Magnetite-enhanced chain elongation via endogenous electron donors through a novel fungi-bacteria microbiome

Anaerobic fermentation of organic waste stream into medium-chain fatty acids (MCFA) through chain elongation (CE) has emerged as a sustainable and eco-friendly approach for resource recovery. Co-culture of yeast with chain elongator achieved a promising endogenous electron donor (ED)-driven CE but was limited by the weak yeast-bacteria synergy. This study presents a novel approach to optimizing the CE process through the regulation of magnetite (0-15 g/L). Results indicated that the 5 g/L of magnetite (Mag-5) achieved MCFA production at 8.42±0.67 g COD/L, which was 2.01 times greater than the blank. In-situ ethanol and lactate served as the ED to drive the CE. Under the optimal condition (Mag-5), Streptococcus (55.83%) and Candida (57.52%) were the dominant ED-producing microorganisms, and Clostridium_sensu_stricto_12 (22.70%) was the dominant chain elongator. In addition, metagenome analysis demonstrated the enhancement in reverse β-oxidization for MCFA production. Furthermore, the truncated tricarboxylic acid cycle was enhanced by magnetite amendment to provide more reduced energy, potentially accelerating the electron transfer within the mixed fungi-bacteria consortia. Finally, the constructed network of fungi and bacteria discerned the substrate competition between yeast and bacteria, and the cooperation among chain elongators, yeast, and mold. This study first proposed to regulate the mixed fungi-bacteria microbiome with magnetite to enhance the in-situ ED-driven CE process, providing a viable approach to the bioconversion of organic streams into high-value products.