autoclaved and sonicated algal biomass. Diluted acid in combination with autoclaving is the most commonly used method for the pretreatment of microalgal
feedstock due to its simple operation and high yield of reducing sugar (Nguyen
et al. 2010; Liu et al. 2012; Kumar et al. 2013; Roy et al. 2014).
Biological pretreatment by enzymes is considered as an efficient method for the
hydrolysis of microalgal biomass due to the high substrate specificity, milder
operating conditions, less energy consumption and no generation of inhibitory
by-products. The effectiveness of this method depends upon the substrate characteristics, enzyme dosage, temperature, pH and treatment duration. For hydrolysis,
selection of enzyme is based on the microalgal cell wall composition. The commonly used enzymes for microalgal pretreatment include cellulases, a-amylases,
amyloglucosidases, xylanases and proteases (Hom-Diaz et al. 2016). The pretreatment of biomass by biological method is usually carried out after physical or
chemical method. Cheng et al. (2014) studied the combined effect of cellulase and
glucoamylase on the reducing sugar yield from acid–heat and alkali–heat pretreated
algal biomass. In combination both enzymes gave better sugar yield than cellulase
alone. To increase the conversion of starch, Nguyen et al. (2010) carried out the
enzymatic hydrolysis of C. reinhardtii biomass by utilizing Termamyl (endoglucanase) enzyme. Under the optimized enzymatic hydrolysis condition, maximum
H 2 yield of 2.5 mol/mol glucose equivalent was achieved via separate hydrolysis
and fermentation (SHF) process. However, pure enzymes are expensive and use of
such enzymes for biomass pretreatment makes the H 2 production process economically unattractive. Therefore, beside commercial enzymes, bacterial or fungal
crude enzymes can be used as cheaper alternative for microalgal biomass pretreatment. Many bacterial and fungal species possess the unique ability of producing wide variety of extracellular hydrolytic enzymes. Prajapati et al. (2015)
reported that crude enzyme obtained from Aspergillus lentulus can efficiently solubilize the microalgal sugars. Soluble sugar concentration of 57 mg/L and 29%
COD solubilization were obtained when biomass of Chroococcus sp. was pretreated by the fungal crude enzyme concentration of 20% v/v. Nevertheless, biological pretreatment is a green approach of obtaining high sugar yield from
microalgal biomass, and the lower rate of hydrolysis makes this process time
consuming and unsuitable for commercialization. Research on fermentative
hydrogen production from microalgal feedstock has just started, and most of the
studies have been conducted in batch systems. The main findings on biohydrogen
production using microalgal biomass as substrate are presented in Table 2.
2.3 Molecular Approaches Towards Improvement
in Biohydrogen Production from Microalgae
Production of H 2 from microalgae is an attractive process, although this renewable
energy system is limited by low H 2 yield and productivity. There are several
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H. Singh and D. Das
feedstock due to its simple operation and high yield of reducing sugar (Nguyen
et al. 2010; Liu et al. 2012; Kumar et al. 2013; Roy et al. 2014).
Biological pretreatment by enzymes is considered as an efficient method for the
hydrolysis of microalgal biomass due to the high substrate specificity, milder
operating conditions, less energy consumption and no generation of inhibitory
by-products. The effectiveness of this method depends upon the substrate characteristics, enzyme dosage, temperature, pH and treatment duration. For hydrolysis,
selection of enzyme is based on the microalgal cell wall composition. The commonly used enzymes for microalgal pretreatment include cellulases, a-amylases,
amyloglucosidases, xylanases and proteases (Hom-Diaz et al. 2016). The pretreatment of biomass by biological method is usually carried out after physical or
chemical method. Cheng et al. (2014) studied the combined effect of cellulase and
glucoamylase on the reducing sugar yield from acid–heat and alkali–heat pretreated
algal biomass. In combination both enzymes gave better sugar yield than cellulase
alone. To increase the conversion of starch, Nguyen et al. (2010) carried out the
enzymatic hydrolysis of C. reinhardtii biomass by utilizing Termamyl (endoglucanase) enzyme. Under the optimized enzymatic hydrolysis condition, maximum
H 2 yield of 2.5 mol/mol glucose equivalent was achieved via separate hydrolysis
and fermentation (SHF) process. However, pure enzymes are expensive and use of
such enzymes for biomass pretreatment makes the H 2 production process economically unattractive. Therefore, beside commercial enzymes, bacterial or fungal
crude enzymes can be used as cheaper alternative for microalgal biomass pretreatment. Many bacterial and fungal species possess the unique ability of producing wide variety of extracellular hydrolytic enzymes. Prajapati et al. (2015)
reported that crude enzyme obtained from Aspergillus lentulus can efficiently solubilize the microalgal sugars. Soluble sugar concentration of 57 mg/L and 29%
COD solubilization were obtained when biomass of Chroococcus sp. was pretreated by the fungal crude enzyme concentration of 20% v/v. Nevertheless, biological pretreatment is a green approach of obtaining high sugar yield from
microalgal biomass, and the lower rate of hydrolysis makes this process time
consuming and unsuitable for commercialization. Research on fermentative
hydrogen production from microalgal feedstock has just started, and most of the
studies have been conducted in batch systems. The main findings on biohydrogen
production using microalgal biomass as substrate are presented in Table 2.
2.3 Molecular Approaches Towards Improvement
in Biohydrogen Production from Microalgae
Production of H 2 from microalgae is an attractive process, although this renewable
energy system is limited by low H 2 yield and productivity. There are several
216
H. Singh and D. Das