Microalgae-derived biohydrogen as a sustainable fuel with advances in production pathways process optimisation and techno-economic assessment.

Biohydrogen synthesis from microalgae represents a promising sustainable energy pathway, attributable to the rapid growth, high photosynthetic efficiency, and carbon dioxide sequestration capabilities of microalgae. This review systematically synthesises and evaluates the principal biological hydrogen production pathways-direct and indirect bio photolysis, photo fermentation, dark fermentation, and microbial electrochemical systems-emphasizing their operational efficiency and recent advancements. Specialised focus is directed towards innovative strategies to augment hydrogen yields, including genetic engineering to elevate hydrogenase expression and refine photosynthetic electron flow, optimisation of culture conditions (light intensity, nutrient limitation, pH), and material-based improvements such as immobilisation and nanoparticle-mediated electron transfer. Genetic alterations have resulted in yield enhancements of up to 60%, whereas optimised cultivation can increase production by 2 to 4 times among microalgal strains. The paper provides an economic analysis of scalability problems and compares the costs of biohydrogen with those of conventional and other renewable hydrogen sources. A SWOT analysis consolidates essential technological strengths, weaknesses, opportunities, and threats, offering a definitive forward-looking framework. This thorough and current synthesis enhances the comprehension of microalgal biohydrogen production and delineates strategic approaches towards feasible, scalable solutions.