Mechanisms of enhanced biohydrogen production from macroalgae by ferrous ion: Insights into correlations of microbes and metabolites

This study explored the mechanisms of the enhanced hydrogen production from macroalgae by Fe2+ supplementation. Highest hydrogen yield of 19.47 mL/g VSadded was achieved at Fe2+ supplementation of 400 mg/L, which was 6.25 times of the control test. In depth analysis of substrate degradation, microbial distribution and metabolites formation was conducted. The results showed that Fe2+-supplemented group was dominated by Clostridium butyricum (67.2%) and Ruminococcus gnavus (24.2%), which stimulated hydrogen generation and volatile organic acids accumulation. In contrast, Fe2+-deficient group had a microbial community dominated by Exiguobacterium sp. (29.0%), Acinetobacter lwoffii (24.5%) and Clostridium stricto 13 (23.4%), which induced higher efficiency of both biomass hydrolysis and mineralization. Microbes from a single system were mutually cooperative, while microbes from Fe2+-deficient and those from Fe2+-supplemented systems were mutually exclusive. This study suggested that Fe2+ is critical in macroalgae fermentation system to affect the microbial community structure and subsequently switch the metabolic pathways.