Low-strength ultrasonication positively affects methanogenic granules toward higher AD performance: Implications from microbial community shift

• Low-strength ultrasonication enhanced methane production from organic wastes.
• More even microbial community was observed from ultrasonicated granules.
• Potential syntrophs within δ-Proteobacteria were enriched in ultrasonicated sludge.
• Acetate- and hydrogen-using methanogens were also enriched after ultrasonication.
• Enhanced hydrogen mass transfer was hypothesized to support syntrophism.

To elucidate the enhanced methane yield from organic wastes, the effects of low-strength ultrasonication on the microbial community structures in upflow anaerobic sludge blanket reactors were for the first time analyzed using pyrosequencing. Interestingly, a more even microbial community was observed in the ultrasonicated granules than in the control, which could compensate for the decreased richness and resulted in comparable (archaea) or even higher (bacteria) diversity. The ultrasonicated granules contained higher levels of δ-Proteobacteria, of which many are reportedly potential syntrophs, as well as methanogenic genera Methanosaeta, Methanotorris, and Methanococcus. The increased presence of syntrophic bacteria with their methanogenic partners was discussed with respect to hydrogen flux; their selective proliferation seems to be responsible for the enhanced anaerobic performance. This study is the first research shedding light on the novel function of low-strength ultrasound shifting the microbial structure towards better biogas production performance, and will facilitate application of low-strength ultrasound to other bioprocesses.