106
M. Scarsini et al.
Beside the visible light, nonvisible light such as UV radiations, alone or/and as
supplementing PAR can enhance carotenogenesis when compared with visible light.
In Dunaliella salina and Haematococcus pluvialis, UV-C (200–280 nm) irradiation
triggers the accumulation of secondary carotenoids, a phenomenon that is accompanied by the typical events of cell settlement i.e. detachment of the flagella and cell
volume increase (Sharma et al. 2015). The studies performed with Dunaliella salina
revealed that the UV-A (320–400 nm) effect might be related to the presence of a
specific photoreceptor because neither UV-B (290–320 nm) nor blue light was as
effective in triggering β-carotene accumulation (Jahnke 1999).
5.4.1.5 The Availability of Other Biochemical Pathways
As already explained, carotenoids are made of carbon atoms and therefore their
accumulation requires a functional carbon metabolism. Indeed, this is crucial for the
accumulation of astaxanthin molecules because nearly all the astaxanthin molecules
that accumulate are esterified with one or two fatty acids (Haematococcus pluvialis:
Lemoine et al. (2008); Schoefs et al. (2001); Scenedesmus sp.: Aburai et al. (2015),
Chromochloris zofingiensis: Zhang et al. (2016)). Several reports have established
that an active tricarboxylic acid cycle (Wang et al. 2014; Li et al. 2017; Wu et al. 2013;
Recht et al. 2012, 2014; Zhekisheva et al. 2002) and an active biosynthetic pathway
are sine qua non conditions for astaxanthin accumulation in Haematococcus pluvialis (Gwak et al. 2014; Chen et al. 2015; Lemoine et al. 2008; Schoefs et al. 2001;
Hu et al. 2019). However, strains such as Coelastrum sp. HA-1 (Liu et al. 2013b)
and Ankistrodesmus sp. (Kopecky et al. 2000) that accumulate 50% and nearly all
their astaxanthin as a nonesterified form, respectively, have been described. This
suggests that the mandatory character of the activity of the fatty acid biosynthetic
pathway is relative. To get further insight in this biochemical control, Liu et al.
(2020) studied the impact of externally added linoleic acid on the astaxanthin esterification in Coelastrum sp. HA-1. Interestingly, the presence of exogenously added
linoleic acid downregulated the fatty acid biosynthetic pathway while stimulating the
astaxanthin esterification. The authors concluded that the presence of a high level
of nonesterified astaxanthin in this taxon results of a low activity of the fatty acid
biosynthetic pathway and suggests that the regulation could operate through a fatty
acid threshold that would be reached to allow astaxanthin accumulation. To obtain
further insights on the involvement of the primary metabolism during the process of
in vivo astaxanthin accumulation in Haematococcus pluvialis, Hu et al. (2020) used
a metabolic approach that has revealed changes in the level of numerous metabolite
including nucleotides, organic acids etc. (Table 5.9). In Chromochloris zofingiensis,
another taxon accumulating astaxanthin, transcriptome analyses revealed the upregulation of the genes coding for astaxanthin biosynthesis enzymes whereas the genes
coding enzymes involved in side pathways were down-regulated (Huang et al. 2016).
M. Scarsini et al.
Beside the visible light, nonvisible light such as UV radiations, alone or/and as
supplementing PAR can enhance carotenogenesis when compared with visible light.
In Dunaliella salina and Haematococcus pluvialis, UV-C (200–280 nm) irradiation
triggers the accumulation of secondary carotenoids, a phenomenon that is accompanied by the typical events of cell settlement i.e. detachment of the flagella and cell
volume increase (Sharma et al. 2015). The studies performed with Dunaliella salina
revealed that the UV-A (320–400 nm) effect might be related to the presence of a
specific photoreceptor because neither UV-B (290–320 nm) nor blue light was as
effective in triggering β-carotene accumulation (Jahnke 1999).
5.4.1.5 The Availability of Other Biochemical Pathways
As already explained, carotenoids are made of carbon atoms and therefore their
accumulation requires a functional carbon metabolism. Indeed, this is crucial for the
accumulation of astaxanthin molecules because nearly all the astaxanthin molecules
that accumulate are esterified with one or two fatty acids (Haematococcus pluvialis:
Lemoine et al. (2008); Schoefs et al. (2001); Scenedesmus sp.: Aburai et al. (2015),
Chromochloris zofingiensis: Zhang et al. (2016)). Several reports have established
that an active tricarboxylic acid cycle (Wang et al. 2014; Li et al. 2017; Wu et al. 2013;
Recht et al. 2012, 2014; Zhekisheva et al. 2002) and an active biosynthetic pathway
are sine qua non conditions for astaxanthin accumulation in Haematococcus pluvialis (Gwak et al. 2014; Chen et al. 2015; Lemoine et al. 2008; Schoefs et al. 2001;
Hu et al. 2019). However, strains such as Coelastrum sp. HA-1 (Liu et al. 2013b)
and Ankistrodesmus sp. (Kopecky et al. 2000) that accumulate 50% and nearly all
their astaxanthin as a nonesterified form, respectively, have been described. This
suggests that the mandatory character of the activity of the fatty acid biosynthetic
pathway is relative. To get further insight in this biochemical control, Liu et al.
(2020) studied the impact of externally added linoleic acid on the astaxanthin esterification in Coelastrum sp. HA-1. Interestingly, the presence of exogenously added
linoleic acid downregulated the fatty acid biosynthetic pathway while stimulating the
astaxanthin esterification. The authors concluded that the presence of a high level
of nonesterified astaxanthin in this taxon results of a low activity of the fatty acid
biosynthetic pathway and suggests that the regulation could operate through a fatty
acid threshold that would be reached to allow astaxanthin accumulation. To obtain
further insights on the involvement of the primary metabolism during the process of
in vivo astaxanthin accumulation in Haematococcus pluvialis, Hu et al. (2020) used
a metabolic approach that has revealed changes in the level of numerous metabolite
including nucleotides, organic acids etc. (Table 5.9). In Chromochloris zofingiensis,
another taxon accumulating astaxanthin, transcriptome analyses revealed the upregulation of the genes coding for astaxanthin biosynthesis enzymes whereas the genes
coding enzymes involved in side pathways were down-regulated (Huang et al. 2016).