glucose transporter. The insertion of HUP1 facilitated the import of glucose
(1 mM) inside the stm6 cells. The transformed C. reinhardtii stm6Glc4 produced
H 2 by simultaneously utilizing the water (66%) and glucose (33%) and showed
fivefold increase in H 2 production than wild type (Doebbe et al. 2007).
2.7 Carbohydrate Metabolism of Microalgae
Carbohydrate-rich microalgal biomass is a suitable substrate for the fermentative H 2
production. The synthesis and accumulation of carbohydrates in microalgae occur
due to CO 2 fixation, through a cyclic metabolic pathway known as Calvin cycle.
CO 2 is reduced at the expense of ATP and NADPH generated during the
light-dependent reaction of photosynthesis. In microalgae, the biosynthetic and
catabolic pathways of energy storage molecules (starch and lipid) are closely
linked. Some research findings suggest that a competition exist for the allocation of
microalgal carbon between the carbohydrate and lipid synthesis (Rismani-Yazdi
et al. 2011; Ho et al. 2012). Moreover, starch degradation provides main precursor
(glycerol-3-phosphate, G3P) for triacylglycerol (TAG) synthesis. Thus, to enhance
the biohydrogen production from microalgal feedstock, understanding and
manipulating the starch metabolism become vital. The rate-limiting step in carbohydrate synthesis is catalysed by the enzyme ADP-glucose pyrophosphorylase
(AGPase). An allosteric activator of AGPase is 3-phosphoglyceric acid (3-PGA)
which is the intermediate product of CO 2 fixation reaction. Therefore, enhancing
the photosynthetic efficiency might prove helpful to improve the carbohydrate
synthesis and accumulation. In some studies, genetic modification in the RuBisCO
subunits increased the photosynthetic efficiency of Chlamydomonas (Genkov et al.
2010; Zhu et al. 2010). An alternative strategy to enhance the microalgal starch
accumulation is to decrease the starch degradation. The mechanism of microalgal
carbohydrate catabolism is not completely understood, but it is well inferred in
Arabidopsis thaliana. Phosphorolytic and/or hydrolytic enzymes play major role in
starch degradation mechanism. Targeting these enzymes for gene knockout probably helps in developing microalgae with desirable phenotype (high carbohydrate
content) (Radakovits et al. 2010). Except the starch stored in plastids, carbohydrates
in algae are also found entrapped within the cell wall mainly in form of cellulose.
The process of cellulose biosynthesis is complicated and involves several enzymatic
reactions. It is synthesized by cellulase synthase utilizing UDP-glucose as precursor
(Chen et al. 2013).
Due to the poor understanding of carbon partitioning between the biosynthetic
pathways of energy-rich molecules, in comparison with genetic engineering, process engineering methodologies have greatly helped in the increment of microalgal
carbohydrate content. However, few studies with molecular approaches have been
carried out in cyanobacteria. In one such study, to enhance the cellulose yield, the
genes for cellulose synthesis (acsAB) were transferred from A. xylinum into
Anabaena sp. PCC 7120 via conjugation (Su et al. 2011). The mutant produced
10 Biofuels from Microalgae: Biohydrogen
221
(1 mM) inside the stm6 cells. The transformed C. reinhardtii stm6Glc4 produced
H 2 by simultaneously utilizing the water (66%) and glucose (33%) and showed
fivefold increase in H 2 production than wild type (Doebbe et al. 2007).
2.7 Carbohydrate Metabolism of Microalgae
Carbohydrate-rich microalgal biomass is a suitable substrate for the fermentative H 2
production. The synthesis and accumulation of carbohydrates in microalgae occur
due to CO 2 fixation, through a cyclic metabolic pathway known as Calvin cycle.
CO 2 is reduced at the expense of ATP and NADPH generated during the
light-dependent reaction of photosynthesis. In microalgae, the biosynthetic and
catabolic pathways of energy storage molecules (starch and lipid) are closely
linked. Some research findings suggest that a competition exist for the allocation of
microalgal carbon between the carbohydrate and lipid synthesis (Rismani-Yazdi
et al. 2011; Ho et al. 2012). Moreover, starch degradation provides main precursor
(glycerol-3-phosphate, G3P) for triacylglycerol (TAG) synthesis. Thus, to enhance
the biohydrogen production from microalgal feedstock, understanding and
manipulating the starch metabolism become vital. The rate-limiting step in carbohydrate synthesis is catalysed by the enzyme ADP-glucose pyrophosphorylase
(AGPase). An allosteric activator of AGPase is 3-phosphoglyceric acid (3-PGA)
which is the intermediate product of CO 2 fixation reaction. Therefore, enhancing
the photosynthetic efficiency might prove helpful to improve the carbohydrate
synthesis and accumulation. In some studies, genetic modification in the RuBisCO
subunits increased the photosynthetic efficiency of Chlamydomonas (Genkov et al.
2010; Zhu et al. 2010). An alternative strategy to enhance the microalgal starch
accumulation is to decrease the starch degradation. The mechanism of microalgal
carbohydrate catabolism is not completely understood, but it is well inferred in
Arabidopsis thaliana. Phosphorolytic and/or hydrolytic enzymes play major role in
starch degradation mechanism. Targeting these enzymes for gene knockout probably helps in developing microalgae with desirable phenotype (high carbohydrate
content) (Radakovits et al. 2010). Except the starch stored in plastids, carbohydrates
in algae are also found entrapped within the cell wall mainly in form of cellulose.
The process of cellulose biosynthesis is complicated and involves several enzymatic
reactions. It is synthesized by cellulase synthase utilizing UDP-glucose as precursor
(Chen et al. 2013).
Due to the poor understanding of carbon partitioning between the biosynthetic
pathways of energy-rich molecules, in comparison with genetic engineering, process engineering methodologies have greatly helped in the increment of microalgal
carbohydrate content. However, few studies with molecular approaches have been
carried out in cyanobacteria. In one such study, to enhance the cellulose yield, the
genes for cellulose synthesis (acsAB) were transferred from A. xylinum into
Anabaena sp. PCC 7120 via conjugation (Su et al. 2011). The mutant produced
10 Biofuels from Microalgae: Biohydrogen
221