Revised version BITE-D17-03462 Microalgal hydrogen production-a review 1 2

36 Bio-hydrogen from microalgae including cyanobacteria has attracted commercial awareness 37 due to its potential as an alternative, reliable and renewable energy source. Photosynthetic 38 hydrogen production from microalgae can be interesting and promising options for clean 39 energy. Advances in hydrogen-fuel-cell technology may attest an eco-friendly way of biofuel 40 production, since, the use of H2 to generate electricity releases only water as a by-product. 41 Progress in genetic/metabolic engineering may significantly enhance the photobiological 42 hydrogen production from microalgae. Manipulation of competing metabolic pathways by 43 modulating the certain key enzymes such as hydrogenase and nitrogenase may enhance the 44 evolution of H2 from photoautotrophic cells. Moreover, biological H2 production at low 45 operating costs is requisite for economic viability. Several photobioreactors have been 46 developed for large-scale biomass and hydrogen production. This review highlights the recent 47 technological progress, enzymes involved and genetic as well as metabolic engineering 48 approaches towards sustainable hydrogen production from microalgae. 49 50