Effect of fungal hydrolysis of lemon peels on biohydrogen production

Citrus residues, particularly lemon peel, pose significant environmental challenges due to disposal difficulties. Despite this, they represent an abundant biomass resource with high recovery potential. One promising valorization route is their use for biohydrogen production via dark fermentation. However, their complex structure and the presence of inhibitory compounds can limit biological conversion. This study evaluated the effect of fungal hydrolysis as a pretreatment for dark fermentation of lemon peel, using strains of Colletotrichum gloeosporioides , Phytophthora sp ., Fusarium oxysporum , and Alternaria alternata , and assessed its impact on hydrogen production. The results showed that fungal pretreatment promoted carbohydrate solubilization, with A. alternata demonstrating the highest capacity for releasing soluble carbohydrates, and the presence of phenolic compounds had a minimal effect. Subsequent dark fermentation of pretreated substrates resulted in significant increases in hydrogen production of 69% and 36% in treatments with Alternaria and Fusarium , respectively, compared with the control. Metabolite profile analysis revealed that the acetate pathway was the primary metabolic route for hydrogen production, while the presence of ethanol confirmed a characteristic reductive metabolism of citrus residues. Overall, fungal hydrolysis emerges as a sustainable and effective strategy for improving substrate bioavailability and enhancing biohydrogen production, offering an alternative to physicochemical pretreatments that often produce inhibitory byproducts.