Metagenomic and proteomic insights into graphene oxide-boosted anaerobic co-fermentation of food waste and sewage sludge for volatile fatty acids production

[Display omitted] •VFAs production was boosted by GO during anaerobic co-fermentation of FW and SS.•The activities of key enzymes and IET were improved in the presence of GO.•The Stickland reaction of amino acids was facilitated by GO as redox mediator.•Metabolic pathways for VFAs production were regulated by triggering suitable IROS.•Proteomics of Proteiniphilum acetatigenes revealed the enhanced acetate production. The work investigated the volatile fatty acids (VFAs) production during anaerobic co-fermentation system of food waste (FW) and sewage sludge (SS), facilitated by graphene oxide (GO) as redox mediator (RM). In the presence of GO, there was a 35.4 % increase in VFAs production, with 26.5 % of acetate compared to the Control. Protease activity increased by 20.7 % to promote protein hydrolysis, while acidification was supported by 10.1 % increase in the activities of acetate kinase, phosphate transacetylase and oxaloacetic acid carboxylase, coupled with 3.5-fold enhancement in intracellular electron transfer activity, augment of amino acid redox reactions and a favorable intracellular redox state (IROS). The microbiota conducive to VFAs production (e.g., Bacteroides and Proteobacteria) was also enriched. Metagenomic analysis indicated that the relative abundance of functional genes related to substrate hydrolysis (e.g., BglX and CLPP) and transport (e.g., gtsA, gltK, and gltI) and VFAs production (e.g., kor, pta, ackA and atoD) was upgraded by 3.5–29.2 %, along with a downregulation (4.7–20.8 %) of genes (e.g., GLO1, gloB and pflD) associated with by-products (e.g., lactate and ethanol). Proteomics of proteiniphilum acetatigenes demonstrated acetate accumulation was boosted through GO-mediated protein hydrolysis and inhibition of by-products that consume pyruvate. This study provided a promising strategy for VFAs production by GO as RM, which reinforced the redox reaction of amino acid degradation, modulated metabolic pathways and maintained lower reduced nicotinamide adenine dinucleotide (NADH) favorable for VFAs production during anaerobic co-fermentation of FW and SS.