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Anatomy
retinoid represents the eyespot and is made up of pigment containing droplets enclosed in a vesicular layer. Small droplets contain carotenoid pigments, whereas large droplets contain melanoid
pigments. The retinoid is an extremely complex membranous construction made up of numerous
regularly arranged layers giving an almost paracrystalline appearance.
No data are available on the structure and localization of the photoreceptive system in the divisions and classes of algae other than those listed above. It seems unlikely that Type II and III systems have not been identified so far in other algal groups, although it is more reasonable to assume
that this lack of information is mainly due to the difficulty to reveal photoreceptor systems belonging to Type I. We can conclude that those algae should possess photoreceptor systems that can be
taken back to Type I.
phoTorecepTiVe proTeins
Over the past 10 years, genome sequencing and sequence comparison tools have revealed that
genes encoding rhodopsin-like photoreceptive proteins are shared among distant taxa, in all three
domains of life: Archaea, Eubacteria, and Eukarya. As far as algal world is concerned, reports from
different research groups worldwide have provided examples of the broad distribution of Type I
rhodopsin-based photoreceptors among the different algal division and allowed the assumption that
these proteins are present in all the algae of the supergroups of Plantae (Glaucophyta, Rhodophyta,
Chlorophyta) and Chromoalveolata (Haptophyta, Cryptophyta, Ochrophyta, and Dinophyceae), that
is, those originating from a primary symbiotic event, and those originating from a secondary or tertiary symbiotic event. In fact, genes encoding functional rhodopsins probably related to photoreception or ionic transport have been detected in Cyanophyceae (prokaryotic algae), and Cryptophyceae,
Glaucophyceae, Chlorophyceae, Dasycladophyceae, Mesostygmatophyceae, Mamiellophyceae,
prasinophytes, Trebouxiophyceae, Coccolithophyceae, and Dinophyceae, all eukaryotic algae. The
presence of an integral membrane protein in the membrane of the muroplast of Cyanophora paradoxa, a glaucophyte, the most primitive of all plastids, could indicate that the common primordial
rhodopsin existed in preeukaryotic cells before the appearance of the first photosynthetic eukaryotic cell about 1.55 × 10 9 years ago (Figure 2.75). Moreover, spectroscopical and biochemical evidence of rhodopsin-based photoreceptor is available for algae belonging also to Euglenophyceae,
Chrysophyceae, and Phaeophyceae.
All rhodopsins consist of a proteic part, the opsin, organized in seven transmembrane
α-helices, and a light absorbing group, the retinal (i.e., the chromophore). The retinal is located
inside a pocket of the opsin, approximately at its center. These proteins can be considered special
for many reasons. First, retinal–opsin complex has an intense absorption band whose maximum
can be shifted to the visible region of the spectrum, over the entire range from 380 to 640 nm.
Second, light isomerizes the retinal inside the protein very efficiently and rapidly. This onemolecule isomerization, that is, the event initiating the vision reaction cascade, can be triggered
almost exclusively by light. In the dark, it occurs about only once in a thousand years. Third,
remarkable structural changes (movements of single α-helix) are produced by isomerization of
retinal. Light is converted into atomic motion of sufficient magnitude to trigger a signal reliably
and reproducibly. Fourth, the photocycle (the photoreceptive protein upon light excitation undergoes a series of conformational changes that can be driven back to the original conformational
state) is very fast, and hence the intracellular photoreceptive machinery is immediately reset for
a new response. Fifth, retinal is derived from β-carotene, a precursor with a widespread biological distribution.
FUndamenTal behaVioral and physiological FeaTUres
The structural features described above should be accompanied by necessary behavioral and physiological characteristics (sampling strategies, trajectory control, and signal transmission).
Anatomy
retinoid represents the eyespot and is made up of pigment containing droplets enclosed in a vesicular layer. Small droplets contain carotenoid pigments, whereas large droplets contain melanoid
pigments. The retinoid is an extremely complex membranous construction made up of numerous
regularly arranged layers giving an almost paracrystalline appearance.
No data are available on the structure and localization of the photoreceptive system in the divisions and classes of algae other than those listed above. It seems unlikely that Type II and III systems have not been identified so far in other algal groups, although it is more reasonable to assume
that this lack of information is mainly due to the difficulty to reveal photoreceptor systems belonging to Type I. We can conclude that those algae should possess photoreceptor systems that can be
taken back to Type I.
phoTorecepTiVe proTeins
Over the past 10 years, genome sequencing and sequence comparison tools have revealed that
genes encoding rhodopsin-like photoreceptive proteins are shared among distant taxa, in all three
domains of life: Archaea, Eubacteria, and Eukarya. As far as algal world is concerned, reports from
different research groups worldwide have provided examples of the broad distribution of Type I
rhodopsin-based photoreceptors among the different algal division and allowed the assumption that
these proteins are present in all the algae of the supergroups of Plantae (Glaucophyta, Rhodophyta,
Chlorophyta) and Chromoalveolata (Haptophyta, Cryptophyta, Ochrophyta, and Dinophyceae), that
is, those originating from a primary symbiotic event, and those originating from a secondary or tertiary symbiotic event. In fact, genes encoding functional rhodopsins probably related to photoreception or ionic transport have been detected in Cyanophyceae (prokaryotic algae), and Cryptophyceae,
Glaucophyceae, Chlorophyceae, Dasycladophyceae, Mesostygmatophyceae, Mamiellophyceae,
prasinophytes, Trebouxiophyceae, Coccolithophyceae, and Dinophyceae, all eukaryotic algae. The
presence of an integral membrane protein in the membrane of the muroplast of Cyanophora paradoxa, a glaucophyte, the most primitive of all plastids, could indicate that the common primordial
rhodopsin existed in preeukaryotic cells before the appearance of the first photosynthetic eukaryotic cell about 1.55 × 10 9 years ago (Figure 2.75). Moreover, spectroscopical and biochemical evidence of rhodopsin-based photoreceptor is available for algae belonging also to Euglenophyceae,
Chrysophyceae, and Phaeophyceae.
All rhodopsins consist of a proteic part, the opsin, organized in seven transmembrane
α-helices, and a light absorbing group, the retinal (i.e., the chromophore). The retinal is located
inside a pocket of the opsin, approximately at its center. These proteins can be considered special
for many reasons. First, retinal–opsin complex has an intense absorption band whose maximum
can be shifted to the visible region of the spectrum, over the entire range from 380 to 640 nm.
Second, light isomerizes the retinal inside the protein very efficiently and rapidly. This onemolecule isomerization, that is, the event initiating the vision reaction cascade, can be triggered
almost exclusively by light. In the dark, it occurs about only once in a thousand years. Third,
remarkable structural changes (movements of single α-helix) are produced by isomerization of
retinal. Light is converted into atomic motion of sufficient magnitude to trigger a signal reliably
and reproducibly. Fourth, the photocycle (the photoreceptive protein upon light excitation undergoes a series of conformational changes that can be driven back to the original conformational
state) is very fast, and hence the intracellular photoreceptive machinery is immediately reset for
a new response. Fifth, retinal is derived from β-carotene, a precursor with a widespread biological distribution.
FUndamenTal behaVioral and physiological FeaTUres
The structural features described above should be accompanied by necessary behavioral and physiological characteristics (sampling strategies, trajectory control, and signal transmission).