Azadeh Mirvakili

Hydrogen production from microalgal biomass via dark fermentation (DF) has attracted considerable scientific attention as a suitable mechanism for renewable energy production within the framework of a circular bioeconomy. This paper systematically reviews contemporary advances in DF-mediated biohydrogen production using microalgae as feedstock, focusing on feedstock characteristics, pretreatment methods, process optimization, and innovative scale-up strategies. Microalgal species such as Chlorella sp. and Scenedesmus sp. have attracted attention due to their high productivity and beneficial biochemical profiles. However, the presence of strong cell walls, high harvesting costs, and limited carbohydrate bioavailability hinders the results of hydrogen production. This review emphasizes the pivotal importance of physical, chemical, biological, and combined pretreatment techniques in increasing substrate availability, along with the increasing use of nanoparticles and catalysts to enhance microbial performance and electron transfer during the fermentation process. Process integration strategies, including combining DF with downstream biorefineries and wastewater treatment processes, are described as potential methods to increase overall efficiency and economic profitability. Despite the observed improvements in hydrogen production capabilities, barriers related to energy consumption, inhibitor formation, scalability, and environmental safety remain. The paper concludes by stating that the realization of commercially viable microalgae-based DF systems requires low-energy pretreatment processes, engineered microbial consortia, safe and recyclable catalytic agents, as well as comprehensive techno-economic and life-cycle assessments to facilitate large-scale application.