Preliminary analysis of Chloroflexi populations in full-scale UASB methanogenic reactors

The phylum Chloroflexi is frequently found in high abundance in methanogenic reactors, but their role is still unclear as most of them remain uncultured and understudied. Hence, a detailed analysis was performed in samples from five up-flow anaerobic sludge blanket (UASB) full-scale reactors fed different industrial wastewaters.

Quantitative PCR show that the phylum Chloroflexi was abundant in all UASB methanogenic reactors, with higher abundance in the reactors operated for a long period of time, which presented granular biomass. Both terminal restriction fragment length polymorphism and 16S rRNA gene amplicon sequencing revealed diverse Chloroflexi populations apparently determined by the different inocula. According to the phylogenetic analysis, the sequences from the dominant Chloroflexi were positioned in branches where no sequences of the cultured representative strains were placed. Fluorescent in situ hybridization analysis performed in two of the reactors showed filamentous morphology of the hybridizing cells.

While members of the Anaerolineae class within phylum Chloroflexi were predominant, their diversity is still poorly described in anaerobic reactors. Due to their filamentous morphology, Chloroflexi may have a key role in the granulation in methanogenic UASB reactors.

Our results bring new insights about the diversity, stability, dynamics and abundance of this phylum in full-scale UASB reactors which aid in understanding their function within the reactor biomass. However, new methodological approaches and analysis of bulking biomass are needed to completely unravel their role in these reactors. Combining all this knowledge with reactor operational parameters will allow to understand their participation in granulation and bulking episodes and design strategies to prevent Chloroflexi overgrowth.