Effect of substrate structure on medium chain fatty acids production and reactor microbiome

The medium chain fatty acids (MCFAs) produced from organic wastes can replace part fossil-fuel-based products to promote the sustainable development of economy and environment. However, the selection and collocation of feedstocks for MCFAs production are lack of reference basis. This study thereby aimed to investigate how the commonly used electron donor (ED) and substrate configuration affect MCFAs synthesis and then obtain the optimal substrate composition. It was found that the optimized ratios for ethanol/acetate, lactate/acetate, and ethanol/lactate/acetate were 3/1, 2/1, and 2/1/1, respectively, and the optimal substrate concentration was 400 mM C. Combining ethanol and lactate as co-EDs effectively concentrated substrate-carbon-flow (increased by 20-28% than sole ED) on MCFAs synthesis by promoting the elongation of butyrate and reutilization of byproducts. As a result, the higher MCFAs yield of 646.22 mg COD/g COD and selectivity of 67.72% were obtained from co-EDs than those from sole ED. Moreover, the key functional bacteria enriched under different ED were also discrepant, which were Clostridium sensu stricto for ethanol, Corynebacterium for lactate, and Veillonella and Oscillibacter for ethanol-lactate, respectively. This study provided a basic but significant reference for the scale-up MCFAs production.