Hematite and multi-walled carbon nanotubes stimulate a faster syntrophic pathway during methanogenic beet sugar industrial wastewater degradation

The quest to understand and subsequently improve the role played by bacteria and archaea in the degradation of organic matter both in natural and engineered anaerobic ecosystems has intensified the utilization of nanoparticles. Microbial communities are known to syntrophically cooperate during the anaerobic conversion of substrates into methane gas via the direct exchange of electrons. In this study, the role of hematite (Fe2O3-750 mg/L) and multi-walled carbon nanotubes (MWCNTs-1500 mg/L) during the degradation of beet sugar industrial wastewater (BSIW) in a batch experiment was investigated. Hematite and MWCNTs enhanced methane gas generation by 35 and 20%, respectively. Furthermore, microbial syntrophic communities might have exchanged metabolic electrons more directly, with hematite and MWCNTs serving as electron conduits between the homoacetogens and methanogens, thereby establishing a direct interspecies electron transfer (DIET) pathway. Additionally, hematite and MWCNTs enriched the bacteria Firmicutes while Chloroflexi reduced in abundance. Scanning electron microscopy and confocal laser scanning microscopy demonstrated that extracellular polymeric substances had noticeable interactions with both hematite and MWCNTs. Our findings provide vital information for more understanding of the response of microbes to hematite and MWCNTs in a complex natural environment.