100
M. Scarsini et al.
or S251N) resulted in the non-functionality of BKT1 in Chromochloris zofigiensis
(Ye and Huang 2019).
5.3.3 Fucoxanthin, an Abundant Allelic Carotenoid
Fucoxanthin is probably the most abundant carotenoid produced on Earth (Haugan
and Liaaen Jensen 1994). It is characterized by the presence of an allelic bond.
Beside its role in vivo as light-harvesting pigment, it shows a huge potential for
medical treatment (Gateau et al. 2017). Despite its important potential, the biochemical pathways along which it is produced in vivo are not yet elucidated. Three pathways have been proposed. The first one involves neoxanthin, the end carotenoid
product of the β-carotene to violaxanthin transformation. Neoxanthin would serve
as a substrate for fucoxanthin, diadinoxanthin or/and abscisic acid syntheses (Gong
and Bassi 2016). The second pathway is very similar to the first one but would
use diadinoxanthin as the precursor of fucoxanthin (Coesel et al. 2008). The 3rd
possibility differs from the second pathway by the steps transforming β-carotene to
violaxanthin that would involve β-cryptoxanthin and β-cryptoxanthin-5,6-epoxide
instead than zeaxanthin and antheraxanthin (Bertrand 2010) (Fig. 5.3). Cui et al.
(2019) and Kuczynska and Jemiola-Rzeminska (2017) disagreed on the activity of
the 3rd pathway in diatoms. The absence of this pathway is likely because when the
genes encoding CYP97b1 and CYP97b2 were cloned in a strain of Escherichia coli
producing β-carotene, only CYP97b1 showed an hydroxylase activity, producing
zeaxanthin and no β-cryptoxanthin (Cui et al. 2019).
No gene coding for BCH enzyme was found in the genome of Phaeodactylum
tricornutum CCAP 1055/1 and only a partial sequence was found in Thalassiosira pseudonana CCMP 1335 (Coesel et al. 2008; Gong and Bassi 2016). Interestingly, two genes coding for CYP97b or LUT-like hydroxylase P450 enzyme
(CYP97b1/LUT-like 1 and CYP97b2/LUT-like 2) have been predicted in Phaeodactylum tricornutum and might be involved in either pathways (Gong and Bassi
2016; Coesel et al. 2008). Recently, Cui et al. (2019) cloned CYP97b1 and CYP97b2
genes to study their cellular localization. Both enzymes were targeted in the
chloroplast but CYP97b2 could possibly also targeted in the cytoplasm.
5.4 Stress Controls Carotenoid Biosynthesis: Biochemical
and Molecular Control
Carotenoids are crucial molecules for the development of microalgal cells. Their
biosynthesis depends on the presence of adequate physico-chemical conditions in the
cell environment. For instance, when ions serving as cofactor for enzymes involved
in the carotenoid biosynthetic pathway are deficient, the carotenoid production is
M. Scarsini et al.
or S251N) resulted in the non-functionality of BKT1 in Chromochloris zofigiensis
(Ye and Huang 2019).
5.3.3 Fucoxanthin, an Abundant Allelic Carotenoid
Fucoxanthin is probably the most abundant carotenoid produced on Earth (Haugan
and Liaaen Jensen 1994). It is characterized by the presence of an allelic bond.
Beside its role in vivo as light-harvesting pigment, it shows a huge potential for
medical treatment (Gateau et al. 2017). Despite its important potential, the biochemical pathways along which it is produced in vivo are not yet elucidated. Three pathways have been proposed. The first one involves neoxanthin, the end carotenoid
product of the β-carotene to violaxanthin transformation. Neoxanthin would serve
as a substrate for fucoxanthin, diadinoxanthin or/and abscisic acid syntheses (Gong
and Bassi 2016). The second pathway is very similar to the first one but would
use diadinoxanthin as the precursor of fucoxanthin (Coesel et al. 2008). The 3rd
possibility differs from the second pathway by the steps transforming β-carotene to
violaxanthin that would involve β-cryptoxanthin and β-cryptoxanthin-5,6-epoxide
instead than zeaxanthin and antheraxanthin (Bertrand 2010) (Fig. 5.3). Cui et al.
(2019) and Kuczynska and Jemiola-Rzeminska (2017) disagreed on the activity of
the 3rd pathway in diatoms. The absence of this pathway is likely because when the
genes encoding CYP97b1 and CYP97b2 were cloned in a strain of Escherichia coli
producing β-carotene, only CYP97b1 showed an hydroxylase activity, producing
zeaxanthin and no β-cryptoxanthin (Cui et al. 2019).
No gene coding for BCH enzyme was found in the genome of Phaeodactylum
tricornutum CCAP 1055/1 and only a partial sequence was found in Thalassiosira pseudonana CCMP 1335 (Coesel et al. 2008; Gong and Bassi 2016). Interestingly, two genes coding for CYP97b or LUT-like hydroxylase P450 enzyme
(CYP97b1/LUT-like 1 and CYP97b2/LUT-like 2) have been predicted in Phaeodactylum tricornutum and might be involved in either pathways (Gong and Bassi
2016; Coesel et al. 2008). Recently, Cui et al. (2019) cloned CYP97b1 and CYP97b2
genes to study their cellular localization. Both enzymes were targeted in the
chloroplast but CYP97b2 could possibly also targeted in the cytoplasm.
5.4 Stress Controls Carotenoid Biosynthesis: Biochemical
and Molecular Control
Carotenoids are crucial molecules for the development of microalgal cells. Their
biosynthesis depends on the presence of adequate physico-chemical conditions in the
cell environment. For instance, when ions serving as cofactor for enzymes involved
in the carotenoid biosynthetic pathway are deficient, the carotenoid production is