total extractable glucose of 0.53–0.66 mg/mL/OD750 which could be used for
biohydrogen production. Recently, Patel et al. (2016) applied random mutagenesis
on Synechocystis PCC 6803 to develop strain with high biomass and carbohydrate
productivity. The mutant produced 3.6-fold more biomass and carbohydrate yield
of 225 mg/L, indicating its potential to be used as fermentative feedstock.
Finally, it could be inferred that advances made in genetic and metabolic
engineering have brought a major breakthrough in microalgal H 2 production process by overcoming several barriers associated with the low hydrogen yield. Indeed,
there are some other problems that must be resolved to increase the overall feasibility of the process. For instance, most of the studies on photobiological H 2 production are carried out at bench-scale photobioreactors (PBRs). Due to the data
scarcity, addressing several engineering issues for the scaling up of the PBR
becomes challenging (Fernández-Sevilla et al. 2014). Another major problem in H 2
production is the incomplete conversion of organic substrate into H 2 and CO 2 via
dark fermentation. H 2 production through this process is associated with the production of some soluble metabolites (volatile fatty acids and alcohols). This leads to
low gaseous energy recovery, and the spent media rich in organic acids may pose
threat to environment. To overcome this problem, an integrative system can be
devised where the effluent of dark fermentation can be integrated with anaerobic
digestion, photofermentation and bioelectrochemical systems (Sambusiti et al.
2015). Interestingly, volatile fatty acids rich spent media can be efficiently utilized
as substrate for the mixotrophic cultivation of microalgae (Ghosh et al. 2017).
Furthermore, utilization of wastewater grown and lipid/value-added product
extracted microalgae as feedstock for biohydrogen production and could make the
process more economically alluring.
3 Conclusion
Hydrogen production through biological routes is considered as the cleanest way of
renewable energy generation. Most of the green microalgae and cyanobacteria
possess novel metabolic features to carry out photobiological hydrogen evolution.
Moreover, microalgal biomass has great potential to be used as substrate for fermentative biohydrogen production. Nevertheless, an efficient and economical
method of biomass pretreatment is critical for carbohydrate saccharification and its
utilization by hydrogen-producing bacteria. Oxygen sensitivity of hydrogenases
and low photon conversion efficiency are two major bottlenecks of microalgal
hydrogen production via biophotolysis, while incomplete knowledge of carbohydrate metabolism presents a challenge for developing sugar-enriched microalgal
feedstock for dark fermentation. Although the application of genetic tools to
enhance the biohydrogen production from microalgae is currently in its infancy,
promising advances have been made to develop the genetically engineered
microalgae with unprecedented precision. It is likely that further research in this
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biohydrogen production. Recently, Patel et al. (2016) applied random mutagenesis
on Synechocystis PCC 6803 to develop strain with high biomass and carbohydrate
productivity. The mutant produced 3.6-fold more biomass and carbohydrate yield
of 225 mg/L, indicating its potential to be used as fermentative feedstock.
Finally, it could be inferred that advances made in genetic and metabolic
engineering have brought a major breakthrough in microalgal H 2 production process by overcoming several barriers associated with the low hydrogen yield. Indeed,
there are some other problems that must be resolved to increase the overall feasibility of the process. For instance, most of the studies on photobiological H 2 production are carried out at bench-scale photobioreactors (PBRs). Due to the data
scarcity, addressing several engineering issues for the scaling up of the PBR
becomes challenging (Fernández-Sevilla et al. 2014). Another major problem in H 2
production is the incomplete conversion of organic substrate into H 2 and CO 2 via
dark fermentation. H 2 production through this process is associated with the production of some soluble metabolites (volatile fatty acids and alcohols). This leads to
low gaseous energy recovery, and the spent media rich in organic acids may pose
threat to environment. To overcome this problem, an integrative system can be
devised where the effluent of dark fermentation can be integrated with anaerobic
digestion, photofermentation and bioelectrochemical systems (Sambusiti et al.
2015). Interestingly, volatile fatty acids rich spent media can be efficiently utilized
as substrate for the mixotrophic cultivation of microalgae (Ghosh et al. 2017).
Furthermore, utilization of wastewater grown and lipid/value-added product
extracted microalgae as feedstock for biohydrogen production and could make the
process more economically alluring.
3 Conclusion
Hydrogen production through biological routes is considered as the cleanest way of
renewable energy generation. Most of the green microalgae and cyanobacteria
possess novel metabolic features to carry out photobiological hydrogen evolution.
Moreover, microalgal biomass has great potential to be used as substrate for fermentative biohydrogen production. Nevertheless, an efficient and economical
method of biomass pretreatment is critical for carbohydrate saccharification and its
utilization by hydrogen-producing bacteria. Oxygen sensitivity of hydrogenases
and low photon conversion efficiency are two major bottlenecks of microalgal
hydrogen production via biophotolysis, while incomplete knowledge of carbohydrate metabolism presents a challenge for developing sugar-enriched microalgal
feedstock for dark fermentation. Although the application of genetic tools to
enhance the biohydrogen production from microalgae is currently in its infancy,
promising advances have been made to develop the genetically engineered
microalgae with unprecedented precision. It is likely that further research in this
222
H. Singh and D. Das