5 Carotenoid Overproduction in Microalgae: Biochemical …
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5.4.1.3 The Carbon Source
Organic molecules, such as carotenoids are exclusively or mostly composed of carbon
atoms. Microalgae being photosynthetic organisms, they need a carbon source to feed
their metabolism with this compound (Schoefs et al. 2017). A few microalgae are able
to use gaseous CO 2 and nearly all use predominantly the dissolved inorganic CO 2
(DIC), the availability of which is not only crucial for biomass production but also
for the production of biomolecules, including carotenoids. DIC deficiency limiting
growth and the production of biomolecules (e.g. Heydarizadeh et al. (2017), (2019)),
a sufficient DIC availability is required for efficient growth and carotenoid production (Juneja et al. 2013; Gardner et al. 2013). Thanks to the carbon concentration
mechanisms, microalgae are able to utilize up to 50 times more efficiently CO 2 that
land plants (Anto et al. 2019). Optimizing DIC for lutein production by Chlorella
pyrenoidosa triggered an upregulation of the pds, lyc and chy genes and resulted in the
improvement of the production of carotenoid production by 300% (Sampathkumar
and Gothandam 2019). A few studies have been dedicated to the effect of aeration on
Haematococcus pluvialis (Göksan et al. 2011). Azizi et al. (2020) showed that cell
growth under aeration with air at 380 ppm CO 2 at 0.5 vvm instead of 1 vvm favoured
biomass production of Haematococcus pluvialis and astaxanthin production under
high light. Other carbon sources than inorganic carbon can be used depending of the
taxon. For instance, Haematococcus pluvialis can develop on different organic carbon
sources (DOC) such as acetate, malonate and glycerol (Table 5.8). In the presence
of DOC, microalgae are developing in mixotrophic conditions that may impact the
production of secondary carotenoids (Orosa et al. 2001). Interestingly, the utilization
of glycerol by the strain H 6 of Haematococcus pluvialis did not change either the
biomass or the photosynthetic activity when grown under low light (150 μmol m
−2
s
−1 ). In these conditions, the carbon flux was oriented toward the accumulation of
pyruvate that in turn was transformed to acetyl CoA, a precursor of fatty acids. The
formation of acetyl CoA occurred through the action of a glycerol dehydrogenase that
transforms glycerol to dihydroxyacetone (DHA) and glycerol-3-phosphate dehydrogenase, respectively. DHA is then converted to DHA phosphate (DHAP) by a DHA
kinase or through the formation of glyceric acid, that would be ultimately converted
to DHAP, the precursor of acetyl CoA (Zhang et al. 2020). Such a reorientation of the
carbon flux under low light was also observed in the diatom Phaeodactylum tricornutum grown under carbon starvation condition (Heydarizadeh et al. 2017, 2019).
Altogether, this suggests that lipid accumulation constitutes a default answer to stress
conditions in these organisms. It is well established that an active fatty acid biosynthesis is required for the accumulation of astaxanthin in Haematococcus pluvialis,
the fatty acids being used for astaxanthin esterification (Lemoine et al. 2008; Schoefs
et al. 2001). It is therefore not completely surprising that in the presence of exogenous glycerol, astaxanthin accumulated in Haematococcus pluvialis H 6 even under
low light intensity, even though that in these conditions, astaxanthin cellular quota
will not as high than under high-light conditions (Zhang et al. 2020). The ability of
microalgae to use other carbon sources than DIC renders the development of biotechnological processes using heterotrophy and mixotrophic cultivation modes. Because
103
5.4.1.3 The Carbon Source
Organic molecules, such as carotenoids are exclusively or mostly composed of carbon
atoms. Microalgae being photosynthetic organisms, they need a carbon source to feed
their metabolism with this compound (Schoefs et al. 2017). A few microalgae are able
to use gaseous CO 2 and nearly all use predominantly the dissolved inorganic CO 2
(DIC), the availability of which is not only crucial for biomass production but also
for the production of biomolecules, including carotenoids. DIC deficiency limiting
growth and the production of biomolecules (e.g. Heydarizadeh et al. (2017), (2019)),
a sufficient DIC availability is required for efficient growth and carotenoid production (Juneja et al. 2013; Gardner et al. 2013). Thanks to the carbon concentration
mechanisms, microalgae are able to utilize up to 50 times more efficiently CO 2 that
land plants (Anto et al. 2019). Optimizing DIC for lutein production by Chlorella
pyrenoidosa triggered an upregulation of the pds, lyc and chy genes and resulted in the
improvement of the production of carotenoid production by 300% (Sampathkumar
and Gothandam 2019). A few studies have been dedicated to the effect of aeration on
Haematococcus pluvialis (Göksan et al. 2011). Azizi et al. (2020) showed that cell
growth under aeration with air at 380 ppm CO 2 at 0.5 vvm instead of 1 vvm favoured
biomass production of Haematococcus pluvialis and astaxanthin production under
high light. Other carbon sources than inorganic carbon can be used depending of the
taxon. For instance, Haematococcus pluvialis can develop on different organic carbon
sources (DOC) such as acetate, malonate and glycerol (Table 5.8). In the presence
of DOC, microalgae are developing in mixotrophic conditions that may impact the
production of secondary carotenoids (Orosa et al. 2001). Interestingly, the utilization
of glycerol by the strain H 6 of Haematococcus pluvialis did not change either the
biomass or the photosynthetic activity when grown under low light (150 μmol m
−2
s
−1 ). In these conditions, the carbon flux was oriented toward the accumulation of
pyruvate that in turn was transformed to acetyl CoA, a precursor of fatty acids. The
formation of acetyl CoA occurred through the action of a glycerol dehydrogenase that
transforms glycerol to dihydroxyacetone (DHA) and glycerol-3-phosphate dehydrogenase, respectively. DHA is then converted to DHA phosphate (DHAP) by a DHA
kinase or through the formation of glyceric acid, that would be ultimately converted
to DHAP, the precursor of acetyl CoA (Zhang et al. 2020). Such a reorientation of the
carbon flux under low light was also observed in the diatom Phaeodactylum tricornutum grown under carbon starvation condition (Heydarizadeh et al. 2017, 2019).
Altogether, this suggests that lipid accumulation constitutes a default answer to stress
conditions in these organisms. It is well established that an active fatty acid biosynthesis is required for the accumulation of astaxanthin in Haematococcus pluvialis,
the fatty acids being used for astaxanthin esterification (Lemoine et al. 2008; Schoefs
et al. 2001). It is therefore not completely surprising that in the presence of exogenous glycerol, astaxanthin accumulated in Haematococcus pluvialis H 6 even under
low light intensity, even though that in these conditions, astaxanthin cellular quota
will not as high than under high-light conditions (Zhang et al. 2020). The ability of
microalgae to use other carbon sources than DIC renders the development of biotechnological processes using heterotrophy and mixotrophic cultivation modes. Because