Metabolic pathways for microalgal biohydrogen production: Current progress and future prospectives

Biohydrogen Production From Biomass Sources: Metabolic Pathways and Economic Analysis

“…The dark and anoxic environment induce ([Fe]-H 2 ase) and help to produce a high yield of biohydrogen. When DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) is present, incubating microalgae culture in these conditions is beneficial for biohydrogen production (El-Dalatony et al, 2020;Mu et al, 2020). Compared to dark fermentation, photofermentation is considered a better process to produce biohydrogen.…”

“…In the process of dark fermentation, biohydrogen is produced by two mechanisms. One of them is by a catabolic transformation of formic acid while the other one is through re-oxidation of nicotinamide adenine dinucleotide hydride (NADH), which is catalysed by the H 2 ase pathway (El-Dalatony et al, 2020). Pyruvate ferredoxin oxidoreductase (PFR) is used to catalyse pyruvate oxidation.…”

“…To enhance the performance of the dark fermentation process, Song et al (2020) employed enzymolysis and acid pretreatment to discharge reducing sugars from A. philoxeroides. The reducing FIGURE 1 | Metabolic pathways of biohydrogen production by micro-algal biomass, modified from El-Dalatony et al (2020). These are mainly classified into three categories: i) the photobiological process through which biohydrogen is produced via direct and indirect photolysis in the microalgae; ii) fermentation; and iii) the electrochemical process that comprises photoelectrochemical and electrolytic.…”

“…Under these conditions, microalgae can produce biohydrogen through fermentation or respiration. The indirect method is not continuous as a return of the light period makes the growth become photosynthetic and hinders H 2 ase (El-Dalatony et al, 2020). Indirect biophotolysis is a two-step process: i) the first stage involves oxygen producing and carbon dioxide fixing into chemical energy carbohydrates, and lipids; and ii) the second stage is functionally the same and a small sealed photo-bioreactor is used with CO 2 synthesis which is divided periodically in the absence of light exposure (Razu et al, 2019).…”

“…Nevertheless, difficulties in process engineering, low productivity of microalgae, oxygen sensitivity, and operational costs, and inadequate insights on strain capacity, hinder the commercialization of microalgae-based biohydrogen production. Even though there are several metabolic routes to produce biohydrogen from microalgae with an improved yield, recent reviews (Buitrón et al, 2017;Sharma and Arya, 2017;Shobana et al, 2017;Show et al, 2018;Razu et al, 2019;Show et al, 2019b;El-Dalatony et al, 2020;Jiménez-Llanos et al, 2020;Mona et al, 2020;Salakkam et al, 2021) have not provided the comprehensive economic evaluation of such strategies, which has resulted in a dearth in the understanding of process feasibility. Therefore, in addressing this knowledge gap, this mini-review explicates the metabolic routes that enhance microalgae-based biohydrogen production and provides insights into their economic considerations to guide future work in the scaling-up of these technologies.…”