Biohydrogen from microalgae

Nowadays, it becomes necessary to consider future fuel alternatives due to the increasing energy demand and the exhaustion of conventional fossil energy sources. Microalgae have attracted considerable interest in the production of biomass, biofuels, and high-value products. Microalgae produce hydrogen by the splitting of water and primarily transferring the electrons to protons instead of reducing CO2. As an energy carrier, hydrogen is considered one of the most promising alternatives due to its high-energy content and carbon-free combustion properties. This chapter focuses on key aspects of hydrogen production by microalgae, from the chemical, metabolic, and technical perspectives. An overview of the fundamental concepts about direct and indirect biophotolyses and the biological barriers to obtaining high-yield H2 will be covered, including the metabolic and genetic engineering approaches used to overcome these bottlenecks. The most common photobioreactor configurations employed will be discussed, but also cultivation strategies, key operational parameters, and scale-up challenges. On the contrary, the production of hydrogen by dark fermentation using microalgal biomass as feedstock from the stoichiometric and energetic points of view will be reviewed. The different biomass pretreatments to achieve high sugar content of hydrolysates and the conditions for enhancing hydrogen production, considering the importance of the inoculum, will be included. Also, the advantages and disadvantages of different reactor configurations from a biorefinery perspective will be covered. Finally, concluding remarks will be provided, assessing the future challenges and opportunities for the production of biohydrogen from microalgae.