Deciphering the function of Fe3O4 in alleviating propionate inhibition during high-solids anaerobic digestion: insights of physiological response and energy conservation

Abstract

Fe₃O₄ is a recognized addictive to enhance low solid anaerobic digestion (AD), while for high solid AD challenged by acidity inhibition, its feasibility and mechanism remain unclear. In this study, the positive effect of Fe₃O₄ on high solid AD of food waste by regulating microbial physiology and energy conservation to enhance mutualistic propionate methanation was documented. The methane yield was increased by 36.7% with Fe₃O₄, which because Fe₃O₄ alleviated propionate stress on methane generation, along with improved propionate degradation and methanogenic metabolism. Fe₃O₄ facilitated the production of extracellular polymeric substances and the formation of tightly bio-aggregates, fostering an enriched microbial population (e.g., Smithella and Methanosaeta) to resist propionate stress. Also, Fe₃O₄ up-regulated the genes in stress defense system, cytomembrane biosynthesis/function, metal irons transporter, cell division and enzyme synthesis, verifying its superiority on cellular physiology. In addition, energy-conservation strategies related to intracellular and extracellular electron transfer were enhanced by Fe₃O₄. Specifically, the enzyme expressions involved in reversed electron transfer and electron bifurcation coupled with direct interspecies electron transfer (DIET) were upregulated by at least 2.2 times with Fe₃O₄, providing sufficient energy to drive thermodynamic adverse methanogenesis from propionate-stressed condition. Consequently, the reinforced enzyme expression in the dismutation and DIET pathway make it to be the predominant drivers for enhanced methanogenic propionate metabolism. This study fills the knowledge gaps of Fe₃O₄-induced microbial physiological and energetic strategies to resist environmental stress, and has remarkable practical implicated for restoring inhibited bioactivities.