5 Carotenoid Overproduction in Microalgae: Biochemical …
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reduced (e.g., Mn: phytoene synthase, Fe: β-carotene hydroxylase) (Lee and SchmidtDannert 2002; Dineshkumar et al. 2015). Other unfavourable conditions, such as
cell aging (Gu et al. 2013), temperature (Tjahjono et al. 1994; Giannelli et al. 2015),
irradiance level (Gu et al. 2014; Schoefs et al. 2001), UV-B irradiation, salinity
(Gao et al. 2015), nutrient deprivation (Sampathkumar et al. 2019; Azadeh et al.
2017; Wu et al. 2013; Scibilia et al. 2015) and drought, alone or in combination
(Hagen et al. 2000; Su et al. 2014; Zhekisheva et al. 2002; Lemoine and Schoefs
2010; Hong et al. 2015; Dominguez-Bocanegra et al. 2004) generate a stress against
which the cells must react under penalty of dying. Among the different response
mechanisms triggered by stress, the metabolic reorientation allows the accumulation
of compounds such as lipids (Sayanova et al. 2017) and carotenoids (Lemoine and
Schoefs 2010). If the metabolic network along which the carotenoids are synthesized
is rather well established, the regulation pathways controlling the activity of the
biochemical network remain mostly unknown. In this section, the impacts of stress
on the carotenoid production are summarized as well as the signalling and some
biochemical and molecular regulation aspects.
5.4.1 The Composition of the Growth Medium
In the natural environment, water evaporation triggers nutrient precipitation, reducing
nutrient availability for the microalgae. Consequently, the concentration in sodium
chloride increases and together with the decrease in nutrient availability triggers a
stress. Simultaneously, the light intensity can increase, adding to the stress perception by the microalgae. These natural conditions to which each microalgal cell is
exposed can be somehow mimicked by changing the culture environment, including
the composition of the growth medium, to induce the carotenoid production by
microalgae (Srinivasan et al. 2015, 2018; Sampathkumar et al. 2019).
5.4.1.1 The Ionic Composition
Ions are crucial for nearly all cellular activities. For instance, magnesium is vital
for chlorophyll biosynthesis (Schoefs 2005b) and more generally for photosynthesis
and other cell processes. Two types of ions can be defined: the essential and the
nonessential ions. Essential ions are nitrate, phosphate (Boussiba et al. 1999) and
sulfur ions (He et al. 2007) and nonessential are calcium, magnesium, chromium,
cobalt and iodine ions (Fabregas et al. 2000). To ensure an adequate ion cellular homeostasis within the different cell compartments, each of them hosts an arsenal of ion
transporters (Marchand et al. 2018, 2020). Unbalance of the ionic composition may
promote stress effects (Masmoudi et al. 2013), impacting directly or indirectly the
carotenoid production. For instance, a high magnesium concentration in the growth
medium of the green alga Chlorella vulgaris promotes phytohormone production
101
reduced (e.g., Mn: phytoene synthase, Fe: β-carotene hydroxylase) (Lee and SchmidtDannert 2002; Dineshkumar et al. 2015). Other unfavourable conditions, such as
cell aging (Gu et al. 2013), temperature (Tjahjono et al. 1994; Giannelli et al. 2015),
irradiance level (Gu et al. 2014; Schoefs et al. 2001), UV-B irradiation, salinity
(Gao et al. 2015), nutrient deprivation (Sampathkumar et al. 2019; Azadeh et al.
2017; Wu et al. 2013; Scibilia et al. 2015) and drought, alone or in combination
(Hagen et al. 2000; Su et al. 2014; Zhekisheva et al. 2002; Lemoine and Schoefs
2010; Hong et al. 2015; Dominguez-Bocanegra et al. 2004) generate a stress against
which the cells must react under penalty of dying. Among the different response
mechanisms triggered by stress, the metabolic reorientation allows the accumulation
of compounds such as lipids (Sayanova et al. 2017) and carotenoids (Lemoine and
Schoefs 2010). If the metabolic network along which the carotenoids are synthesized
is rather well established, the regulation pathways controlling the activity of the
biochemical network remain mostly unknown. In this section, the impacts of stress
on the carotenoid production are summarized as well as the signalling and some
biochemical and molecular regulation aspects.
5.4.1 The Composition of the Growth Medium
In the natural environment, water evaporation triggers nutrient precipitation, reducing
nutrient availability for the microalgae. Consequently, the concentration in sodium
chloride increases and together with the decrease in nutrient availability triggers a
stress. Simultaneously, the light intensity can increase, adding to the stress perception by the microalgae. These natural conditions to which each microalgal cell is
exposed can be somehow mimicked by changing the culture environment, including
the composition of the growth medium, to induce the carotenoid production by
microalgae (Srinivasan et al. 2015, 2018; Sampathkumar et al. 2019).
5.4.1.1 The Ionic Composition
Ions are crucial for nearly all cellular activities. For instance, magnesium is vital
for chlorophyll biosynthesis (Schoefs 2005b) and more generally for photosynthesis
and other cell processes. Two types of ions can be defined: the essential and the
nonessential ions. Essential ions are nitrate, phosphate (Boussiba et al. 1999) and
sulfur ions (He et al. 2007) and nonessential are calcium, magnesium, chromium,
cobalt and iodine ions (Fabregas et al. 2000). To ensure an adequate ion cellular homeostasis within the different cell compartments, each of them hosts an arsenal of ion
transporters (Marchand et al. 2018, 2020). Unbalance of the ionic composition may
promote stress effects (Masmoudi et al. 2013), impacting directly or indirectly the
carotenoid production. For instance, a high magnesium concentration in the growth
medium of the green alga Chlorella vulgaris promotes phytohormone production