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S. Kato and T. Shinomura
4.1 Introduction
Carotenoids, isoprenoid compounds with 40 carbon backbones, are the most
widespread pigments in nature. They are found in various organisms including
bacteria, archaea, fungi, algae, plants, and animals. So far, more than 750 naturally
occurring carotenoids have been isolated and structurally identified from photosynthetic and non-photosynthetic organisms (Britton et al. 2004). Carotenoids are
indispensable components of photosynthetic machinery in photoautotrophs, forming
chlorophyll- and carotenoid-binding protein complexes in thylakoid membranes of
cyanobacteria, eukaryotic algae, and land plants. They serve crucial roles in light
harvesting to drive photosynthesis and protecting photosystem II (PSII) from photodamage by thermal dissipation and scavenging of reactive oxygen species (Müller
et al. 2001; Triantaphylidès and Havaux 2009; Telfer 2014).
Photosynthetic organisms are exposed to various abiotic stresses such as high
irradiance that can induce photodamage to PSII and heat, low-temperature, drought,
salinity, and oxidative stress that can enhance the extent of photoinhibition by
inhibiting the repair of photodamaged PSII (Nishiyama et al. 2008; Takahashi and
Murata 2008). Those environmental stimuli can alter the biosynthesis of photosynthetic pigments. Here, we summarize studies on the regulation of biosynthesis and
accumulation of carotenoids in eukaryotic microalgae in response to light and temperature. In addition, we would like to introduce our recent findings on the physiology
of carotenoid biosynthesis of Euglena gracilis in response to those environmental
stimuli.
4.2 Carotenoids in Photosynthetic Machinery
Photosystem I (PSI) and PSII core complexes contain carotenoids. X-ray structural
studies identified 22 and 12 carotenoid molecules in the monomeric PSI at 2.5 Å resolution and PSII at 2.9 Å resolution, respectively, of Thermosynechococcus elongatus
(Jordan et al. 2001; Guskov et al. 2009). Umena et al. (2011) assigned 11 β-carotenes
in the monomeric PSII of Thermosynechococcus vulcanus at 1.9 Å resolution. PSII
reaction center of T. vulcanus contains two molecules of β-carotene in the vicinity
of reaction center chlorophylls (Kamiya and Shen 2003). β-carotenes in the PSII
reaction center are considered to function primarily in scavenging of singlet oxygen
produced via triplet-state P 680 (Triantaphylidès and Havaux 2009; Telfer 2014). The
core light-harvesting complex (LHC) of PSII also contains β-carotene and peripheral
LHC of PSII contains several xanthophylls.
Major xanthophylls in the peripheral LHC vary depending on algal divisions/classes: e.g., lutein, violaxanthin, and 9
-cis neoxanthin in land plants; alloxanthin in Cryptophyta; fucoxanthin in Chrysophyceae, Bacillariophyceae, Phaeophyceae, and Haptophyta; peridinin in Dinophyta; and diadinoxanthin and diatoxanthin in Haptophyta, Heterokontophyta, Dinophyta, and Euglenophyta (Takaichi
S. Kato and T. Shinomura
4.1 Introduction
Carotenoids, isoprenoid compounds with 40 carbon backbones, are the most
widespread pigments in nature. They are found in various organisms including
bacteria, archaea, fungi, algae, plants, and animals. So far, more than 750 naturally
occurring carotenoids have been isolated and structurally identified from photosynthetic and non-photosynthetic organisms (Britton et al. 2004). Carotenoids are
indispensable components of photosynthetic machinery in photoautotrophs, forming
chlorophyll- and carotenoid-binding protein complexes in thylakoid membranes of
cyanobacteria, eukaryotic algae, and land plants. They serve crucial roles in light
harvesting to drive photosynthesis and protecting photosystem II (PSII) from photodamage by thermal dissipation and scavenging of reactive oxygen species (Müller
et al. 2001; Triantaphylidès and Havaux 2009; Telfer 2014).
Photosynthetic organisms are exposed to various abiotic stresses such as high
irradiance that can induce photodamage to PSII and heat, low-temperature, drought,
salinity, and oxidative stress that can enhance the extent of photoinhibition by
inhibiting the repair of photodamaged PSII (Nishiyama et al. 2008; Takahashi and
Murata 2008). Those environmental stimuli can alter the biosynthesis of photosynthetic pigments. Here, we summarize studies on the regulation of biosynthesis and
accumulation of carotenoids in eukaryotic microalgae in response to light and temperature. In addition, we would like to introduce our recent findings on the physiology
of carotenoid biosynthesis of Euglena gracilis in response to those environmental
stimuli.
4.2 Carotenoids in Photosynthetic Machinery
Photosystem I (PSI) and PSII core complexes contain carotenoids. X-ray structural
studies identified 22 and 12 carotenoid molecules in the monomeric PSI at 2.5 Å resolution and PSII at 2.9 Å resolution, respectively, of Thermosynechococcus elongatus
(Jordan et al. 2001; Guskov et al. 2009). Umena et al. (2011) assigned 11 β-carotenes
in the monomeric PSII of Thermosynechococcus vulcanus at 1.9 Å resolution. PSII
reaction center of T. vulcanus contains two molecules of β-carotene in the vicinity
of reaction center chlorophylls (Kamiya and Shen 2003). β-carotenes in the PSII
reaction center are considered to function primarily in scavenging of singlet oxygen
produced via triplet-state P 680 (Triantaphylidès and Havaux 2009; Telfer 2014). The
core light-harvesting complex (LHC) of PSII also contains β-carotene and peripheral
LHC of PSII contains several xanthophylls.
Major xanthophylls in the peripheral LHC vary depending on algal divisions/classes: e.g., lutein, violaxanthin, and 9
-cis neoxanthin in land plants; alloxanthin in Cryptophyta; fucoxanthin in Chrysophyceae, Bacillariophyceae, Phaeophyceae, and Haptophyta; peridinin in Dinophyta; and diadinoxanthin and diatoxanthin in Haptophyta, Heterokontophyta, Dinophyta, and Euglenophyta (Takaichi