7 Carotenoids
Carotenoids constitute a structurally diverse group of colored lipidic pigments,
which are widespread in nature and play crucial roles in many physiological
processes. Carotenoids represent a long aliphatic polyene chain composed of “n”
isoprene units (C 5 H 8 ), which are sometimes terminated by rings, and may or may not
have additional oxygen atom attached. They are initially synthesized with a chain of
30 or 40 carbon atoms from prenyl pyrophosphates, and they may be extended at a
later stage up to 50 carbon atoms [57]. There are two general classes of carotenoids:
carotenes and xanthophylls. The familiar carotenes are β-carotene and lycopene.
Both are strict hydrocarbon carotenoids and do not possess any substituent (or even
oxygen) in their structures. Xanthophylls or oxycarotenoids, which belong to the
second group, are oxygen-containing molecules. Lutein and zeaxanthin are two
xanthophylls with –OH groups in their structures, whereas canthaxanthin and
echinenone contain ¼O groups. Astaxanthin has both –OH and ¼O groups in its
structure [58].
Carotenoids have a wide range of application in the health care and nutraceutical
industry; they are produced as food colorants, feed supplements, and nutraceuticals,
for cosmetic and for pharmaceutical purposes. Carotenoids are essential for humans
and must be supplied through diet. The growing importance of carotenoid has also
led to an increase in the demand for carotenoids in the global market, which was
estimated to be ~1.24 billion USD in 2016, and is projected to increase to ~1.53
billion USD by 2021, at a compound annual growth rate (CAGR) of 3.8% from 2016
to 2021 [58].
Carotenoids are essential components of all photosynthetic organisms due to
their eminent photoprotective and antioxidant properties. However, their occurrence
is not restricted to plants, algae, and cyanobacteria, as some fungi and nonphotosynthetic bacteria can synthesize carotenoids as well [59]. Recently, microbial
sources of carotenoids have received attention due to the stringent rules and regulations applied to chemically synthesized/purified pigments (like replacing synthetic
astaxanthin for salmon feed). Microorganisms for the industrial production of
carotenoids are preferred over other natural sources (such as vegetables and fruits)
due to problems of seasonal and geographic variability in production of the
latter [60].
According to the Carotenoids Database providing chemical information on 1,117
natural carotenoids with 683 source organisms, carotenoids seem to have been
spread largely in bacteria, as they produce C30, C40, C45, and C50 carotenoids
(based on the molecular carbon number), with the widest range of end groups, and
they share a small portion of C40 carotenoids with eukaryotes. Archaea share an
even smaller portion with eukaryotes. Eukaryotes have evolved a considerable
variety of C40 carotenoids. Considering carotenoids, eukaryotes seem more closely
related to bacteria than to archaea [61].
Carotenoids of diverse structures have been also found among alkaliphilic bacteria. Previous reviewers have reported about triterpenoid carotenoids from yellow
174
E. Khalikova et al.
Carotenoids constitute a structurally diverse group of colored lipidic pigments,
which are widespread in nature and play crucial roles in many physiological
processes. Carotenoids represent a long aliphatic polyene chain composed of “n”
isoprene units (C 5 H 8 ), which are sometimes terminated by rings, and may or may not
have additional oxygen atom attached. They are initially synthesized with a chain of
30 or 40 carbon atoms from prenyl pyrophosphates, and they may be extended at a
later stage up to 50 carbon atoms [57]. There are two general classes of carotenoids:
carotenes and xanthophylls. The familiar carotenes are β-carotene and lycopene.
Both are strict hydrocarbon carotenoids and do not possess any substituent (or even
oxygen) in their structures. Xanthophylls or oxycarotenoids, which belong to the
second group, are oxygen-containing molecules. Lutein and zeaxanthin are two
xanthophylls with –OH groups in their structures, whereas canthaxanthin and
echinenone contain ¼O groups. Astaxanthin has both –OH and ¼O groups in its
structure [58].
Carotenoids have a wide range of application in the health care and nutraceutical
industry; they are produced as food colorants, feed supplements, and nutraceuticals,
for cosmetic and for pharmaceutical purposes. Carotenoids are essential for humans
and must be supplied through diet. The growing importance of carotenoid has also
led to an increase in the demand for carotenoids in the global market, which was
estimated to be ~1.24 billion USD in 2016, and is projected to increase to ~1.53
billion USD by 2021, at a compound annual growth rate (CAGR) of 3.8% from 2016
to 2021 [58].
Carotenoids are essential components of all photosynthetic organisms due to
their eminent photoprotective and antioxidant properties. However, their occurrence
is not restricted to plants, algae, and cyanobacteria, as some fungi and nonphotosynthetic bacteria can synthesize carotenoids as well [59]. Recently, microbial
sources of carotenoids have received attention due to the stringent rules and regulations applied to chemically synthesized/purified pigments (like replacing synthetic
astaxanthin for salmon feed). Microorganisms for the industrial production of
carotenoids are preferred over other natural sources (such as vegetables and fruits)
due to problems of seasonal and geographic variability in production of the
latter [60].
According to the Carotenoids Database providing chemical information on 1,117
natural carotenoids with 683 source organisms, carotenoids seem to have been
spread largely in bacteria, as they produce C30, C40, C45, and C50 carotenoids
(based on the molecular carbon number), with the widest range of end groups, and
they share a small portion of C40 carotenoids with eukaryotes. Archaea share an
even smaller portion with eukaryotes. Eukaryotes have evolved a considerable
variety of C40 carotenoids. Considering carotenoids, eukaryotes seem more closely
related to bacteria than to archaea [61].
Carotenoids of diverse structures have been also found among alkaliphilic bacteria. Previous reviewers have reported about triterpenoid carotenoids from yellow
174
E. Khalikova et al.