a first cultivation stage using nitrogen- and iron-supplemented medium, followed by
a second cultivation stage in a nitrogen- and iron-free medium. The high starch
content obtained (up to 41.0% of dry cell weight) suggests C. vulgaris P12 as a very
promising feedstock for bioethanol production.
Carbohydrates are the major products derived from photosynthesis and the
carbon fixation metabolism (Calvin cycle), which are either accumulated in the
plastids as reserve materials (starch), or become the main component of cell walls
(cellulose, pectin, and sulfated polysaccharides). However, the composition and
metabolism of carbohydrates (mainly starch and cellulose) in microalgae may differ
significantly from species to species. Microalgae that contain glucose-based carbohydrates are the most feasible feedstock for bioethanol production (Chen et al.
2013). The cell walls of microalgae primarily consist of an inner cell wall layer and
an outer cell wall layer. The composition of the outer cell wall varies from species
to species, but usually contains specific polysaccharides, such as pectin, agar, and
alginate, while the inner cell wall layer is mainly composed of cellulose and other
materials. Table 3 shows the compositions of the cell walls and the storage products. For some microalgae, the glucose polymers produced via cellulose/starch are
the predominant component in the cell walls and stored products of microalgae.
Starch and most cell wall polysaccharides can be converted into fermentable sugars
for subsequent bioethanol production via microbial fermentation.
The accumulation of carbohydrates in microalgae is due to CO 2 fixation during
the photosynthetic process (Fig. 2). Photosynthesis is a biological process utilizing
ATP/NADPH to fix and convert CO 2 captured from the air to produce glucose and
other sugars through a metabolic pathway known as the Calvin cycle. The metabolic pathways of energy-rich molecules are closely linked. Some studies demonstrated that there was a competition between lipid and starch synthesis because the
major precursor for triacylglycerols synthesis is glycerol-3-phosphate (G3P), which
is produced via catabolism of glucose (glycolysis). Thus, to enhance biofuels’
production from microalgae-based carbohydrates, it is vital to understand and
manipulate the related metabolisms to achieve higher microalgal carbohydrate
Table 3 Composition of microalgal cell wall and storage products (Chen et al. 2013)
Division
Cell wall
Storage products
Cyanophyta
Lipopolysaccharides, peptidoglycan
Cyanophycean
starch
Chlorophyta
Cellulose, hemicellulose
Starch/lipid
Dinophyta
Absence or contain few cellulose
Starch
Cryptophyta
Periplast
Starch
Euglenophyta
Absence
Paramylum/lipid
Rhodophyta
Agar, carrageenan, cellulose, calcium carbonate
Floridean starch
Heterokontophyta
Naked or covered by scales or with large quantities
of silica
Leucosin/lipid
236
R. G. Bastos
a second cultivation stage in a nitrogen- and iron-free medium. The high starch
content obtained (up to 41.0% of dry cell weight) suggests C. vulgaris P12 as a very
promising feedstock for bioethanol production.
Carbohydrates are the major products derived from photosynthesis and the
carbon fixation metabolism (Calvin cycle), which are either accumulated in the
plastids as reserve materials (starch), or become the main component of cell walls
(cellulose, pectin, and sulfated polysaccharides). However, the composition and
metabolism of carbohydrates (mainly starch and cellulose) in microalgae may differ
significantly from species to species. Microalgae that contain glucose-based carbohydrates are the most feasible feedstock for bioethanol production (Chen et al.
2013). The cell walls of microalgae primarily consist of an inner cell wall layer and
an outer cell wall layer. The composition of the outer cell wall varies from species
to species, but usually contains specific polysaccharides, such as pectin, agar, and
alginate, while the inner cell wall layer is mainly composed of cellulose and other
materials. Table 3 shows the compositions of the cell walls and the storage products. For some microalgae, the glucose polymers produced via cellulose/starch are
the predominant component in the cell walls and stored products of microalgae.
Starch and most cell wall polysaccharides can be converted into fermentable sugars
for subsequent bioethanol production via microbial fermentation.
The accumulation of carbohydrates in microalgae is due to CO 2 fixation during
the photosynthetic process (Fig. 2). Photosynthesis is a biological process utilizing
ATP/NADPH to fix and convert CO 2 captured from the air to produce glucose and
other sugars through a metabolic pathway known as the Calvin cycle. The metabolic pathways of energy-rich molecules are closely linked. Some studies demonstrated that there was a competition between lipid and starch synthesis because the
major precursor for triacylglycerols synthesis is glycerol-3-phosphate (G3P), which
is produced via catabolism of glucose (glycolysis). Thus, to enhance biofuels’
production from microalgae-based carbohydrates, it is vital to understand and
manipulate the related metabolisms to achieve higher microalgal carbohydrate
Table 3 Composition of microalgal cell wall and storage products (Chen et al. 2013)
Division
Cell wall
Storage products
Cyanophyta
Lipopolysaccharides, peptidoglycan
Cyanophycean
starch
Chlorophyta
Cellulose, hemicellulose
Starch/lipid
Dinophyta
Absence or contain few cellulose
Starch
Cryptophyta
Periplast
Starch
Euglenophyta
Absence
Paramylum/lipid
Rhodophyta
Agar, carrageenan, cellulose, calcium carbonate
Floridean starch
Heterokontophyta
Naked or covered by scales or with large quantities
of silica
Leucosin/lipid
236
R. G. Bastos