Emerging technologies for sustainable production of biohydrogen production from microalgae: A state-of-the-art review of upstream and downstream processes.

Biohydrogen (BioH) is considered as one of the most environmentally friendly fuels and a strong candidate to meet the future demand for a sustainable source of energy. Presently, the production of BioH from photosynthetic organisms has raised a lot of hopes in the fuel industry. Moreover, microalgal-based BioH synthesis not only helps to combat current global warming by capturing greenhouse gases but also plays a key role in wastewater treatment. Hence, this manuscript provides a state-of-the-art review of the upstream and downstream BioH production processes. Different metabolic routes such as direct and indirect photolysis, dark fermentation, photofermentation, and microbial electrolysis are covered in detail. Upstream processes (e.g. growth techniques, growth media) also have a great impact on BioH productivity and economics, which is also explored. Technical and scientific obstacles of microalgae BioH systems are finally addressed, allowing the technology to become more innovative and commercial.