Toward sustained and efficient n-caproate production: Insights into magnetite-encapsulated biochar mediated microbial...

Decarbonization of the chemical sector may be facilitated by microbial chain elongation, through conversion of organic waste into chemical products. Challenges in efficient interspecies electron transfer is a barrier to this technology. Here we introduce a magnetite-encapsulated biochar (Fe@biochar) amendment strategically integrated with chain elongation to facilitate sustained and efficient n-caproate production. This strategy increased n-caproate yields to 13.92 g COD/L, 199 % and 71 % higher than the control (4.66 g COD/L) and biochar (8.13 g COD/L) groups, respectively. Most importantly, Fe@biochar consistently enhanced n-caproate production after recycle, a continuity not observed with biochar. This difference may be attributed to the inhibitory effects of high concentrations of undissociated n-caproic acid (up to 9.06 mM or 2.32 g COD/L) on functional microbes in the biochar group. Microbial community analysis identified Clostridium_sensu_stricto as the key genus involved in n-caproate production. Comparative metagenomic and microbial activity analyses revealed the energy metabolism, where Fe@biochar promoted extracellular iron-based electron transfer, and further accelerated intracellular electron sinks. This is substantiated by a 3.6 fold increase in electron transfer system activity and evaluated relative abundances of key genes encoding acetyl-CoA synthetase (15.0 % increase), cytochrome c biosynthesis (28.7 % increase) and NADH dehydrogenase (48.0 % increase). This study offers a new paradigm for sustained n-caproate production.