A review on biohydrogen production through dark fermentation via the lactate pathway.

Biohydrogen production via dark fermentation is traditionally limited by lactate accumulation, which is often regarded as an inhibitory metabolic sink. This review reframes the role of lactate and lactic acid bacteria by analyzing the transition from process inhibition to strategic utilization through lactate-driven dark fermentation. By evaluating metabolic pathways and microbial cross-feeding dynamics, the work demonstrates how lactate acts as a high-energy intermediate that can be reassimilated by hydrogen-producing bacteria to enhance yields. The pyruvate:ferredoxin oxidoreductase-based pathway and electron bifurcation are identified as central mechanisms linking lactate oxidation to superior hydrogen evolution. Various biological factors and operational strategies, such as inoculum selection, pH regulation, and reactor configurations, are evaluated to determine the favorable conditions that support robust microbial cross-feeding. Findings highlight that efficient lactate-driven dark fermentation is governed by functional community balance rather than absolute lactate concentrations, with optimal performance occurring when lactate-oxidizing hydrogen-producing bacteria match lactate formation rates. Ultimately, this work provides a roadmap for process optimization, emphasizing that managing lactic acid and hydrogen-producing bacteria syntrophy is essential for transforming lactate into a driver of sustainable bioenergy production.