Inoculation of Megasphaera elsdenii influences microbial community structure and drives metabolic pathways in lactate-driven dark fermentation from lactic acid-rich effluent

Lactate-driven dark fermentation holds strong potential for hydrogen production from lactic-acid-rich substrates, but its microbial mechanism has been mostly explored in batch culture. Under steady-state conditions, microbial interactions may shift lactate conversion, potentially suppressing lactic acid-consuming bacteria ( Megasphaera elsdenii ). In this study, hydrogen production was assessed in sequencing batch reactors fed with a lactic-acid-rich food waste effluent and using either the native community or the external addition of M. elsdenii . The native inoculum achieved the highest hydrogen production (1039.7 mL/L reactor ), whereas M. elsdenii inoculum rapidly transitioned to methanogenesis, reaching 660.6 ± 54.6 mL CH 4 /L reactor . A contrasting microbial network was observed: a restricted community dominated by emergent M. elsdenii , favoring hydrogenogenesis in the native system; and a smaller and centralized network composed of M. elsdenii and Prevotella , coupled to methanogenesis in the inoculated reactor. This revealed how lactic acid metabolism was influenced by the microbial community shaped by the initial inoculum.