Biohydrogen Production Using Microalgae: A Comprehensive Review of Mechanisms, Optimization Strategies, and Future Prospects

The escalating global demand for energy and the pressing need to mitigate climate change have intensified interest in sustainable alternatives to fossil fuels. Biohydrogen, produced through photobiological processes in microalgae, presents a compelling renewable energy solution owing to its high energy density and near-zero carbon emissions. This review comprehensively examines the current state of biohydrogen production using microalgae, with particular focus on Chlorella vulgaris and Scenedesmus obliquus—two high-performing species extensively documented in the literature. Key production pathways examined include direct biophotolysis, indirect biophotolysis, and photo-fermentation. Critical operational parameters governing hydrogen evolution efficiency are reviewed in detail, including light intensity, wavelength (blue and purple spectra), temperature, pH, and sulfur deprivation strategies. The review synthesizes published experimental findings on hydrogen yields, reactor design principles, and anaerobic management strategies. The current body of evidence underscores the potential of algal biohydrogen as a scalable, low-carbon energy vector, while identifying critical research gaps—including hydrogenase oxygen sensitivity, process scalability, and genetic optimization—that must be addressed to enable commercial viability.