Metatranscriptomic evidences for magnetite nanoparticle-stimulated acetoclastic methanogenesis under continuous agitation

Conductive nano-materials 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 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 MPN-supplemented 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.