibacter clades, in four replicate mini-sequencing batch reactors (mSBRs) operated for EBPR over a 35-day period. The ability of the T-RFLP technique to detect trends was confirmed using fluorescence in situ hybridisation (FISH). EBPR performance varied between reactors and over time; by day 35, performance was maintained in mSBR2 whilst it had deteriorated in mSBR1. However, reproducible trends in structure-function relationships were detected in the mSBRs. EBPR performance was strongly associated with the relative abundance of total Accumulibacter. A shift in the ratio of the dominant Accumulibacter clades was also detected, with Type IA associated with good EBPR performance and Type IIC associated with poor EBPR performance. Changes in ecosystem function of the mSBRs in the early stages of the experiment were more closely associated with changes in the abundance of (unknown) members of the flanking community than of either Accumulibacter or Candidatus "Competibacter phosphatis". This study therefore reveals a hitherto unrecorded and complex relationship between Accumulibacter clades, the flanking community and ecosystem function of laboratory-scale EBPR systems.