272
R. Halim
under the same taxon generally sharing common cell wall features and structures
(Baudelet et al. 2017; Gerken et al. 2013).
The cell walls of green microalgae are known to have rigid wall components
embedded within a plastic polymer-based matrix (Baudelet et al. 2017; Gerken et al.
2013). The rigid fraction is resistant to hydrolysis by either sodium hydroxide or
trifluoroacetic acid (TFA) and is composed of glucosamine-derived polymer (such as
chitin), glucose-mannose polysaccharides or other biopolymers (such as algaenan)
(Baudelet et al. 2017; Gerken et al. 2013). The plastic matrix is hydrolysable by
sodium hydroxide or TFA and is generally composed of uronic acids and neutral
sugars, such as rhamnose, arabinose, fucose, xylose, mannose, galactose or glucose
(Baudelet et al. 2017; Gerken et al. 2013).
Chitin is a crystalline polymer commonly found in shellfish, the exoskeletons of
crustaceans, fish scales and the cell walls of fungi. Algaenan is a non-hydrolyzable
biopolymer that is commonly found in the rigid fraction of microalgal cell walls
across different taxonomical groups (Baudelet et al. 2017; Gerken et al. 2013). It
normally forms part of the trilaminar structure (TLS) that is located in the thin
outer layer of the cell wall (10–20 nm). The biopolymer is able to withstand harsh
acid/alkali hydrolyses and energy-intensive mechanical processes. The presence of
algaenan in microalgal cell wall has often been postulated to be the reason for the
wall’s recalcitrance to cell rupture and resistance to bacterial degradation. Even
though its composition is yet to be fully elucidated, algaenan has been shown to
comprise long, straight-chain, saturated aliphatic compounds (~ C30) that are joined
together with ether cross linkages to form a structure closely resembling that of cutan
in vascular plants (Baudelet et al. 2017; Gerken et al. 2013).
12.1.3.1 Cell Wall of Haematococcus Pluvialis
Under nutrient deprivation, H.pluvialis cells enter encystment stage and begin to
accumulate starch, lipid droplets and astaxanthin. During this stage, the cells increase
their volumes and transform themselves from an ellipsoidal flagellate to a spherical
red cyst (also known as aplanospore) (Baudelet et al. 2017; Desai et al. 2016).
The cells also synthesise a robust and physically resilient multi-layered cell wall
(Fig. 12.1a) as part of the encystment process. This new cell wall (up to 16 wt%
of the biomass) consists of an outer trilaminar sheath (TLS) layer and a thick inner
secondary wall. The trilaminar sheath is composed of algaenan, while the secondary
wall is made of non-fibrillar mannan polymers (Baudelet et al. 2017; Desai et al.
2016). As mentioned above, algaenan is a long-chain alipathic hydrocarbon that is
able to withstand harsh oxidative treatments and resists acid/alkali hydrolyses. It is
this algaenan that confers H.pluvialis cell walls with rigidity and recalcitrance to
breakage (Baudelet et al. 2017; Desai et al. 2016). In terms of mass composition,
carbohydrate accounts for 70 wt% of the cyst cell wall, while protein and acetolysisresistant materials (algaenan) make up 6 and 3 wt% of the cell wall respectively.
Mannose is the principal monosaccharide of the cell wall (89 wt% of carbohydrates).
Glucose, arabinose and xylose are also present in smaller quantities in the cell wall
R. Halim
under the same taxon generally sharing common cell wall features and structures
(Baudelet et al. 2017; Gerken et al. 2013).
The cell walls of green microalgae are known to have rigid wall components
embedded within a plastic polymer-based matrix (Baudelet et al. 2017; Gerken et al.
2013). The rigid fraction is resistant to hydrolysis by either sodium hydroxide or
trifluoroacetic acid (TFA) and is composed of glucosamine-derived polymer (such as
chitin), glucose-mannose polysaccharides or other biopolymers (such as algaenan)
(Baudelet et al. 2017; Gerken et al. 2013). The plastic matrix is hydrolysable by
sodium hydroxide or TFA and is generally composed of uronic acids and neutral
sugars, such as rhamnose, arabinose, fucose, xylose, mannose, galactose or glucose
(Baudelet et al. 2017; Gerken et al. 2013).
Chitin is a crystalline polymer commonly found in shellfish, the exoskeletons of
crustaceans, fish scales and the cell walls of fungi. Algaenan is a non-hydrolyzable
biopolymer that is commonly found in the rigid fraction of microalgal cell walls
across different taxonomical groups (Baudelet et al. 2017; Gerken et al. 2013). It
normally forms part of the trilaminar structure (TLS) that is located in the thin
outer layer of the cell wall (10–20 nm). The biopolymer is able to withstand harsh
acid/alkali hydrolyses and energy-intensive mechanical processes. The presence of
algaenan in microalgal cell wall has often been postulated to be the reason for the
wall’s recalcitrance to cell rupture and resistance to bacterial degradation. Even
though its composition is yet to be fully elucidated, algaenan has been shown to
comprise long, straight-chain, saturated aliphatic compounds (~ C30) that are joined
together with ether cross linkages to form a structure closely resembling that of cutan
in vascular plants (Baudelet et al. 2017; Gerken et al. 2013).
12.1.3.1 Cell Wall of Haematococcus Pluvialis
Under nutrient deprivation, H.pluvialis cells enter encystment stage and begin to
accumulate starch, lipid droplets and astaxanthin. During this stage, the cells increase
their volumes and transform themselves from an ellipsoidal flagellate to a spherical
red cyst (also known as aplanospore) (Baudelet et al. 2017; Desai et al. 2016).
The cells also synthesise a robust and physically resilient multi-layered cell wall
(Fig. 12.1a) as part of the encystment process. This new cell wall (up to 16 wt%
of the biomass) consists of an outer trilaminar sheath (TLS) layer and a thick inner
secondary wall. The trilaminar sheath is composed of algaenan, while the secondary
wall is made of non-fibrillar mannan polymers (Baudelet et al. 2017; Desai et al.
2016). As mentioned above, algaenan is a long-chain alipathic hydrocarbon that is
able to withstand harsh oxidative treatments and resists acid/alkali hydrolyses. It is
this algaenan that confers H.pluvialis cell walls with rigidity and recalcitrance to
breakage (Baudelet et al. 2017; Desai et al. 2016). In terms of mass composition,
carbohydrate accounts for 70 wt% of the cyst cell wall, while protein and acetolysisresistant materials (algaenan) make up 6 and 3 wt% of the cell wall respectively.
Mannose is the principal monosaccharide of the cell wall (89 wt% of carbohydrates).
Glucose, arabinose and xylose are also present in smaller quantities in the cell wall