266
R. Halim
12.1 Overview of Microalgal Pigments, Pigment Extraction
and Cell Walls
12.1.1 Pigment Class and Solubility
Microalgal pigments can be classified into 3 different groups: chlorophylls,
carotenoids and phycobiliproteins (Cuellar-Bermudez et al. 2015; Khanra et al.
2018). Chlorophylls are green photosynthetic pigments found universally in all
microalgal and cyanobacterial species. Chlorophyll molecules have a porphyrin ring
coordinated to a central magnesium ion as their basic structure. Chlorophylls are
soluble in organic solvents (such as methanol, ethanol, acetone) and supercritical
carbon dioxide.
Carotenoids are a group of accessory pigments made of linear hydrocarbon
polyenes (some of which have oxygenated derivatives) that are able to absorb light
between 400 and 550 nm (Cuellar-Bermudez et al. 2015; Khanra et al. 2018).
Carotenoids are divided into separate classes based on their molecular structure and
functionality. In terms of molecular structure, carotenoids composed of a pure hydrocarbon chain are classified as carotenes (e.g. β-carotene), while those with molecules
that include oxygenated derivatives (such as–OH and–CO substitutes) are classified as xanthophylls (e.g. astaxanthin). In terms of functionality, carotenoids can be
grouped as either primary or secondary carotenoids (Cuellar-Bermudez et al. 2015;
Khanra et al. 2018). Primary carotenoids (such as β-carotene, lutein and violaxanthin) transfer absorbed light energy to chlorophylls and thus act as energy harvesters
that expand the light-absorbing spectrum of the cells. On the other hand, secondary
carotenoids (such as astaxanthin and canthaxanthin) serve to protect chlorophylls
from photo-damage by forming a protective layer over the microalgal cells upon
exposure to extreme light intensities (Cuellar-Bermudez et al. 2015; Khanra et al.
2018). The synthesis of secondary carotenoids in the cells generally intensifies under
environmental stresses. All carotenoids demonstrate strong antioxidant properties
and are able to scavenge reactive oxygen species (ROS) to protect the cells from
free radical attacks and oxidation under environmental stresses (Cuellar-Bermudez
et al. 2015; Khanra et al. 2018). Even though a microalgal species may have a
number of different carotenoids, it normally has a dominant carotenoid (between
70—80wt% of its total carotenoid content) (Cuellar-Bermudez et al. 2015; Khanra
et al. 2018). Research works on the species are thus often focused on the recovery
of this primary carotenoid (β-carotene from D.salina, astaxanthin from H. pluvialis, lutein from Scenedesmus sp.). Similar to chlorophylls, carotenoids are soluble
in organic solvents (such as methanol, ethanol, acetone) and supercritical carbon
dioxide.
Phycobiliproteins are coloured antennae-protein pigments bound to supramolecular complexes (known as phycobillisomes) on the outer surface of the thylakoid
membranes. The pigments absorb energy in the visible light spectrum (450—650 nm)
and function as accessory pigments that enhance cellular light collection for photosynthesis (Cuellar-Bermudez et al. 2015; Khanra et al. 2018; Rammuni et al. 2019).
R. Halim
12.1 Overview of Microalgal Pigments, Pigment Extraction
and Cell Walls
12.1.1 Pigment Class and Solubility
Microalgal pigments can be classified into 3 different groups: chlorophylls,
carotenoids and phycobiliproteins (Cuellar-Bermudez et al. 2015; Khanra et al.
2018). Chlorophylls are green photosynthetic pigments found universally in all
microalgal and cyanobacterial species. Chlorophyll molecules have a porphyrin ring
coordinated to a central magnesium ion as their basic structure. Chlorophylls are
soluble in organic solvents (such as methanol, ethanol, acetone) and supercritical
carbon dioxide.
Carotenoids are a group of accessory pigments made of linear hydrocarbon
polyenes (some of which have oxygenated derivatives) that are able to absorb light
between 400 and 550 nm (Cuellar-Bermudez et al. 2015; Khanra et al. 2018).
Carotenoids are divided into separate classes based on their molecular structure and
functionality. In terms of molecular structure, carotenoids composed of a pure hydrocarbon chain are classified as carotenes (e.g. β-carotene), while those with molecules
that include oxygenated derivatives (such as–OH and–CO substitutes) are classified as xanthophylls (e.g. astaxanthin). In terms of functionality, carotenoids can be
grouped as either primary or secondary carotenoids (Cuellar-Bermudez et al. 2015;
Khanra et al. 2018). Primary carotenoids (such as β-carotene, lutein and violaxanthin) transfer absorbed light energy to chlorophylls and thus act as energy harvesters
that expand the light-absorbing spectrum of the cells. On the other hand, secondary
carotenoids (such as astaxanthin and canthaxanthin) serve to protect chlorophylls
from photo-damage by forming a protective layer over the microalgal cells upon
exposure to extreme light intensities (Cuellar-Bermudez et al. 2015; Khanra et al.
2018). The synthesis of secondary carotenoids in the cells generally intensifies under
environmental stresses. All carotenoids demonstrate strong antioxidant properties
and are able to scavenge reactive oxygen species (ROS) to protect the cells from
free radical attacks and oxidation under environmental stresses (Cuellar-Bermudez
et al. 2015; Khanra et al. 2018). Even though a microalgal species may have a
number of different carotenoids, it normally has a dominant carotenoid (between
70—80wt% of its total carotenoid content) (Cuellar-Bermudez et al. 2015; Khanra
et al. 2018). Research works on the species are thus often focused on the recovery
of this primary carotenoid (β-carotene from D.salina, astaxanthin from H. pluvialis, lutein from Scenedesmus sp.). Similar to chlorophylls, carotenoids are soluble
in organic solvents (such as methanol, ethanol, acetone) and supercritical carbon
dioxide.
Phycobiliproteins are coloured antennae-protein pigments bound to supramolecular complexes (known as phycobillisomes) on the outer surface of the thylakoid
membranes. The pigments absorb energy in the visible light spectrum (450—650 nm)
and function as accessory pigments that enhance cellular light collection for photosynthesis (Cuellar-Bermudez et al. 2015; Khanra et al. 2018; Rammuni et al. 2019).