Biological Routes for Biohydrogen Production: A Clean and Carbon-Free Fuel.

Hydrogen (H) is a clean, renewable, and sustainable energy source that holds great promise as an alternative fuel and is expected to play a central role in the future transportation energy economy. However, the hydrogen yield from microorganisms remains insufficient, presenting a significant challenge. Biohydrogen (bio-H) production pathways are well established and can be categorized into four main processes: (1) direct biological photolysis of water by green algae; (2) indirect biological photolysis by cyanobacteria, a combination of green algae and photosynthetic microorganisms, or a separate two-stage photolysis using only green algae; (3) photo-fermentation by purple bacteria, photosynthetic bacteria, or fermentative bacteria; and (4) dark anaerobic fermentation by fermentative bacteria. Among these processes, dark fermentation shows great potential for practical applications, such as organic waste treatment. This review summarizes recent advances in bio-H production, including both fundamental research and applied studies.