Metatranscriptomic Evidence for Magnetite Nanoparticle-Stimulated Acetoclastic Methanogenesis under Continuous Agitation

Abstract

Conductive nanomaterials have been reported to accelerate methanogenesis by promoting direct interspecies electron transfer (DIET), while their effects seem to vary depending on operational conditions. The present study examined the effects of magnetite nanoparticles (MNPs) on methanogenesis from acetate by soil-derived anaerobic cultures under continuous agitation. We found that MNPs accelerated methanogenesis in agitated cultures, as has been observed previously for static cultures. Metabarcoding of 16S rRNA gene amplicons showed that Methanosarcina substantially increased in the presence of MNPs, while DIET-related Geobacter did not occur. Metagenomic and metatranscriptomic analyses confirmed the predominance of Methanosarcina in MNP-supplemented agitated cultures. In addition, genes coding for acetoclastic methanogenesis, but not those for hydrogenotrophic methanogenesis, were abundantly expressed in the dominant Methanosarcina in the presence of MNPs. These results suggest that MNPs stimulate acetoclastic methanogenesis under continuous agitation. IMPORTANCE Previous studies have shown that conductive nanoparticles, such as MNPs, accelerate methanogenesis and suggested that MNPs facilitate DIET between exoelectrogenic bacteria and methanogenic archaea. In these methanogens, electrons thus obtained are considered to be used for hydrogenotrophic methanogenesis. However, the present work provides evidence that shows that MNPs accelerate DIET-independent acetoclastic methanogenesis under continuous agitation. Since most of previous studies have examined effects of MNPs in static or weakly agitated methanogenic cultures, results obtained in the present work suggest that hydraulic conditions definitively determine how MNPs accelerate methanogenesis. In addition, the knowledge obtained in this study is useful for engineers operating stirred-tank anaerobic digesters, since we show that MNPs accelerate methanogenesis under continuous agitation.

Methane production by acetate dismutation stimulated by Shewanella oneidensis and carbon materials: An alternative to classical CO2 reduction

“…For instance, S. oneidensis MR-1 increased methane production only by 5% after 6 days of incubation with CH 3 F, whereas the increase reached 22.5% without CH 3 F. Calculations shows that acetoclastic methanogenesis accounted for 70-80% of produced methane, and, conversely, CO 2 reduction accounted for only 20-30% of produced methane (Table 1). Our findings are supported by pure culture and metatranscriptomic data showing that Fe 3 O 4 nanoparticles act as electron shuttles to stimulate acetoclastic methanogenesis [20,21]. However, for this study and the two studies mentioned above [20,21], the experiments were performed with acetate as the sole carbon source.…”

“…Our findings are supported by pure culture and metatranscriptomic data showing that Fe 3 O 4 nanoparticles act as electron shuttles to stimulate acetoclastic methanogenesis [20,21]. However, for this study and the two studies mentioned above [20,21], the experiments were performed with acetate as the sole carbon source. The observations may be limited to the simplified experimental conditions, not to real environments containing different VFAs (e.g., propionate and butyrate) and other organic compounds whose metabolism can involve DIET (via GAC or CNTs).…”

“…To date, long-standing concept that DIET-CO 2 reduction to generate methane introduced by conductive materials seems to be fully applicable to almost all scenario. Recently, some studies indicated the benefit of increased electron transfer on direct acetate dismutation [20,21,28]. However, these data are not enough to provide evidence that exoelectrogenic activity can affect direct acetate dismutation to generate methane.…”

“…are not always detected in improved methane production systems [19]. Two recent studies suggested that conductive magnetite accelerates acetoclastic methanogenesis [20,21]. First, Fu et al showed that nanoFe 3 O 4 acts as solid electron shuttles to accelerate acetoclastic methanogenesis by Methanosarcina barkeri in pure cultures [20].…”

“…First, Fu et al showed that nanoFe 3 O 4 acts as solid electron shuttles to accelerate acetoclastic methanogenesis by Methanosarcina barkeri in pure cultures [20]. Second, Inaba et al provided metatranscriptomic evidence for stimulated acetoclastic methanogenesis by magnetite nanoparticle under continuous agitation [21]. These findings show that conductive minerals enhance the dismutation of acetate and, in turn, increase methanogenesis.…”