Enhancing chain-elongating microbiomes: Sequential fungi-bacteria cooperation for efficient food waste upgrading using endogenous electron donors

The upgrading of organic waste such as food waste (FW) into medium-chain carboxylates (MCC) is within a bio-based circular economy concept. An efficient chain elongation (CE) microbiome is difficult to obtain, which normally requires long-term acclimatization and an exogenous supplement of electron donor (ED, especially ethanol). In this study, the CE microbiome was rapidly shaped within 18 days and an efficient endogenous ethanol-based CE was achieved through the amendment of distiller yeast (DY) during FW fermentation. Multiple FW feeding accumulated the caproate concentration to 5.76 ± 0.33 g COD/L, which also regulated the CE process from ethanol-dependent to lactate-dependent metabolism. During the lactate-based CE process, the caproate production increased linearly (R2 = 0.82) with the rising D-lactate content. Ethanol-producing fungi yeast mainly consisted of Wickerhamomyces (66.54 %), Saccharomycopsis (5.37 %), Issatchenkia (2.21 %), unclassified_f_Metschnikowiaceae (1.73 %), and Saccharomyces (1.33 %). These yeasts were eliminated by FW feeding, which was in line with the alteration of ED preference from ethanol to lactate. Metagenome analysis indicated that the total relative abundance of lactate-based chain elongators including Ruminococcaceae bacterium CPB6, Clostridium luticellarii, Caproiciproducens galactitolivorans, and Megasphaera elsdenii was increased by 28.6 %, while the ethanol-based chain elongator Clostridium kluyveri kept stable after FW feeding. In addition, the functional genes related to the oxidation of lactate to acetyl-CoA were enriched after FW feeding, which further enhanced the subsequent CE pathway of acetyl-CoA-based reverse β oxidation rather than the fatty acids synthesis. This study provides a new insight into upgrading FW into valuable MCC without exogenous ED addition.