Enhancement Of Propionate Bioconversion To Methane By Amino Acids Modified Fe₃O₄ During Anaerobic Digestion

As a green and cost-effective technology,anaerobic digestion（AD）can meet the target for global sustainable development goals with simultaneous reduction of organic wastes and production of renewable energy（i.e.,methane）.Propionate is an important metabolite in the process of anaerobic digestion,and 35%of the total methane produced is attributed to the metabolism of propionate.Nevertheless,the bioconversion of propionate to methane is difficult,and the instability of AD reactor is often accompanied by the propionate accumulation.In the present study a new method to remarkably promote the biotransformation of propionate to methane by modifying the amino acid on the surface of Fe3O4 was reported.The mechanism of methionine modified Fe₃O₄（Met@Fe₃O₄）and lysine modified Fe₃O₄（Lys@Fe₃O₄）remarkably enhancing propionate biotransformation and methane production were investigated,and further application of ML@Fe₃O₄ to anaerobic digestion of organic waste fermentation liquid containing propionate were explored.First,the influence of methionine modified Fe₃O₄,namely Met@Fe₃O₄,propionate bioconversion to methane was explored.It was observed that in the presence of Met@Fe₃O₄ the CH₄ yield and the maximal CH₄ generation rate were 81.2%and152.1%higher than the Blank,and 37.6%and 82.7%greater than Fe₃O₄ respectively.The mechanisms investigations indicated that the lower electrochemical impedance spectroscopy of Met@Fe₃O₄ making it more effective on enhancing the intracellular electron transfport in acetoclastic methanogenesis and extracellular DIET than Fe₃O₄,leading to the increase of enzyme activity in the propionate degradation—MMC pathway.Further investigation indicated that the protein of EPS was more likely to adsorb in Met@Fe₃O₄ because of the hydrophilic nature,thus the attachment of Met@Fe₃O₄ to microbia was promoted,resulting in the faste DIET.Then,the effect of lysine-modified Fe₃O₄（Lys@Fe₃O₄）was investigated,it was found that Lys@Fe₃O₄ remarkably increase propionate bioconversion to methane,with an average methane yield of 278.7%and 202.8%of that of blank and Fe₃O₄,respectively.The metagenomic and metaproteomic investigation indicated that Lys@Fe₃O₄ increased not only active Smithella by 2.6-fold but cell surface and adhesion proteins,and promoted syntrophic interaction of Smithella and methanogens,facilitating electron and acetate transfer from Smithella to methanogens.Also,the expression of proteins related to bacteria interspecies communication was enhanced,benefiting symbiotic association of Smithella and Syntrophomonas and transformation of butyrate from Smithella to Syntrophomonas.Furthermore,the expressions of key enzymes related to metabolism and electron transfer in propionate and butyrate oxidation and CO₂ and acetate methanogenesis were all significantly up-regulated.The effect of modified Fe₃O₄ with methione and lysine（ML@Fe₃O₄）on propionate bioconversion to methane was explored.It’s indicated that promotion effect of ML@Fe₃O₄ on propionate bioconversion to methane was higher than Met@Fe3O4and Lys@Fe3O4,and the cumulative methane generation was 259.2%,169.9%,152.5%and 47.2%greater than that of Blank,Fe₃O₄,Met@Fe₃O₄ and Lys@Fe₃O₄,respectively.In addition,ML@Fe₃O₄ enhanced the bioconversion of propionate to acetate,AM and DIET processes.Finally,the application of ML@Fe₃O₄ to anaerobic digestion of organic waste fermentation liquid containing propionate were explored.And ML@Fe₃O₄ promoted the anaerobic digestion of organic waste fermentation liquid containing propionate,with the methane yield of 237.2%and 83.2%higher than that of Blank and Fe₃O₄,respectively.The metagenomic and metaproteomic investigation indicated that ML@Fe₃O₄ increased the relative abundance of Syntrophobacter、Smithella、Methanosarcina、Methanothix and Methanospirillum,while Fe₃O₄ only increased the relative abundance Syntrophobacter and Methanosarcina.In addition,ML@Fe₃O₄increased the syntrophic interaction of propionate oxidizing bacteria and methanogenic bacteria by promoting the protein expression of amino acid transport,cell division and biofilm forming.ML@Fe₃O₄ increased the proteins expression related to ferritin and protein synthesis,and promoted the expression of enzymes related to substance metabolism and electron transport in the MMC pathway and dismutation pathway;Furthermore,the expressions of key enzymes related to metabolism and electron transfer in AM and DIET were all significantly up-regulated.