Abstract
Dark fermentation of organic waste offers a means of coupling molecular hydrogen (H2) production with the bioconversion of wet organic feedstocks. However, its practical performance remains limited by unstable carbon and electron-flow distribution, substrate heterogeneity, and dynamic shifts in microbial community structure. This review critically evaluates microbial consortium engineering as a strategy to improve the selectivity, stability, and controllability of dark-fermentative H2 production. Available experimental evidence indicates that H2 productivity is determined not by a single high-performing strain but by the coordinated activity of microorganisms responsible for hydrolysis, fermentation of soluble compounds, and conversion through H2-producing pathways. Disruption of this coordination redirects carbon and electrons toward lactate, alcohols, methane, and homoacetogenic pathways. Synthetic microbial consortia enable more targeted control over community composition and the allocation of metabolic functions; nevertheless, their operational performance and reproducibility may decline substantially when defined model substrates are replaced by chemically heterogeneous, non-sterile waste-derived substrates. The most reproducible performance is achieved when community composition is matched to feedstock chemistry and reactor operating conditions. Further progress will require a transition from empirical culture selection to substrate-specific engineering of microbial functions supported by metabolite profiling, molecular analyses, and mass- and electron-balance data.